1FLAWFINDER(1) Flawfinder FLAWFINDER(1)
2
6 flawfinder - lexically find potential security flaws ("hits") in source
7 code
8
10 flawfinder [--help|-h] [--version] [--listrules]
11 [--allowlink] [--followdotdir] [--nolink]
12 [--patch=filename|-P filename]
13 [--inputs|-I] [ --minlevel=X | -m X ] [--falsepositive|-F]
14 [--neverignore|-n]
15 [--regex=PATTERN | -e PATTERN]
16 [--context|-c] [--columns|-C] [--csv] [--dataonly|-D] [--html|-H]
17 [--immediate|-i] [--singleline|-S] [--omittime] [--quiet|-Q] [--error-
18 level=LEVEL]
19 [--loadhitlist=F] [--savehitlist=F] [--diffhitlist=F]
20 [--] [ source code file or source root directory ]+
21
23 Flawfinder searches through C/C++ source code looking for potential
24 security flaws. To run flawfinder, simply give flawfinder a list of
25 directories or files. For each directory given, all files that have
26 C/C++ filename extensions in that directory (and its subdirectories,
27 recursively) will be examined. Thus, for most projects, simply give
28 flawfinder the name of the source code's topmost directory (use ``.''
29 for the current directory), and flawfinder will examine all of the
30 project's C/C++ source code. Flawfinder does not require that you be
31 able to build your software, so it can be used even with incomplete
32 source code. If you only want to have changes reviewed, save a unified
33 diff of those changes (created by GNU "diff -u" or "svn diff" or "git
34 diff") in a patch file and use the --patch (-P) option.
35 Flawfinder will produce a list of ``hits'' (potential security flaws,
37 also called findings), sorted by risk; the riskiest hits are shown
38 first. The risk level is shown inside square brackets and varies from
39 0, very little risk, to 5, great risk. This risk level depends not
40 only on the function, but on the values of the parameters of the func‐
41 tion. For example, constant strings are often less risky than fully
42 variable strings in many contexts, and in those contexts the hit will
43 have a lower risk level. Flawfinder knows about gettext (a common
44 library for internationalized programs) and will treat constant strings
45 passed through gettext as though they were constant strings; this
46 reduces the number of false hits in internationalized programs.
47 Flawfinder will do the same sort of thing with _T() and _TEXT(), common
48 Microsoft macros for handling internationalized programs. Flawfinder
49 correctly ignores text inside comments and strings. Normally
50 flawfinder shows all hits with a risk level of at least 1, but you can
51 use the --minlevel option to show only hits with higher risk levels if
52 you wish. Hit descriptions also note the relevant Common Weakness Enu‐
53 meration (CWE) identifier(s) in parentheses, as discussed below.
54 Flawfinder is officially CWE-Compatible. Hit descriptions with "[MS-
55 banned]" indicate functions that are in the banned list of functions
56 released by Microsoft; see http://msdn.microsoft.com/en-
57 us/library/bb288454.aspx for more information about banned functions.
58 Not every hit (aka finding) is actually a security vulnerability, and
60 not every security vulnerability is necessarily found. Nevertheless,
61 flawfinder can be an aid in finding and removing security vulnerabili‐
62 ties. A common way to use flawfinder is to first apply flawfinder to a
63 set of source code and examine the highest-risk items. Then, use
64 --inputs to examine the input locations, and check to make sure that
65 only legal and safe input values are accepted from untrusted users.
66 Once you've audited a program, you can mark source code lines that are
68 actually fine but cause spurious warnings so that flawfinder will stop
69 complaining about them. To mark a line so that these warnings are sup‐
70 pressed, put a specially-formatted comment either on the same line
71 (after the source code) or all by itself in the previous line. The
72 comment must have one of the two following formats:
73 · // Flawfinder: ignore
75 · /* Flawfinder: ignore */
77 For compatibility's sake, you can replace "Flawfinder:" with "ITS4:" or
79 "RATS:" in these specially-formatted comments. Since it's possible
80 that such lines are wrong, you can use the --neverignore option, which
81 causes flawfinder to never ignore any line no matter what the comment
82 directives say (more confusingly, --neverignore ignores the ignores).
83 Flawfinder uses an internal database called the ``ruleset''; the rule‐
85 set identifies functions that are common causes of security flaws. The
86 standard ruleset includes a large number of different potential prob‐
87 lems, including both general issues that can impact any C/C++ program,
88 as well as a number of specific Unix-like and Windows functions that
89 are especially problematic. The --listrules option reports the list of
90 current rules and their default risk levels. As noted above, every
91 potential security flaw found in a given source code file (matching an
92 entry in the ruleset) is called a ``hit,'' and the set of hits found
93 during any particular run of the program is called the ``hitlist.''
94 Hitlists can be saved (using --savehitlist), reloaded back for redis‐
95 play (using --loadhitlist), and you can show only the hits that are
96 different from another run (using --diffhitlist).
97 Flawfinder is a simple tool, leading to some fundamental pros and cons.
99 Flawfinder works by doing simple lexical tokenization (skipping com‐
100 ments and correctly tokenizing strings), looking for token matches to
101 the database (particularly to find function calls). Flawfinder is thus
102 similar to RATS and ITS4, which also use simple lexical tokenization.
103 Flawfinder then examines the text of the function parameters to esti‐
104 mate risk. Unlike tools such as splint, gcc's warning flags, and
105 clang, flawfinder does not use or have access to information about con‐
106 trol flow, data flow, or data types when searching for potential vul‐
107 nerabilities or estimating the level of risk. Thus, flawfinder will
108 necessarily produce many false positives for vulnerabilities and fail
109 to report many vulnerabilities. On the other hand, flawfinder can find
110 vulnerabilities in programs that cannot be built or cannot be linked.
111 It can often work with programs that cannot even be compiled (at least
112 by the reviewer's tools). Flawfinder also doesn't get as confused by
113 macro definitions and other oddities that more sophisticated tools have
114 trouble with. Flawfinder can also be useful as a simple introduction
115 to static analysis tools in general, since it is easy to start using
116 and easy to understand.
