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The GNU C Library Security Policy

This document describes the policy followed by the GNU C Library maintainers to handle bugs that may have a security impact. This includes determining if a bug has a security impact, reporting such bugs to the community and handling such bugs all the way to resolution. This policy may evolve over time, so if you're reading this from a release tarball, be sure to check the latest copy of the SECURITY.md in the repository, especially for instructions on reporting issues privately.

What is a security bug?

Most security vulnerabilities in the GNU C Library materialize only after an application uses functionality in a specific way. Therefore, it is sometimes difficult to determine if a defect in the GNU C Library constitutes a vulnerability as such. The follow guidelines can help with a decision.

  • Buffer overflows should be treated as security bugs if it is conceivable that the data triggering them can come from an untrusted source.
  • Other bugs that cause memory corruption which is likely exploitable should be treated as security bugs.
  • Information disclosure can be security bugs, especially if exposure through applications can be determined.
  • Memory leaks and races are security bugs if they cause service breakage.
  • Stack overflow through unbounded alloca calls or variable-length arrays are security bugs if it is conceivable that the data triggering the overflow could come from an untrusted source.
  • Stack overflow through deep recursion and other crashes are security bugs if they cause service breakage.
  • Bugs that cripple the whole system (so that it doesn't even boot or does not run most applications) are not security bugs because they will not be exploitable in practice, due to general system instability.
  • Bugs that crash nscd are generally security bugs, except if they can only be triggered by a trusted data source (DNS is not trusted, but NIS and LDAP probably are).
  • The Security Exceptions section below describes subsystems for which determining the security status of bugs is especially complicated.
  • For consistency, if the bug has received a CVE name attributing it to the GNU C library, it should be flagged security+.
  • Duplicates of security bugs (flagged with security+) should be flagged security-, to avoid cluttering the reporting.

In this context, service breakage means client-side privilege escalation (code execution) or server-side denial of service or privilege escalation through actual, concrete, non-synthetic applications. Or put differently, if the GNU C Library causes a security bug in an application (and the application uses the library in a standard-conforming manner or according to the manual), the GNU C Library bug should be treated as security-relevant.

Security Exceptions

It may be especially complicated to determine the security status of bugs in some subsystems in the GNU C Library. This subsection describes such subsystems and the special considerations applicable during security bug classification in them.

Regular expression processing

Regular expression processing comes in two parts, compilation (through regcomp) and execution (through regexec).

Implementing regular expressions efficiently, in a standard-conforming way, and without denial-of-service vulnerabilities is very difficult and impossible for Basic Regular Expressions. Most implementation strategies have issues dealing with certain classes of patterns.

Consequently, certain issues which can be triggered only with crafted patterns (either during compilation or execution) are treated as regular bugs and not security issues. Examples of such issues would include (but is not limited to):

  • Running out of memory through valid use of malloc
  • Quadratic or exponential behaviour resulting in slow execution time
  • Stack overflows due to recursion when processing patterns

Crashes, infinite loops (and not merely exponential behavior), buffer overflows and overreads, memory leaks and other bugs resulting from the regex implementation relying on undefined behavior should be treated as security vulnerabilities.

wordexp patterns

wordexp inherently has exponential memory consumption in terms of the input size. This means that denial of service flaws from crafted patterns are not security issues (even if they lead to other issues, such as NULL pointer dereferences).

Asynchronous I/O

The GNU C Library tries to implement asynchronous I/O without kernel support, which means that several operations are not fully standard conforming. Several known races can cause crashes and resource leaks. Such bugs are only treated as security bugs if applications (as opposed to synthetic test cases) have security exposures due to these bugs.

Asynchronous cancellation

The implementation of asynchronous cancellation is not fully standard-conforming and has races and leaks. Again, such bugs are only treated as security bugs if applications (as opposed to synthetic test cases) have security exposures due to these bugs.

Crafted binaries and ldd

The ldd tool is not expected to be used with untrusted executables.

Post-exploitation countermeasures

Certain features have been added to the library only to make exploitation of security bugs (mainly for code execution) more difficult. Examples includes the stack smashing protector, function pointer obfuscation, vtable validation for stdio stream handles, and various heap consistency checks. Failure of such countermeasures to stop exploitation of a different vulnerability is not a security vulnerability in itself. By their nature, these countermeasures are based on heuristics and will never offer complete protection, so the original vulnerability needs to be fixed anyway.

Reporting security bugs

The process to report security bugs is documented on the glibc security page. In general, most security bugs may be reported publicly in the glibc bugzilla, but if in doubt, please feel free to report security issues privately first.

Triaging security bugs

This section is aimed at developers, not reporters.

Security-relevant bugs should be marked with security+, as per the Bugzilla security flag documentation, following the guidelines above. If you set the security+ flag, you should make sure the following information is included in the bug (usually in a bug comment):

  • The first glibc version which includes the vulnerable code. If the vulnerability was introduced before glibc 2.4 (released in 2006), this information is not necessary.
  • The commit or commits (identified by hash) that fix this vulnerability in the master branch, and (for historic security bugs) the first release that includes this fix.
  • The summary should include the CVE names (if any), in parentheses at the end.
  • If there is a single CVE name assigned to this bug, it should be set as an alias.

The following links are helpful for finding untriaged bugs:

Fixing security bugs

For changes to master, the regular consensus-driven process must be followed. It makes sense to obtain consensus in private, to ensure that the patch is likely in a committable state, before disclosing an emboargoed vulnerability.

Security backports to release branches need to follow the release process.

Contact the website maintainers and have them draft a news entry for the website frontpage to direct users to the bug, the fix, or the mailing list discussions.

CVE assignment

Security bugs flagged with security+ should have CVE identifiers. Please reach out to the glibc security team using the documented security process and they work on getting a CVE number.