1libcurl-security(3) libcurl security libcurl-security(3)
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6 libcurl-security - security considerations when using libcurl
7
9 The libcurl project takes security seriously. The library is written
10 with caution and precautions are taken to mitigate many kinds of risks
11 encountered while operating with potentially malicious servers on the
12 Internet. It is a powerful library, however, which allows application
13 writers to make trade-offs between ease of writing and exposure to po‐
14 tential risky operations. If used the right way, you can use libcurl to
15 transfer data pretty safely.
16 Many applications are used in closed networks where users and servers
18 can (possibly) be trusted, but many others are used on arbitrary
19 servers and are fed input from potentially untrusted users. Following
20 is a discussion about some risks in the ways in which applications com‐
21 monly use libcurl and potential mitigations of those risks. It is not
22 comprehensive, but shows classes of attacks that robust applications
23 should consider. The Common Weakness Enumeration project at
24 https://cwe.mitre.org/ is a good reference for many of these and simi‐
25 lar types of weaknesses of which application writers should be aware.
26
28 If you use a command line tool (such as curl) that uses libcurl, and
29 you give options to the tool on the command line those options can get
30 read by other users of your system when they use 'ps' or other tools to
31 list currently running processes.
32 To avoid these problems, never feed sensitive things to programs using
34 command line options. Write them to a protected file and use the -K op‐
35 tion to avoid this.
36
38 .netrc is a pretty handy file/feature that allows you to login quickly
39 and automatically to frequently visited sites. The file contains pass‐
40 words in clear text and is a real security risk. In some cases, your
41 .netrc is also stored in a home directory that is NFS mounted or used
42 on another network based file system, so the clear text password will
43 fly through your network every time anyone reads that file.
44 For applications that enable .netrc use, a user who manage to set the
46 right URL might then be possible to pass on passwords.
47 To avoid these problems, do not use .netrc files and never store pass‐
49 words in plain text anywhere.
50
52 Many of the protocols libcurl supports send name and password unen‐
53 crypted as clear text (HTTP Basic authentication, FTP, TELNET etc). It
54 is easy for anyone on your network or a network nearby yours to just
55 fire up a network analyzer tool and eavesdrop on your passwords. do not
56 let the fact that HTTP Basic uses base64 encoded passwords fool you.
57 They may not look readable at a first glance, but they are easily "de‐
58 ciphered" by anyone within seconds.
59 To avoid this problem, use an authentication mechanism or other proto‐
61 col that does not let snoopers see your password: Digest, CRAM-MD5,
62 Kerberos, SPNEGO or NTLM authentication. Or even better: use authenti‐
63 cated protocols that protect the entire connection and everything sent
64 over it.
65
67 Protocols that do not have any form of cryptographic authentication
68 cannot with any certainty know that they communicate with the right re‐
69 mote server.
70 If your application is using a fixed scheme or fixed host name, it is
72 not safe as long as the connection is un-authenticated. There can be a
73 man-in-the-middle or in fact the whole server might have been replaced
74 by an evil actor.
75 Un-authenticated protocols are unsafe. The data that comes back to curl
77 may have been injected by an attacker. The data that curl sends might
78 be modified before it reaches the intended server. If it even reaches
79 the intended server at all.
80 Remedies:
82 Restrict operations to authenticated transfers
84 Ie use authenticated protocols protected with HTTPS or SSH.
85 Make sure the server's certificate etc is verified
87 Never ever switch off certificate verification.
88
90 The CURLOPT_FOLLOWLOCATION(3) option automatically follows HTTP redi‐
91 rects sent by a remote server. These redirects can refer to any kind of
92 URL, not just HTTP. libcurl restricts the protocols allowed to be used
93 in redirects for security reasons: only HTTP, HTTPS, FTP and FTPS are
94 enabled by default. Applications may opt to restrict that set further.
95 A redirect to a file: URL would cause the libcurl to read (or write)
97 arbitrary files from the local filesystem. If the application returns
98 the data back to the user (as would happen in some kinds of CGI
99 scripts), an attacker could leverage this to read otherwise forbidden
100 data (e.g. file://localhost/etc/passwd).
101 If authentication credentials are stored in the ~/.netrc file, or Ker‐
103 beros is in use, any other URL type (not just file:) that requires au‐
104 thentication is also at risk. A redirect such as ftp://some-internal-
105 server/private-file would then return data even when the server is
106 password protected.
107 In the same way, if an unencrypted SSH private key has been configured
109 for the user running the libcurl application, SCP: or SFTP: URLs could
110 access password or private-key protected resources, e.g.
111 sftp://user@some-internal-server/etc/passwd
112 The CURLOPT_REDIR_PROTOCOLS(3) and CURLOPT_NETRC(3) options can be used
114 to mitigate against this kind of attack.