117 Any filename given on the command line will be examined (even if it
119 doesn't have a usual C/C++ filename extension); thus you can force
120 flawfinder to examine any specific files you desire. While searching
121 directories recursively, flawfinder only opens and examines regular
122 files that have C/C++ filename extensions. Flawfinder presumes that
123 files are C/C++ files if they have the extensions ".c", ".h", ".ec",
124 ".ecp", ".pgc", ".C", ".cpp", ".CPP", ".cxx", ".c++", ".cc", ".CC",
125 ".pcc", ".hpp", or ".H". The filename ``-'' means the standard input.
126 To prevent security problems, special files (such as device special
127 files and named pipes) are always skipped, and by default symbolic
128 links are skipped (the --allowlink option follows symbolic links).
129 After the list of hits is a brief summary of the results (use -D to
131 remove this information). It will show the number of hits, lines ana‐
132 lyzed (as reported by wc -l), and the physical source lines of code
133 (SLOC) analyzed. A physical SLOC is a non-blank, non-comment line. It
134 will then show the number of hits at each level; note that there will
135 never be a hit at a level lower than minlevel (1 by default). Thus,
136 "[0] 0 [1] 9" means that at level 0 there were 0 hits reported, and
137 at level 1 there were 9 hits reported. It will next show the number of
138 hits at a given level or larger (so level 3+ has the sum of the number
139 of hits at level 3, 4, and 5). Thus, an entry of "[0+] 37" shows that
140 at level 0 or higher there were 37 hits (the 0+ entry will always be
141 the same as the "hits" number above). Hits per KSLOC is next shown;
142 this is each of the "level or higher" values multiplied by 1000 and
143 divided by the physical SLOC. If symlinks were skipped, the count of
144 those is reported. If hits were suppressed (using the "ignore" direc‐
145 tive in source code comments as described above), the number suppressed
146 is reported. The minimum risk level to be included in the report is
147 displayed; by default this is 1 (use --minlevel to change this). The
148 summary ends with important reminders: Not every hit is necessarily a
149 security vulnerability, and there may be other security vulnerabilities
150 not reported by the tool.
151 Flawfinder can easily integrate into a continuous integration system.
153 You might want to check out the --error-level option to help do that.
154 Flawfinder is released under the GNU GPL license version 2 or later
156 (GPLv2+).
157 Flawfinder works similarly to another program, ITS4, which is not fully
159 open source software (as defined in the Open Source Definition) nor
160 free software (as defined by the Free Software Foundation). The author
161 of Flawfinder has never seen ITS4's source code. Flawfinder is similar
162 in many ways to RATS, if you are familiar with RATS.
163
166 Here's a brief example of how flawfinder might be used. Imagine that
167 you have the C/C++ source code for some program named xyzzy (which you
168 may or may not have written), and you're searching for security vulner‐
169 abilities (so you can fix them before customers encounter the vulnera‐
170 bilities). For this tutorial, I'll assume that you're using a Unix-
171 like system, such as Linux, OpenBSD, or MacOS X.
172 If the source code is in a subdirectory named xyzzy, you would probably
174 start by opening a text window and using flawfinder's default settings,
175 to analyze the program and report a prioritized list of potential secu‐
176 rity vulnerabilities (the ``less'' just makes sure the results stay on
177 the screen):
178 flawfinder xyzzy | less
179 At this point, you will see a large number of entries. Each entry has
182 a filename, a colon, a line number, a risk level in brackets (where 5
183 is the most risky), a category, the name of the function, and a
184 description of why flawfinder thinks the line is a vulnerability.
185 Flawfinder normally sorts by risk level, showing the riskiest items
186 first; if you have limited time, it's probably best to start working on
187 the riskiest items and continue until you run out of time. If you want
188 to limit the display to risks with only a certain risk level or higher,
189 use the --minlevel option. If you're getting an extraordinary number
190 of false positives because variable names look like dangerous function
191 names, use the -F option to remove reports about them. If you don't
192 understand the error message, please see documents such as the Secure
193 Programming HOWTO ⟨https://dwheeler.com/secure-programs⟩ at
194 https://dwheeler.com/secure-programs which provides more information on
195 writing secure programs.
196 Once you identify the problem and understand it, you can fix it. Occa‐
198 sionally you may want to re-do the analysis, both because the line num‐
199 bers will change and to make sure that the new code doesn't introduce
200 yet a different vulnerability.
201 If you've determined that some line isn't really a problem, and you're
203 sure of it, you can insert just before or on the offending line a com‐
204 ment like
205 /* Flawfinder: ignore */
206 to keep them from showing up in the output.
207 Once you've done that, you should go back and search for the program's
209 inputs, to make sure that the program strongly filters any of its
210 untrusted inputs. Flawfinder can identify many program inputs by using
211 the --inputs option, like this:
212 flawfinder --inputs xyzzy
213 Flawfinder can integrate well with text editors and integrated develop‐
215 ment environments; see the examples for more information.
216 Flawfinder includes many other options, including ones to create HTML
218 versions of the output (useful for prettier displays). The next sec‐
219 tion describes those options in more detail.
220
223 Flawfinder has a number of options, which can be grouped into options
224 that control its own documentation, select input data, select which
225 hits to display, select the output format, and perform hitlist manage‐
226 ment. The commonly-used flawfinder options support the standard option
227 syntax defined in the POSIX (Issue 7, 2013 Edition) section ``Utility
228 Conventions''. Flawfinder also supports the GNU long options (double-
229 dash options of form --option) as defined in the GNU C Library Refer‐
230 ence Manual ``Program Argument Syntax Conventions'' and GNU Coding
231 Standards ``Standards for Command Line Interfaces''. Long option argu‐
232 ments can be provided as ``--name=value'' or ``-name value''. All
233 options can be accessed using the more readable GNU long option conven‐
234 tions; some less commonly used options can only be accessed using long
235 option conventions.
236 Documentation
239 --help
240 -h Show usage (help) information.
