CVE-2021-27291: Pygments Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-pq64-v7f5-gqh8
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CVSS 7.5
CWE-400
Published
Advisory Summary

CVE-2021-27291 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Pygments libraries in the PyPI ecosystem. The issue involves pygments vulnerable to regular expression denial of service (redos). Remediation is available by upgrading affected packages to version 2.7.4.

CVE-2021-27291 at a Glance

CVE Identifier CVE-2021-27291
GHSA Identifier GHSA-pq64-v7f5-gqh8
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Pygments
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2021-27291

To remediate this vulnerability, upgrade all affected Pygments dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Pygments fixed: 2.7.4 
pip install Pygments>=2.7.4
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show Pygments

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-27291, inspect your installed packages using pip:

pip show Pygments

Analyze the terminal output to locate any version of Pygments in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.7.4), your application is affected.

Note: The pip show Pygments command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Pygments, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2021-27291 Affected Pygments Versions

1 component(s)
Fixed Versions:
2.7.4
Product / Package Ecosystem Affected Versions Fixed Version
Pygments
pypi/Pygments
PyPI
≥ 1.1
< 2.7.4
2.7.4

References

NVD — CVE-2021-27291 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-27291
Security Fix Commit FIX COMMIT
https://github.com/pygments/pygments/commit/2e7e8c4a7b318f4032493773732754e418279a14
Pygments Source Repository REPOSITORY
https://gist.github.com/b-c-ds/b1a2cc0c68a35c57188575eb496de5ce
Pygments Source Repository REPOSITORY
https://github.com/pygments/pygments
Pygments Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/pygments/PYSEC-2021-141.yaml
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2021/03/msg00024.html
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2021/05/msg00003.html
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2021/05/msg00006.html
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/GSJRFHALQ7E3UV4FFMFU2YQ6LUDHAI55
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/WSLD67LFGXOX2K5YNESSWAS4AGZIJTUQ
Pygments Security Information SECURITY
https://www.debian.org/security/2021/dsa-4878
Pygments Security Information SECURITY
https://www.debian.org/security/2021/dsa-4889

Frequently Asked Questions

What is CVE-2021-27291?
CVE-2021-27291 is a security vulnerability in Pygments involving Pygments vulnerable to Regular Expression Denial of Service (ReDoS) under CWE-400: Uncontrolled Resource Consumption.
Which Pygments versions are affected?
Affected versions include releases prior to 2.7.4 across affected packages: Pygments.
What version fixes CVE-2021-27291?
Upgrading to 2.7.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is Pygments affected?
Yes, Pygments is affected in releases prior to 2.7.4. Upgrade your dependency to 2.7.4 or later.
What is the CVSS score of CVE-2021-27291?
CVE-2021-27291 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2021-27291?
CVE-2021-27291 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2021-27291?
Run the dependency tree command: 'pip show Pygments'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-pq64-v7f5-gqh8",
  "modified" : "2024-10-14T16:10:54Z",
  "published" : "2021-03-29T16:33:03Z",
  "aliases" : [ "CVE-2021-27291" ],
  "summary" : "Pygments vulnerable to Regular Expression Denial of Service (ReDoS)",
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "Pygments"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.1"
      }, {
        "fixed" : "2.7.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-27291"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pygments/pygments/commit/2e7e8c4a7b318f4032493773732754e418279a14"
  }, {
    "type" : "WEB",
    "url" : "https://gist.github.com/b-c-ds/b1a2cc0c68a35c57188575eb496de5ce"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pygments/pygments"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pygments/PYSEC-2021-141.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2021/03/msg00024.html"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2021/05/msg00003.html"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2021/05/msg00006.html"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/GSJRFHALQ7E3UV4FFMFU2YQ6LUDHAI55"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/WSLD67LFGXOX2K5YNESSWAS4AGZIJTUQ"
  }, {
    "type" : "WEB",
    "url" : "https://www.debian.org/security/2021/dsa-4878"
  }, {
    "type" : "WEB",
    "url" : "https://www.debian.org/security/2021/dsa-4889"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-03-22T20:28:40Z",
    "nvd_published_at" : "2021-03-17T13:15:00Z"
  }
}