GHSA-5239-wwwm-4pmq CVE-2026-4539 PYSEC-2026-2987
Pygments has Regular Expression Denial of Service (ReDoS) due to Inefficient Regex for GUID Matching
- Severity
- 1.9 / 10
- Low · CVSS 4.0
- Exploit chance (EPSS)
- 0.16%
- Chance of exploitation in the next 30 days. Higher than 5.02% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Fix
- Available
- 2 packages have a fixed version
Description
A security flaw has been discovered in pygments before 2.20.0. The impacted element is the function AdlLexer of the file pygments/lexers/archetype.py. The manipulation results in inefficient regular expression complexity. The attack is only possible with local access. The exploit has been released to the public and may be used for attacks. The project was informed of the problem early through an issue report but has not responded yet.
Affected versions
PyPI
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
Pygments PyPI → 2.20.0
- pip
pip install --upgrade "Pygments>=2.20.0"- poetry
poetry add "Pygments>=2.20.0"- uv
uv add "Pygments>=2.20.0"
pygments PyPI → 2.20.0
- pip
pip install --upgrade "pygments>=2.20.0"- poetry
poetry add "pygments>=2.20.0"- uv
uv add "pygments>=2.20.0"
Severity breakdown
CVSS 4.0 1.9
CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P
- Attack vector
- Local
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- Low
- User interaction
- None
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- None
- Availability (vulnerable system)
- Low
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
- Exploit maturity
- Proof of concept
CVSS 3.1 3.3
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
- Attack vector
- Local
- Attack complexity
- Low
- Privileges required
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- Low
Weakness
CWE-400: Uncontrolled Resource Consumption
The product does not properly control the allocation and maintenance of a limited resource.
How to prevent it
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Implementation: Ensure that all failures in resource allocation place the system into a safe posture.
CWE-1333: Inefficient Regular Expression Complexity
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
How to prevent it
- Architecture and Design: Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
- System Configuration: Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
- Implementation: Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
- Implementation: Limit the length of the input that the regular expression will process.
References
Advisories
Fixes and patches
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-mrwq-x4v8-fh7p Pygments vulnerable to ReDoS | Medium | Also affects pygments |
| GHSA-9w8r-397f-prfh Infinite Loop in Pygments | High | Also affects pygments |
| GHSA-pq64-v7f5-gqh8 Pygments vulnerable to Regular Expression Denial of Service (ReDoS) | High | Also affects pygments |
| GHSA-fff8-4w9p-7v76 Command Injection in Pygments | Critical | Also affects pygments |
| GHSA-cggf-277g-3q54 pypdf has possible large memory usage when parsing page tree | High | Same weakness, CWE-400 |
| GHSA-3g3m-2fjr-6pw4 Glob: AbortSignal cancellation bypass allows post-timeout filesystem traversal | Medium | Same weakness, CWE-400 |
| GHSA-42gh-23hr-vphw pypdf has possible long runtimes when retrieving page labels | High | Same weakness, CWE-400 |
| GHSA-4vqv-7265-gm2w pypdf has possible large memory usage for exceptions in strict mode | High | Same weakness, CWE-400 |
Credits: dnegreira, nzlaura
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