242 --version Shows (just) the version number and exits.
245 --listrules List the terms (tokens) that trigger further examination,
248 their default risk level, and the default warning (includ‐
249 ing the CWE identifier(s), if applicable), all tab-sepa‐
250 rated. The terms are primarily names of potentially-dan‐
251 gerous functions. Note that the reported risk level and
252 warning for some specific code may be different than the
253 default, depending on how the term is used. Combine with
254 -D if you do not want the usual header. Flawfinder version
255 1.29 changed the separator from spaces to tabs, and added
256 the default warning field.
257 Selecting Input Data
260 --allowlink Allow the use of symbolic links; normally symbolic links
261 are skipped. Don't use this option if you're analyzing
262 code by others; attackers could do many things to cause
263 problems for an analysis with this option enabled. For
264 example, an attacker could insert symbolic links to files
265 such as /etc/passwd (leaking information about the file) or
266 create a circular loop, which would cause flawfinder to run
267 ``forever''. Another problem with enabling this option is
268 that if the same file is referenced multiple times using
269 symbolic links, it will be analyzed multiple times (and
270 thus reported multiple times). Note that flawfinder
271 already includes some protection against symbolic links to
272 special file types such as device file types (e.g.,
273 /dev/zero or C:\mystuff\com1). Note that for flawfinder
274 version 1.01 and before, this was the default.
275 --followdotdir
278 Enter directories whose names begin with ".". Normally
279 such directories are ignored, since they normally include
280 version control private data (such as .git/ or .svn/),
281 build metadata (such as .makepp), configuration informa‐
282 tion, and so on.
283 --nolink Ignored. Historically this disabled following symbolic
286 links; this behavior is now the default.
287 --patch=patchfile
290 -P patchfile
292 Examine the selected files or directories, but only report
293 hits in lines that are added or modified as described in
294 the given patch file. The patch file must be in a recog‐
295 nized unified diff format (e.g., the output of GNU "diff -u
296 old new", "svn diff", or "git diff [commit]"). Flawfinder
297 assumes that the patch has already been applied to the
298 files. The patch file can also include changes to irrele‐
299 vant files (they will simply be ignored). The line numbers
300 given in the patch file are used to determine which lines
301 were changed, so if you have modified the files since the
302 patch file was created, regenerate the patch file first.
303 Beware that the file names of the new files given in the
304 patch file must match exactly, including upper/lower case,
305 path prefix, and directory separator (\ vs. /). Only uni‐
306 fied diff format is accepted (GNU diff, svn diff, and git
307 diff output is okay); if you have a different format, again
308 regenerate it first. Only hits that occur on resultant
309 changed lines, or immediately above and below them, are
310 reported. This option implies --neverignore. Warning: Do
311 not pass a patch file without the -P, because flawfinder
312 will then try to treat the file as a source file. This
313 will often work, but the line numbers will be relative to
314 the beginning of the patch file, not the positions in the
315 source code. Note that you must also provide the actual
316 files to analyze, and not just the patch file; when using
317 -P files are only reported if they are both listed in the
318 patch and also listed (directly or indirectly) in the list
319 of files to analyze.
320 Selecting Hits to Display
324 --inputs
325 -I Show only functions that obtain data from outside the program;
327 this also sets minlevel to 0.
328 --minlevel=X
331 -m X Set minimum risk level to X for inclusion in hitlist. This can
333 be from 0 (``no risk'') to 5 (``maximum risk''); the default is
334 1.
335 --falsepositive
338 -F Do not include hits that are likely to be false positives. Cur‐
340 rently, this means that function names are ignored if they're
341 not followed by "(", and that declarations of character arrays
342 aren't noted. Thus, if you have use a variable named "access"
343 everywhere, this will eliminate references to this ordinary
344 variable. This isn't the default, because this also increases
345 the likelihood of missing important hits; in particular, func‐
346 tion names in #define clauses and calls through function point‐
347 ers will be missed.
348 --neverignore
351 -n Never ignore security issues, even if they have an ``ignore''
353 directive in a comment.
354 --regexp=PATTERN
357 -e PATTERN
359 Only report hits with text that matches the regular expression
360 pattern PATTERN. For example, to only report hits containing
361 the text "CWE-120", use ``--regex CWE-120''. These option flag
362 names are the same as grep.
363 Selecting Output Format
367 --columns
368 -C Show the column number (as well as the file name and line
370 number) of each hit; this is shown after the line number by
371 adding a colon and the column number in the line (the first
372 character in a line is column number 1). This is useful
373 for editors that can jump to specific columns, or for inte‐
374 grating with other tools (such as those to further filter
375 out false positives).
376 --context
379 -c Show context, i.e., the line having the "hit"/potential
381 flaw. By default the line is shown immediately after the
382 warning.
383 --csv Generate output in comma-separated-value (CSV) format.
386 This is the recommended format for sending to other tools
387 for processing. It will always generate a header row, fol‐
388 lowed by 0 or more data rows (one data row for each hit).
389 Selecting this option automatically enables --quiet and
390 --dataonly. The headers are mostly self-explanatory.
391 "File" is the filename, "Line" is the line number, "Column"
392 is the column (starting from 1), "Level" is the risk level
393 (0-5, 5 is riskiest), "Category" is the general flawfinder
394 category, "Name" is the name of the triggering rule, "Warn‐
395 ing" is text explaining why it is a hit (finding), "Sugges‐
396 tion" is text suggesting how it might be fixed, "Note" is
397 other explanatory notes, "CWEs" is the list of one or more
398 CWEs, "Context" is the source code line triggering the hit,
399 and "Fingerprint" is the SHA-256 hash of the context once
400 its leading and trailing whitespace have been removed (the
401 fingerprint may help detect and eliminate later duplica‐
402 tions). If you use Python3, the hash is of the context
403 when encoded as UTF-8.
404 --dataonly
407 -D Don't display the header and footer. Use this along with
409 --quiet to see just the data itself.
410 --html
413 -H Format the output as HTML instead of as simple text.
