The Rust Security Response WG was notified that the regex crate did not properly limit the complexity of the regular expressions (regex) it parses. An attacker could use this security issue to perform a denial of service, by sending a specially crafted regex to a service accepting untrusted regexes. No known vulnerability is present when parsing untrusted input with trusted regexes.

This issue has been assigned CVE-2022-24713. The severity of this vulnerability is "high" when the regex crate is used to parse untrusted regexes. Other uses of the regex crate are not affected by this vulnerability.

Overview

The regex crate features built-in mitigations to prevent denial of service attacks caused by untrusted regexes, or untrusted input matched by trusted regexes. Those (tunable) mitigations already provide sane defaults to prevent attacks. This guarantee is documented and it's considered part of the crate's API.

Unfortunately a bug was discovered in the mitigations designed to prevent untrusted regexes to take an arbitrary amount of time during parsing, and it's possible to craft regexes that bypass such mitigations. This makes it possible to perform denial of service attacks by sending specially crafted regexes to services accepting user-controlled, untrusted regexes.

Affected versions

All versions of the regex crate before or equal to 1.5.4 are affected by this issue. The fix is include starting from regex 1.5.5.

Mitigations

We recommend everyone accepting user-controlled regexes to upgrade immediately to the latest version of the regex crate.

Unfortunately there is no fixed set of problematic regexes, as there are practically infinite regexes that could be crafted to exploit this vulnerability. Because of this, we do not recommend denying known problematic regexes.

Acknowledgements

We want to thank Addison Crump for responsibly disclosing this to us according to the Rust security policy, and for helping review the fix.

We also want to thank Andrew Gallant for developing the fix, and Pietro Albini for coordinating the disclosure and writing this advisory.

Issue 1: Failure to quote characters

Affected versions of this crate allowed the bytes { and \xa0 to appear unquoted and unescaped in command arguments.

If the output of quote or join is passed to a shell, then what should be a single command argument could be interpreted as multiple arguments.

This does not directly allow arbitrary command execution (you can't inject a command substitution or similar). But depending on the command you're running, being able to inject multiple arguments where only one is expected could lead to undesired consequences, potentially including arbitrary command execution.

The flaw was corrected in version 1.2.1 by escaping additional characters. Updating to 1.3.0 is recommended, but 1.2.1 offers a more minimal fix if desired.

Workaround: Check for the bytes { and \xa0 in quote/join input or output.

(Note: { is problematic because it is used for glob expansion. \xa0 is problematic because it's treated as a word separator in specific environments.)

Issue 2: Dangerous API w.r.t. nul bytes

Version 1.3.0 deprecates the quote and join APIs in favor of try_quote and try_join, which behave the same except that they have Result return type, returning Err if the input contains nul bytes.

Strings containing nul bytes generally cannot be used in Unix command arguments or environment variables, and most shells cannot handle nul bytes even internally. If you try to pass one anyway, then the results might be security-sensitive in uncommon scenarios. More details here.

Due to the low severity, the behavior of the original quote and join APIs has not changed; they continue to allow nuls.

Workaround: Manually check for nul bytes in quote/join input or output.

Issue 3: Lack of documentation for interactive shell risks

The quote family of functions does not and cannot escape control characters. With non-interactive shells this is perfectly safe, as control characters have no special effect. But if you writing directly to the standard input of an interactive shell (or through a pty), then control characters can cause misbehavior including arbitrary command injection.

This is essentially unfixable, and has not been patched. But as of version 1.3.0, documentation has been added.

Future versions of shlex may add API variants that avoid the issue at the cost of reduced portability.