GHSA-pq64-v7f5-gqh8 CVE-2021-27291 PYSEC-2021-141

Pygments vulnerable to Regular Expression Denial of Service (ReDoS)

High PyPI

Severity: 0.0 / 10; High · CVSS 4.0
Exploit chance (EPSS): 3.91%; Chance of exploitation in the next 30 days. Higher than 90.28% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-400 Uncontrolled Resource Consumption
Fix: 2.7.4; Upgrade pygments to this version or later

Description

In pygments 1.1+, fixed in 2.7.4, the lexers used to parse programming languages rely heavily on regular expressions. Some of the regular expressions have exponential or cubic worst-case complexity and are vulnerable to ReDoS. By crafting malicious input, an attacker can cause a denial of service.

Affected versions

PyPI

Package	Vulnerable	Fixed in
pygments	`>= 1.1, < 2.7.4`	`2.7.4`
pygments	`>= 1.1, < 2.7.4`	`2.7.4`

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 → `2.7.4`

pip

pip install --upgrade "pygments>=2.7.4"

poetry

poetry add "pygments>=2.7.4"

uv

uv add "pygments>=2.7.4"

Severity breakdown

CVSS 4.0 0.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Attack vector: Network
Attack complexity: Low
Attack requirements: None
Privileges required: None
User interaction: None
Confidentiality (vulnerable system): None
Integrity (vulnerable system): None
Availability (vulnerable system): High
Confidentiality (subsequent systems): None
Integrity (subsequent systems): None
Availability (subsequent systems): None

CVSS 3.1 7.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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.

References

Advisories

Packages

github.com/pygments/pygments

Timeline

Mar 17, 2021 Published
Jun 17, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-5239-wwwm-4pmq Pygments has Regular Expression Denial of Service (ReDoS) due to Inefficient Regex for GUID Matching	Low	Also affects pygments
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-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

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