415 --immediate
418 -i Immediately display hits (don't just wait until the end).
420 --singleline
423 -S Display as single line of text output for each hit. Useful
425 for interacting with compilation tools.
426 --omittime Omit timing information. This is useful for regression
429 tests of flawfinder itself, so that the output doesn't vary
430 depending on how long the analysis takes.
431 --quiet
434 -Q Don't display status information (i.e., which files are
436 being examined) while the analysis is going on.
437 --error-level=LEVEL
440 Return a nonzero (false) error code if there is at least
441 one hit of LEVEL or higher. If a diffhitlist is provided,
442 hits noted in it are ignored. This option can be useful
443 within a continuous integration script, especially if you
444 mark known-okay lines as "flawfinder: ignore". Usually you
445 want level to be fairly high, such as 4 or 5. By default,
446 flawfinder returns 0 (true) on a successful run.
447 Hitlist Management
450 --savehitlist=F
451 Save all resulting hits (the "hitlist") to F.
452 --loadhitlist=F
455 Load the hitlist from F instead of analyzing source pro‐
456 grams. Warning: Do not load hitlists from untrusted
457 sources (for security reasons). These are internally
458 implemented using Python's "pickle" facility, which trusts
459 the input. Note that stored hitlists often cannot be read
460 when using an older version of Python, in particular, if
461 savehitlist was used but flawfinder was run using Python 3,
462 the hitlist can't be loaded by running flawfinder with
463 Python 2.
464 --diffhitlist=F
467 Show only hits (loaded or analyzed) not in F. F was pre‐
468 sumably created previously using --savehitlist. Warning:
469 Do not diff hitlists from untrusted sources (for security
470 reasons). If the --loadhitlist option is not provided,
471 this will show the hits in the analyzed source code files
472 that were not previously stored in F. If used along with
473 --loadhitlist, this will show the hits in the loaded
474 hitlist not in F. The difference algorithm is conserva‐
475 tive; hits are only considered the ``same'' if they have
476 the same filename, line number, column position, function
477 name, and risk level.
478 Character Encoding
481 Flawfinder presumes that the character encoding your system uses is
482 also the character encoding used by your source files. Even if this
483 isn't correct, if you run flawfinder with Python 2 these non-conformi‐
484 ties often do not impact processing in practice.
485 However, if you run flawfinder with Python 3, this can be a problem.
487 Python 3 wants the world to always use encodings perfectly correctly,
488 everywhere, even though the world often doesn't care what Python 3
489 wants. This is a problem even if the non-conforming text is in com‐
490 ments or strings (where it often doesn't matter). Python 3 fails to
491 provide useful built-ins to deal with the messiness of the real world,
492 so it's non-trivial to deal with this problem without depending on
493 external libraries (which we're trying to avoid).
494 A symptom of this problem is if you run flawfinder and you see an error
496 message like this:
497 UnicodeDecodeError: 'utf-8' codec can't decode byte ... in position
499 ...: invalid continuation byte
500 If this happens to you, there are several options.
502 The first option is to convert the encoding of the files to be analyzed
504 so that it's a single encoding (usually the system encoding). For
505 example, the program "iconv" can be used to convert encodings. This
506 works well if some files have one encoding, and some have another, but
507 they are consistent within a single file. If the files have encoding
508 errors, you'll have to fix them. I strongly recommend using the UTF-8
509 encoding for all source code and in the system itself; if you do that,
510 many problems disappear.
511 The second option is to tell flawfinder what the encoding of the files
513 is. E.G., you can set the LANG environment variable. You can set
514 PYTHONIOENCODING to the encoding you want your output to be in, if
515 that's different. This in theory would work, but I haven't had much
516 success with this.
517 The third option is to run flawfinder using Python 2 instead of Python
519 3. E.g., "python2 flawfinder ...".
520
523 Here are various examples of how to invoke flawfinder. The first exam‐
524 ples show various simple command-line options. Flawfinder is designed
525 to work well with text editors and integrated development environments,
526 so the next sections show how to integrate flawfinder into vim and
527 emacs.
528 Simple command-line options
531 flawfinder /usr/src/linux-3.16
532 Examine all the C/C++ files in the directory
533 /usr/src/linux-3.16 and all its subdirectories (recur‐
534 sively), reporting on all hits found. By default
535 flawfinder will skip symbolic links and directories with
536 names that start with a period.
537 flawfinder --minlevel=4 .
540 Examine all the C/C++ files in the current directory and
541 its subdirectories (recursively); only report vulnerabili‐
542 ties level 4 and up (the two highest risk levels).
543 flawfinder --inputs mydir
546 Examine all the C/C++ files in mydir and its subdirectories
547 (recursively), and report functions that take inputs (so
548 that you can ensure that they filter the inputs appropri‐
549 ately).
550 flawfinder --neverignore mydir
553 Examine all the C/C++ files in the directory mydir and its
554 subdirectories, including even the hits marked for ignoring
555 in the code comments.
556 flawfinder --csv .
559 Examine the current directory down (recursively), and
560 report all hits in CSV format. This is the recommended
561 form if you want to further process flawfinder output using
562 other tools (such as data correlation tools).
563 flawfinder -QD mydir
566 Examine mydir and report only the actual results (removing
567 the header and footer of the output). This form may be
568 useful if the output will be piped into other tools for
569 further analysis, though CSV format is probably the better
570 choice in that case. The -C (--columns) and -S (--single‐
571 line) options can also be useful if you're piping the data
572 into other tools.
573 flawfinder -QDSC mydir
576 Examine mydir, reporting only the actual results (no header
577 or footer). Each hit is reported on one line, and column
578 numbers are reported. This can be a useful command if you
579 are feeding flawfinder output to other tools.
580 flawfinder --quiet --html --context mydir > results.html
583 Examine all the C/C++ files in the directory mydir and its
584 subdirectories, and produce an HTML formatted version of
585 the results. Source code management systems (such as
586 SourceForge and Savannah) might use a command like this.
587 flawfinder --quiet --savehitlist saved.hits *.[ch]
590 Examine all .c and .h files in the current directory.
591 Don't report on the status of processing, and save the
592 resulting hitlist (the set of all hits) in the file
593 saved.hits.