115 A redirect can also specify a location available only on the machine
117 running libcurl, including servers hidden behind a firewall from the
118 attacker. e.g. http://127.0.0.1/ or http://intranet/delete-
119 stuff.cgi?delete=all or tftp://bootp-server/pc-config-data
120 Applications can mitigate against this by disabling CURLOPT_FOLLOWLOCA‐
122 TION(3) and handling redirects itself, sanitizing URLs as necessary.
123 Alternately, an app could leave CURLOPT_FOLLOWLOCATION(3) enabled but
124 set CURLOPT_REDIR_PROTOCOLS(3) and install a CURLOPT_OPENSOCKETFUNC‐
125 TION(3) or CURLOPT_PREREQFUNCTION(3) callback function in which ad‐
126 dresses are sanitized before use.
127
129 A user who can control the DNS server of a domain being passed in
130 within a URL can change the address of the host to a local, private ad‐
131 dress which a server-side libcurl-using application could then use.
132 e.g. the innocuous URL http://fuzzybunnies.example.com/ could actually
133 resolve to the IP address of a server behind a firewall, such as
134 127.0.0.1 or 10.1.2.3. Applications can mitigate against this by set‐
135 ting a CURLOPT_OPENSOCKETFUNCTION(3) or CURLOPT_PREREQFUNCTION(3) and
136 checking the address before a connection.
137 All the malicious scenarios regarding redirected URLs apply just as
139 well to non-redirected URLs, if the user is allowed to specify an arbi‐
140 trary URL that could point to a private resource. For example, a web
141 app providing a translation service might happily translate file://lo‐
142 calhost/etc/passwd and display the result. Applications can mitigate
143 against this with the CURLOPT_PROTOCOLS(3) option as well as by similar
144 mitigation techniques for redirections.
145 A malicious FTP server could in response to the PASV command return an
147 IP address and port number for a server local to the app running
148 libcurl but behind a firewall. Applications can mitigate against this
149 by using the CURLOPT_FTP_SKIP_PASV_IP(3) option or CURLOPT_FTPPORT(3).
150 Local servers sometimes assume local access comes from friends and
152 trusted users. An application that expects https://exam‐
153 ple.com/file_to_read that and instead gets
154 http://192.168.0.1/my_router_config might print a file that would oth‐
155 erwise be protected by the firewall.
156 Allowing your application to connect to local hosts, be it the same ma‐
158 chine that runs the application or a machine on the same local network,
159 might be possible to exploit by an attacker who then perhaps can "port-
160 scan" the particular hosts - depending on how the application and
161 servers acts.
162
164 Some users might be tempted to filter access to local resources or sim‐
165 ilar based on numerical IPv4 addresses used in URLs. This is a bad and
166 error-prone idea because of the many different ways a numerical IPv4
167 address can be specified and libcurl accepts: one to four dot-separated
168 fields using one of or a mix of decimal, octal or hexadecimal encoding.
169
171 libcurl will normally handle IPv6 addresses transparently and just as
172 easily as IPv4 addresses. That means that a sanitizing function that
173 filters out addresses like 127.0.0.1 is not sufficient--the equivalent
174 IPv6 addresses ::1, ::, 0:00::0:1, ::127.0.0.1 and ::ffff:7f00:1 sup‐
175 plied somehow by an attacker would all bypass a naive filter and could
176 allow access to undesired local resources. IPv6 also has special ad‐
177 dress blocks like link-local and site-local that generally should not
178 be accessed by a server-side libcurl-using application. A poorly con‐
179 figured firewall installed in a data center, organization or server may
180 also be configured to limit IPv4 connections but leave IPv6 connections
181 wide open. In some cases, setting CURLOPT_IPRESOLVE(3) to CURL_IPRE‐
182 SOLVE_V4 can be used to limit resolved addresses to IPv4 only and by‐
183 pass these issues.
184
186 When uploading, a redirect can cause a local (or remote) file to be
187 overwritten. Applications must not allow any unsanitized URL to be
188 passed in for uploads. Also, CURLOPT_FOLLOWLOCATION(3) should not be
189 used on uploads. Instead, the applications should consider handling
190 redirects itself, sanitizing each URL first.
191
193 Use of CURLOPT_UNRESTRICTED_AUTH(3) could cause authentication informa‐
194 tion to be sent to an unknown second server. Applications can mitigate
195 against this by disabling CURLOPT_FOLLOWLOCATION(3) and handling redi‐
196 rects itself, sanitizing where necessary.
197 Use of the CURLAUTH_ANY option to CURLOPT_HTTPAUTH(3) could result in
199 user name and password being sent in clear text to an HTTP server. In‐
200 stead, use CURLAUTH_ANYSAFE which ensures that the password is en‐
201 crypted over the network, or else fail the request.
