Headline
GHSA-c2pc-g5qf-rfrf: league/commonmark's quadratic complexity bugs may lead to a denial of service
Impact
Several polynomial time complexity issues in league/commonmark may lead to unbounded resource exhaustion and subsequent denial of service.
Malicious users could trigger that inefficient code with carefully crafted Markdown inputs that are specifically designed to ensure the worst-case performance is reached. Sending multiple such requests in parallel could tie up all available CPU resources and/or PHP-FPM processes, leading to denial of service for legitimate users.
Patches
These vulnerabilities have been patched in version 2.6.0. All users on older versions are highly encouraged to upgrade as soon as possible.
Workarounds
If you cannot upgrade, you may be able to mitigate the issues by:
- Setting very low
memory_limitandmax_execution_timePHP configurations to prevent runaway resource usage - Implementing rate-limiting, bot protection, or other approaches to reduce the risk of simultaneous bad requests hitting your site
- Limiting the size of inputs fed into this library (specifically the max length of each line)
- Limiting the use of this library to trusted users
References
Most of these issues were discovered in other Markdown parsers. You can read more about them here:
- https://github.com/commonmark/commonmark.js/issues/129
- https://github.com/commonmark/commonmark.js/issues/157
- https://github.com/commonmark/commonmark.js/issues/172
- https://github.com/github/cmark-gfm/security/advisories/GHSA-r572-jvj2-3m8p
- https://github.com/github/cmark-gfm/security/advisories/GHSA-24f7-9frr-5h2r
- https://github.com/github/cmark-gfm/security/advisories/GHSA-29g3-96g3-jg6c
- https://github.com/github/cmark-gfm/security/advisories/GHSA-r8vr-c48j-fcc5
- https://github.com/github/cmark-gfm/security/advisories/GHSA-w4qg-3vf7-m9x5
- https://github.com/github/cmark-gfm/security/advisories/GHSA-66g8-4hjf-77xh
For general information about this type of issue:
- https://en.wikipedia.org/wiki/Time_complexity
- https://cwe.mitre.org/data/definitions/407.html
Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.