594 flawfinder --diffhitlist saved.hits *.[ch]
597 Examine all .c and .h files in the current directory, and
598 show any hits that weren't already in the file saved.hits.
599 This can be used to show only the ``new'' vulnerabilities
600 in a modified program, if saved.hits was created from the
601 older version of the program being analyzed.
602 flawfinder --patch recent.patch .
605 Examine the current directory recursively, but only report
606 lines that were changed or added in the already-applied
607 patchfile named recent.patch.
608 flawfinder --regex "CWE-120|CWE-126" src/
611 Examine directory src recursively, but only report hits
612 where CWE-120 or CWE-126 apply.
613 Invoking from vim
616 The text editor vim includes a "quickfix" mechanism that works well
617 with flawfinder, so that you can easily view the warning messages and
618 jump to the relevant source code.
619 First, you need to invoke flawfinder to create a list of hits, and
621 there are two ways to do this. The first way is to start flawfinder
622 first, and then (using its output) invoke vim. The second way is to
623 start (or continue to run) vim, and then invoke flawfinder (typically
624 from inside vim).
625 For the first way, run flawfinder and store its output in some FLAWFILE
627 (say "flawfile"), then invoke vim using its -q option, like this: "vim
628 -q flawfile". The second way (starting flawfinder after starting vim)
629 can be done a legion of ways. One is to invoke flawfinder using a
630 shell command, ":!flawfinder-command > FLAWFILE", then follow that with
631 the command ":cf FLAWFILE". Another way is to store the flawfinder
632 command in your makefile (as, say, a pseudocommand like "flaw"), and
633 then run ":make flaw".
634 In all these cases you need a command for flawfinder to run. A plausi‐
636 ble command, which places each hit in its own line (-S) and removes
637 headers and footers that would confuse it, is:
638 flawfinder -SQD .
640 You can now use various editing commands to view the results. The com‐
643 mand ":cn" displays the next hit; ":cN" displays the previous hit, and
644 ":cr" rewinds back to the first hit. ":copen" will open a window to
645 show the current list of hits, called the "quickfix window"; ":cclose"
646 will close the quickfix window. If the buffer in the used window has
647 changed, and the error is in another file, jumping to the error will
648 fail. You have to make sure the window contains a buffer which can be
649 abandoned before trying to jump to a new file, say by saving the file;
650 this prevents accidental data loss.
651 Invoking from emacs
654 The text editor / operating system emacs includes "grep mode" and "com‐
655 pile mode" mechanisms that work well with flawfinder, making it easy to
656 view warning messages, jump to the relevant source code, and fix any
657 problems you find.
658 First, you need to invoke flawfinder to create a list of warning mes‐
660 sages. You can use "grep mode" or "compile mode" to create this list.
661 Often "grep mode" is more convenient; it leaves compile mode untouched
662 so you can easily recompile once you've changed something. However, if
663 you want to jump to the exact column position of a hit, compile mode
664 may be more convenient because emacs can use the column output of
665 flawfinder to directly jump to the right location without any special
666 configuration.
667 To use grep mode, enter the command "M-x grep" and then enter the
669 needed flawfinder command. To use compile mode, enter the command "M-x
670 compile" and enter the needed flawfinder command. This is a meta-key
671 command, so you'll need to use the meta key for your keyboard (this is
672 usually the ESC key). As with all emacs commands, you'll need to press
673 RETURN after typing "grep" or "compile". So on many systems, the grep
674 mode is invoked by typing ESC x g r e p RETURN.
675 You then need to enter a command, removing whatever was there before if
677 necessary. A plausible command is:
678 flawfinder -SQDC .
680 This command makes every hit report a single line, which is much easier
682 for tools to handle. The quiet and dataonly options remove the other
683 status information not needed for use inside emacs. The trailing
684 period means that the current directory and all descendents are
685 searched for C/C++ code, and analyzed for flaws.
686 Once you've invoked flawfinder, you can use emacs to jump around in its
688 results. The command C-x ` (Control-x backtick) visits the source code
689 location for the next warning message. C-u C-x ` (control-u control-x
690 backtick) restarts from the beginning. You can visit the source for
691 any particular error message by moving to that hit message in the *com‐
692 pilation* buffer or *grep* buffer and typing the return key. (Techni‐
693 cal note: in the compilation buffer, this invokes compile-goto-error.)
694 You can also click the Mouse-2 button on the error message (you don't
695 need to switch to the *compilation* buffer first).
696 If you want to use grep mode to jump to specific columns of a hit,
698 you'll need to specially configure emacs to do this. To do this, mod‐
699 ify the emacs variable "grep-regexp-alist". This variable tells Emacs
700 how to parse output of a "grep" command, similar to the variable "com‐
701 pilation-error-regexp-alist" which lists various formats of compilation
702 error messages.
703 Invoking from Integrated Development Environments (IDEs)
706 For (other) IDEs, consult your IDE's set of plug-ins.
707
710 The Common Weakness Enumeration (CWE) is ``a formal list or dictionary
711 of common software weaknesses that can occur in software's architec‐
712 ture, design, code or implementation that can lead to exploitable secu‐
713 rity vulnerabilities... created to serve as a common language for
714 describing software security weaknesses''
715 (https://cwe.mitre.org/about/faq.html). For more information on CWEs,
716 see https://cwe.mitre.org.
717 Flawfinder supports the CWE and is officially CWE-Compatible. Hit
719 descriptions typically include a relevant Common Weakness Enumeration
720 (CWE) identifier in parentheses where there is known to be a relevant
721 CWE. For example, many of the buffer-related hits mention CWE-120, the
722 CWE identifier for ``buffer copy without checking size of input'' (aka
723 ``Classic Buffer Overflow''). In a few cases more than one CWE identi‐
724 fier may be listed. The HTML report also includes hypertext links to
725 the CWE definitions hosted at MITRE. In this way, flawfinder is
726 designed to meet the CWE-Output requirement.