202 Use of the CURLUSESSL_TRY option to CURLOPT_USE_SSL(3) could result in
204 user name and password being sent in clear text to an FTP server. In‐
205 stead, use CURLUSESSL_CONTROL to ensure that an encrypted connection is
206 used or else fail the request.
207
209 If cookies are enabled and cached, then a user could craft a URL which
210 performs some malicious action to a site whose authentication is al‐
211 ready stored in a cookie. e.g. http://mail.example.com/delete-
212 stuff.cgi?delete=all Applications can mitigate against this by dis‐
213 abling cookies or clearing them between requests.
214
216 SCP URLs can contain raw commands within the scp: URL, which is a side
217 effect of how the SCP protocol is designed. e.g.
218 scp://user:pass@host/a;date >/tmp/test;
220 Applications must not allow unsanitized SCP: URLs to be passed in for
222 downloads.
223
225 By default curl and libcurl support file:// URLs. Such a URL is always
226 an access, or attempted access, to a local resource. If your applica‐
227 tion wants to avoid that, keep control of what URLs to use and/or pre‐
228 vent curl/libcurl from using the protocol.
229 By default, libcurl prohibits redirects to file:// URLs.
231
234 The Windows operating system will automatically, and without any way
235 for applications to disable it, try to establish a connection to an‐
236 other host over the network and access it (over SMB or other proto‐
237 cols), if only the correct file path is accessed.
238 When first realizing this, the curl team tried to filter out such at‐
240 tempts in order to protect applications for inadvertent probes of for
241 example internal networks etc. This resulted in CVE-2019-15601 and the
242 associated security fix.
243 However, we have since been made aware of the fact that the previous
245 fix was far from adequate as there are several other ways to accomplish
246 more or less the same thing: accessing a remote host over the network
247 instead of the local file system.
248 The conclusion we have come to is that this is a weakness or feature in
250 the Windows operating system itself, that we as an application cannot
251 safely protect users against. It would just be a whack-a-mole race we
252 do not want to participate in. There are too many ways to do it and
253 there's no knob we can use to turn off the practice.
254 If you use curl or libcurl on Windows (any version), disable the use of
256 the FILE protocol in curl or be prepared that accesses to a range of
257 "magic paths" will potentially make your system try to access other
258 hosts on your network. curl cannot protect you against this.
259
261 Applications may find it tempting to let users set the URL that it can
262 work on. That is probably fine, but opens up for mischief and trickery
263 that you as an application author may want to address or take precau‐
264 tions against.
265 If your curl-using script allow a custom URL do you also, perhaps unin‐
267 tentionally, allow the user to pass other options to the curl command
268 line if creative use of special characters are applied?
269 If the user can set the URL, the user can also specify the scheme part
271 to other protocols that you did not intend for users to use and perhaps
272 did not consider. curl supports over 20 different URL schemes.
273 "http://" might be what you thought, "ftp://" or "imap://" might be
274 what the user gives your application. Also, cross-protocol operations
275 might be done by using a particular scheme in the URL but point to a
276 server doing a different protocol on a non-standard port.
277 Remedies:
279 Use --proto
281 curl command lines can use --proto to limit what URL schemes it
282 accepts
283 Use CURLOPT_PROTOCOLS
285 libcurl programs can use CURLOPT_PROTOCOLS(3) to limit what URL
286 schemes it accepts
287 consider not allowing the user to set the full URL
289 Maybe just let the user provide data for parts of it? Or maybe
290 filter input to only allow specific choices?
291
293 curl supports URLs mostly according to how they are defined in RFC
294 3986, and has done so since the beginning.
295 Web browsers mostly adhere to the WHATWG URL Specification.
297 This deviance makes some URLs copied between browsers (or returned over
299 HTTP for redirection) and curl not work the same way. It can also cause
300 problems if an application parses URLs differently from libcurl and
301 makes different assumptions about a link. This can mislead users into
302 getting the wrong thing, connecting to the wrong host or otherwise not
303 working identically.
304 Within an application, this can be mitigated by always using the
306 curl_url(3) API to parse URLs, ensuring that they are parsed the same
307 way as within libcurl itself.
308
310 When performing an FTP transfer, two TCP connections are used: one for
311 setting up the transfer and one for the actual data.
312 FTP is not only un-authenticated, but the setting up of the second
314 transfer is also a weak spot. The second connection to use for data, is
315 either setup with the PORT/EPRT command that makes the server connect
316 back to the client on the given IP+PORT, or with PASV/EPSV that makes
317 the server setup a port to listen to and tells the client to connect to
318 a given IP+PORT.
319 Again, un-authenticated means that the connection might be meddled with
321 by a man-in-the-middle or that there's a malicious server pretending to
322 be the right one.