727 In some cases there are CWE mapping and usage challenges; here is how
729 flawfinder handles them. If the same entry maps to multiple CWEs
730 simultaneously, all the CWE mappings are listed as separated by commas.
731 This often occurs with CWE-20, Improper Input Validation; thus the
732 report "CWE-676, CWE-120" maps to two CWEs. In addition, flawfinder
733 provides additional information for those who are are interested in the
734 CWE/SANS top 25 list 2011 (https://cwe.mitre.org/top25/) when mappings
735 are not directly to them. Many people will want to search for specific
736 CWEs in this top 25 list, such as CWE-120 (classic buffer overflow).
737 The challenge is that some flawfinder hits map to a more general CWE
738 that would include a top 25 item, while in some other cases hits map to
739 a more specific vulnerability that is only a subset of a top 25 item.
740 To resolve this, in some cases flawfinder will list a sequence of CWEs
741 in the format "more-general/more-specific", where the CWE actually
742 being mapped is followed by a "!". This is always done whenever a flaw
743 is not mapped directly to a top 25 CWE, but the mapping is related to
744 such a CWE. So "CWE-119!/CWE-120" means that the vulnerability is
745 mapped to CWE-119 and that CWE-120 is a subset of CWE-119. In con‐
746 trast, "CWE-362/CWE-367!" means that the hit is mapped to CWE-367, a
747 subset of CWE-362. Note that this is a subtle syntax change from
748 flawfinder version 1.31; in flawfinder version 1.31, the form "more-
749 general:more-specific" meant what is now listed as "more-general!/more-
750 specific", while "more-general/more-specific" meant "more-general/more-
751 specific!". Tools can handle both the version 1.31 and the current
752 format, if they wish, by noting that the older format did not use "!"
753 at all (and thus this is easy to distinguish). These mapping mecha‐
754 nisms simplify searching for certain CWEs.
755 CWE version 2.7 (released June 23, 2014) was used for the mapping. The
757 current CWE mappings select the most specific CWE the tool can deter‐
758 mine. In theory, most CWE security elements (signatures/patterns that
759 the tool searches for) could theoretically be mapped to CWE-676 (Use of
760 Potentially Dangerous Function), but such a mapping would not be use‐
761 ful. Thus, more specific mappings were preferred where one could be
762 found. Flawfinder is a lexical analysis tool; as a result, it is
763 impractical for it to be more specific than the mappings currently
764 implemented. This also means that it is unlikely to need much updating
765 for map currency; it simply doesn't have enough information to refine
766 to a detailed CWE level that CWE changes would typically affect. The
767 list of CWE identifiers was generated automatically using "make show-
768 cwes", so there is confidence that this list is correct. Please report
769 CWE mapping problems as bugs if you find any.
770 Flawfinder may fail to find a vulnerability, even if flawfinder covers
772 one of these CWE weaknesses. That said, flawfinder does find vulnera‐
773 bilities listed by the CWEs it covers, and it will not report lines
774 without those vulnerabilities in many cases. Thus, as required for any
775 tool intending to be CWE compatible, flawfinder has a rate of false
776 positives less than 100% and a rate of false negatives less than 100%.
777 Flawfinder almost always reports whenever it finds a match to a CWE
778 security element (a signature/pattern as defined in its database),
779 though certain obscure constructs can cause it to fail (see BUGS
780 below).
781 Flawfinder can report on the following CWEs (these are the CWEs that
783 flawfinder covers; ``*'' marks those in the CWE/SANS top 25 list):
784 · CWE-20: Improper Input Validation
786 · CWE-22: Improper Limitation of a Pathname to a Restricted Directory
788 (``Path Traversal'')
789 · CWE-78: Improper Neutralization of Special Elements used in an OS
791 Command (``OS Command Injection'')*
792 · CWE-119: Improper Restriction of Operations within the Bounds of a
794 Memory Buffer (a parent of CWE-120*, so this is shown as
795 CWE-119!/CWE-120)
796 · CWE-120: Buffer Copy without Checking Size of Input (``Classic Buffer
798 Overflow'')*
799 · CWE-126: Buffer Over-read
801 · CWE-134: Uncontrolled Format String*
803 · CWE-190: Integer Overflow or Wraparound*
805 · CWE-250: Execution with Unnecessary Privileges
807 · CWE-327: Use of a Broken or Risky Cryptographic Algorithm*
809 · CWE-362: Concurrent Execution using Shared Resource with Improper
811 Synchronization (``Race Condition'')
812 · CWE-377: Insecure Temporary File
814 · CWE-676: Use of Potentially Dangerous Function*
816 · CWE-732: Incorrect Permission Assignment for Critical Resource*
818 · CWE-785: Use of Path Manipulation Function without Maximum-sized Buf‐
820 fer (child of CWE-120*, so this is shown as CWE-120/CWE-785)
821 · CWE-807: Reliance on Untrusted Inputs in a Security Decision*
823 · CWE-829: Inclusion of Functionality from Untrusted Control Sphere*
825 You can select a specific subset of CWEs to report by using the
827 ``--regex'' (-e) option. This option accepts a regular expression, so
828 you can select multiple CWEs, e.g., ``--regex "CWE-120|CWE-126"''. If
829 you select multiple CWEs with ``|'' on a command line you will typi‐
830 cally need to quote the parameters (since an unquoted ``|'' is the pipe
831 symbol). Flawfinder is designed to meet the CWE-Searchable require‐
832 ment.
833 If your goal is to report a subset of CWEs that are listed in a file,
835 that can be achieved on a Unix-like system using the ``--regex'' aka
836 ``-e'' option. The file must be in regular expression format. For
837 example, ``flawfinder -e $(cat file1)'' would report only hits that
838 matched the pattern in ``file1''. If file1 contained
839 ``CWE-120|CWE-126'' it would only report hits matching those CWEs.
840 A list of all CWE security elements (the signatures/patterns that
842 flawfinder looks for) can be found by using the ``--listrules'' option.