323 A malicious FTP server can respond to PASV commands with the IP+PORT of
325 a totally different machine. Perhaps even a third party host, and when
326 there are many clients trying to connect to that third party, it could
327 create a Distributed Denial-Of-Service attack out of it. If the client
328 makes an upload operation, it can make the client send the data to an‐
329 other site. If the attacker can affect what data the client uploads, it
330 can be made to work as a HTTP request and then the client could be made
331 to issue HTTP requests to third party hosts.
332 An attacker that manages to control curl's command line options can
334 tell curl to send an FTP PORT command to ask the server to connect to a
335 third party host instead of back to curl.
336 The fact that FTP uses two connections makes it vulnerable in a way
338 that is hard to avoid.
339
341 A malicious server could cause libcurl to effectively hang by sending
342 data slowly, or even no data at all but just keeping the TCP connection
343 open. This could effectively result in a denial-of-service attack. The
344 CURLOPT_TIMEOUT(3) and/or CURLOPT_LOW_SPEED_LIMIT(3) options can be
345 used to mitigate against this.
346 A malicious server could cause libcurl to download an infinite amount
348 of data, potentially causing all of memory or disk to be filled. Set‐
349 ting the CURLOPT_MAXFILESIZE_LARGE(3) option is not sufficient to guard
350 against this. Instead, applications should monitor the amount of data
351 received within the write or progress callback and abort once the limit
352 is reached.
353 A malicious HTTP server could cause an infinite redirection loop, caus‐
355 ing a denial-of-service. This can be mitigated by using the CUR‐
356 LOPT_MAXREDIRS(3) option.
357
359 User-supplied data must be sanitized when used in options like CUR‐
360 LOPT_USERAGENT(3), CURLOPT_HTTPHEADER(3), CURLOPT_POSTFIELDS(3) and
361 others that are used to generate structured data. Characters like em‐
362 bedded carriage returns or ampersands could allow the user to create
363 additional headers or fields that could cause malicious transactions.
364
366 A server can supply data which the application may, in some cases, use
367 as a file name. The curl command-line tool does this with --remote-
368 header-name, using the Content-disposition: header to generate a file
369 name. An application could also use CURLINFO_EFFECTIVE_URL(3) to gener‐
370 ate a file name from a server-supplied redirect URL. Special care must
371 be taken to sanitize such names to avoid the possibility of a malicious
372 server supplying one like "/etc/passwd", "\autoexec.bat", "prn:" or
373 even ".bashrc".
374
376 A secure application should never use the CURLOPT_SSL_VERIFYPEER(3) op‐
377 tion to disable certificate validation. There are numerous attacks that
378 are enabled by applications that fail to properly validate server
379 TLS/SSL certificates, thus enabling a malicious server to spoof a le‐
380 gitimate one. HTTPS without validated certificates is potentially as
381 insecure as a plain HTTP connection.
382
384 Relatedly, be aware that in situations when you have problems with
385 libcurl and ask someone for help, everything you reveal in order to get
386 best possible help might also impose certain security related risks.
387 Host names, user names, paths, operating system specifics, etc. (not to
388 mention passwords of course) may in fact be used by intruders to gain
389 additional information of a potential target.
390 Be sure to limit access to application logs if they could hold private
392 or security-related data. Besides the obvious candidates like user
393 names and passwords, things like URLs, cookies or even file names could
394 also hold sensitive data.
395 To avoid this problem, you must of course use your common sense. Often,
397 you can just edit out the sensitive data or just search/replace your
398 true information with faked data.
399
401 libcurl-using applications that set the 'setuid' bit to run with ele‐
402 vated or modified rights also implicitly give that extra power to
403 libcurl and this should only be done after careful considerations.
404 Giving setuid powers to the application means that libcurl can save
406 files using those new rights (if for example the `SSLKEYLOGFILE` envi‐
407 ronment variable is set). Also: if the application wants these powers
408 to read or manage secrets that the user is otherwise not able to view
409 (like credentials for a login etc), it should be noted that libcurl
410 still might understand proxy environment variables that allow the user
411 to redirect libcurl operations to use a proxy controlled by the user.
412
414 An application that uses libcurl and invokes `fork()` will get all file
415 descriptors duplicated in the child process, including the ones libcurl
416 created.
417 libcurl itself uses `fork()` and `execl()` if told to use the
419 `CURLAUTH_NTLM_WB` authentication method which then will invoke the
420 helper command in a child process with file descriptors duplicated.
421 Make sure that only the trusted and reliable helper program is invoked!
422
424 Should you detect or just suspect a security problem in libcurl or
425 curl, contact the project curl security team immediately. See
426 https://curl.se/dev/secprocess.html for details.
427libcurl 7.82.0 December 15, 2021 libcurl-security(3)