843 Each line lists the signature token (typically a function name) that
844 may lead to a hit, the default risk level, and the default warning
845 (which includes the default CWE identifier). For most purposes this is
846 also enough if you want to see what CWE security elements map to which
847 CWEs, or the reverse. For example, to see the most of the signatures
848 (function names) that map to CWE-327, without seeing the default risk
849 level or detailed warning text, run ``flawfinder --listrules | grep
850 CWE-327 | cut -f1''. You can also see the tokens without a CWE mapping
851 this way by running ``flawfinder -D --listrules | grep -v CWE-''. How‐
852 ever, while --listrules lists all CWE security elements, it only lists
853 the default mappings from CWE security elements to CWE identifiers. It
854 does not include the refinements that flawfinder applies (e.g., by
855 examining function parameters).
856 If you want a detailed and exact mapping between the CWE security ele‐
858 ments and CWE identifiers, the flawfinder source code (included in the
859 distribution) is the best place for that information. This detailed
860 information is primarily of interest to those few people who are trying
861 to refine the CWE mappings of flawfinder or refine CWE in general. The
862 source code documents the mapping between the security elements to the
863 respective CWE identifiers, and is a single Python file. The
864 ``c_rules'' dataset defines most rules, with reference to a function
865 that may make further refinements. You can search the dataset for
866 function names to see what CWE it generates by default; if first param‐
867 eter is not ``normal'' then that is the name of a refining Python
868 method that may select different CWEs (depending on additional informa‐
869 tion). Conversely, you can search for ``CWE-number'' and find what
870 security elements (signatures or patterns) refer to that CWE identi‐
871 fier. For most people, this is much more than they need; most people
872 just want to scan their source code to quickly find problems.
873
877 The whole point of this tool is to help find vulnerabilities so they
878 can be fixed. However, developers and reviewers must know how to
879 develop secure software to use this tool, because otherwise, a fool
880 with a tool is still a fool. My book at https://dwheeler.com/secure-
881 programs may help.
882 This tool should be, at most, a small part of a larger software devel‐
884 opment process designed to eliminate or reduce the impact of vulnera‐
885 bilities. Developers and reviewers need know how to develop secure
886 software, and they need to apply this knowledge to reduce the risks of
887 vulnerabilities in the first place.
888 Different vulnerability-finding tools tend to find different vulnera‐
890 bilities. Thus, you are best off using human review and a variety of
891 tools. This tool can help find some vulnerabilities, but by no means
892 all.
893 You should always analyze a copy of the source program being analyzed,
895 not a directory that can be modified by a developer while flawfinder is
896 performing the analysis. This is especially true if you don't necess‐
897 ily trust a developer of the program being analyzed. If an attacker
898 has control over the files while you're analyzing them, the attacker
899 could move files around or change their contents to prevent the expo‐
900 sure of a security problem (or create the impression of a problem where
901 there is none). If you're worried about malicious programmers you
902 should do this anyway, because after analysis you'll need to verify
903 that the code eventually run is the code you analyzed. Also, do not
904 use the --allowlink option in such cases; attackers could create mali‐
905 cious symbolic links to files outside of their source code area (such
906 as /etc/passwd).
907 Source code management systems (like GitHub, SourceForge, and Savannah)
909 definitely fall into this category; if you're maintaining one of those
910 systems, first copy or extract the files into a separate directory
911 (that can't be controlled by attackers) before running flawfinder or
912 any other code analysis tool.
913 Note that flawfinder only opens regular files, directories, and (if
915 requested) symbolic links; it will never open other kinds of files,
916 even if a symbolic link is made to them. This counters attackers who
917 insert unusual file types into the source code. However, this only
918 works if the filesystem being analyzed can't be modified by an attacker
919 during the analysis, as recommended above. This protection also
920 doesn't work on Cygwin platforms, unfortunately.
921 Cygwin systems (Unix emulation on top of Windows) have an additional
923 problem if flawfinder is used to analyze programs that the analyst can‐
924 not trust. The problem is due to a design flaw in Windows (that it
925 inherits from MS-DOS). On Windows and MS-DOS, certain filenames (e.g.,
926 ``com1'') are automatically treated by the operating system as the
927 names of peripherals, and this is true even when a full pathname is
928 given. Yes, Windows and MS-DOS really are designed this badly.
929 Flawfinder deals with this by checking what a filesystem object is, and
930 then only opening directories and regular files (and symlinks if
931 enabled). Unfortunately, this doesn't work on Cygwin; on at least some
932 versions of Cygwin on some versions of Windows, merely trying to deter‐
933 mine if a file is a device type can cause the program to hang. A work‐
934 around is to delete or rename any filenames that are interpreted as
935 device names before performing the analysis. These so-called
936 ``reserved names'' are CON, PRN, AUX, CLOCK$, NUL, COM1-COM9, and
937 LPT1-LPT9, optionally followed by an extension (e.g., ``com1.txt''), in
938 any directory, and in any case (Windows is case-insensitive).
939 Do not load or diff hitlists from untrusted sources. They are imple‐
941 mented using the Python pickle module, and the pickle module is not
942 intended to be secure against erroneous or maliciously constructed
943 data. Stored hitlists are intended for later use by the same user who
944 created the hitlist; in that context this restriction is not a problem.
945
948 Flawfinder is based on simple text pattern matching, which is part of
949 its fundamental design and not easily changed. This design approach
950 leads to a number of fundamental limitations, e.g., a higher false pos‐
951 itive rate, and is the underlying cause of most of the bugs listed
952 here. On the positive side, flawfinder doesn't get confused by many
953 complicated preprocessor sequences that other tools sometimes choke on;
954 flawfinder can often handle code that cannot link, and sometimes cannot
955 even compile or build.
956 Flawfinder is currently limited to C/C++. In addition, when analyzing
958 C++ it focuses primarily on the C subset of C++. For example,
959 flawfinder does not report on expressions like cin >> charbuf, where
960 charbuf is a char array. That is because flawfinder doesn't have type
961 information, and ">>" is safe with many other types; reporting on all
962 ">>" would lead to too many false positives. That said, it's designed
963 so that adding support for other languages should be easy where its
964 text-based approach can usefully apply.
965 Flawfinder can be fooled by user-defined functions or method names that
967 happen to be the same as those defined as ``hits'' in its database, and
968 will often trigger on definitions (as well as uses) of functions with
969 the same name. This is typically not a problem for C code. In C code,
970 a function with the same name as a common library routine name often
971 indicates that the developer is simply rewriting a common library rou‐
972 tine with the same interface, say for portability's sake. C programs
973 tend to avoid reusing the same name for a different purpose (since in C
974 function names are global by default). There are reasonable odds that
975 these rewritten routines will be vulnerable to the same kinds of mis‐
976 use, and thus, reusing these rules is a reasonable approach. However,
977 this can be a much more serious problem in C++ code which heavily uses
978 classes and namespaces, since the same method name may have many dif‐
979 ferent meanings. The --falsepositive option can help somewhat in this
980 case. If this is a serious problem, feel free to modify the program,
981 or process the flawfinder output through other tools to remove the
982 false positives.
983 Preprocessor commands embedded in the middle of a parameter list of a
985 call can cause problems in parsing, in particular, if a string is
986 opened and then closed multiple times using an #ifdef .. #else con‐
987 struct, flawfinder gets confused. Such constructs are bad style, and
988 will confuse many other tools too. If you must analyze such files, re‐
989 write those lines. Thankfully, these are quite rare.
990 Flawfinder reports vulnerabilities regardless of the parameters of
992 "#if" or "#ifdef". A construct "#if VALUE" will often have VALUE of 0
993 in some cases, and non-zero in others. Similarly, "#ifdef VALUE" will
994 have VALUE defined in some cases, and not defined in others.
995 Flawfinder reports in all cases, which means that flawfinder has a
996 chance of reporting vulnerabilities in all alternatives. This is not a
997 bug, this is intended behavior.
998 Flawfinder will report hits even if they are between a literal "#if 0"
1000 and "#endif". It would be possible to change this particular situa‐
1001 tion, but directly using "#if 0" to comment-out code (other than during
1002 debugging) is itself that the removal is very temporary (in which case
1003 we should report it) or an indicator of a problem with poor code prac‐
1004 tices. If you want to permanently get rid of code, then delete it
1005 instead of using "#if 0", since you can always see what it was using
1006 your version control software. If you don't use version control soft‐
1007 ware, then that's the bug you need to fix right now.
1008 Some complex or unusual constructs can mislead flawfinder. In particu‐
1010 lar, if a parameter begins with gettext(" and ends with ), flawfinder
1011 will presume that the parameter of gettext is a constant. This means
1012 it will get confused by patterns like gettext("hi") + function("bye").
1013 In practice, this doesn't seem to be a problem; gettext() is usually
1014 wrapped around the entire parameter.
1015 The routine to detect statically defined character arrays uses simple
1017 text matching; some complicated expressions can cause it to trigger or
1018 not trigger unexpectedly.
1019 Flawfinder looks for specific patterns known to be common mistakes.
1021 Flawfinder (or any tool like it) is not a good tool for finding inten‐
1022 tionally malicious code (e.g., Trojan horses); malicious programmers
1023 can easily insert code that would not be detected by this kind of tool.
1024 Flawfinder looks for specific patterns known to be common mistakes in
1026 application code. Thus, it is likely to be less effective analyzing
1027 programs that aren't application-layer code (e.g., kernel code or self-
1028 hosting code). The techniques may still be useful; feel free to
1029 replace the database if your situation is significantly different from
1030 normal.
1031 Flawfinder's default output format (filename:linenumber, followed
1033 optionally by a :columnnumber) can be misunderstood if any source files
1034 have very weird filenames. Filenames embedding a newline/linefeed
1035 character will cause odd breaks, and filenames including colon (:) are
1036 likely to be misunderstood. This is especially important if
1037 flawfinder's output is being used by other tools, such as filters or
1038 text editors. If you are using flawfinder's output in other tools,
1039 consider using its CSV format instead (which can handle this). If
1040 you're looking at new code, examine the files for such characters.
1041 It's incredibly unwise to have such filenames anyway; many tools can't
1042 handle such filenames at all. Newline and linefeed are often used as
1043 internal data delimeters. The colon is often used as special charac‐
1044 ters in filesystems: MacOS uses it as a directory separator, Win‐
1045 dows/MS-DOS uses it to identify drive letters, Windows/MS-DOS inconsis‐
1046 tently uses it to identify special devices like CON:, and applications
1047 on many platforms use the colon to identify URIs/URLs. Filenames
1048 including spaces and/or tabs don't cause problems for flawfinder,
1049 though note that other tools might have problems with them.
1050 Flawfinder is not internationalized, so it currently does not support
1052 localization.
1053 In general, flawfinder attempts to err on the side of caution; it tends
1055 to report hits, so that they can be examined further, instead of
1056 silently ignoring them. Thus, flawfinder prefers to have false posi‐
1057 tives (reports that turn out to not be problems) rather than false neg‐
1058 atives (failures to report security vulnerabilities). But this is a
1059 generality; flawfinder uses simplistic heuristics and simply can't get
1060 everything "right".
1061 Security vulnerabilities might not be identified as such by flawfinder,
1063 and conversely, some hits aren't really security vulnerabilities. This
1064 is true for all static security scanners, and is especially true for
1065 tools like flawfinder that use a simple lexical analysis and pattern
1066 analysis to identify potential vulnerabilities. Still, it can serve as
1067 a useful aid for humans, helping to identify useful places to examine
1068 further, and that's the point of this simple tool.
1069
1072 See the flawfinder website at https://dwheeler.com/flawfinder. You
1073 should also see the Secure Programming HOWTO at
1074 https://dwheeler.com/secure-programs.
1075
1078 David A. Wheeler (dwheeler@dwheeler.com).
1079Flawfinder 4 Apr 2018 FLAWFINDER(1)