CVE-2021-20270: Pygments Infinite Loop Vulnerability

HIGH
Advisory Identifier: GHSA-9w8r-397f-prfh
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CVSS 7.5
CWE-835
Published
Advisory Summary

CVE-2021-20270 is a HIGH (CVSS 7.5) infinite loop vulnerability (CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')) affecting Pygments libraries in the PyPI ecosystem. The issue involves infinite loop in pygments. Remediation is available by upgrading affected packages to version 2.7.4.

CVE-2021-20270 at a Glance

CVE Identifier CVE-2021-20270
GHSA Identifier GHSA-9w8r-397f-prfh
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')
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-2021-20270

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-2021-20270, 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-835: Loop with Unreachable Exit Condition ('Infinite Loop'), where the product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. 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

An infinite loop in SMLLexer in Pygments versions 1.5 to 2.7.3 may lead to denial of service when performing syntax highlighting of a Standard ML (SML) source file, as demonstrated by input that only contains the "exception" keyword.

CVE-2021-20270 Affected Pygments Versions

1 component(s)
Fixed Versions:
2.7.4
Product / Package Ecosystem Affected Versions Fixed Version
Pygments
pypi/Pygments
PyPI
≥ 1.5
< 2.7.4
2.7.4

References

NVD — CVE-2021-20270 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-20270
Security Fix Commit FIX COMMIT
https://github.com/pygments/pygments/commit/f91804ff4772e3ab41f46e28d370f57898700333
bugzilla.redhat.com/show_bug.cgi WEB
https://bugzilla.redhat.com/show_bug.cgi?id=1922136
Pygments Source Repository REPOSITORY
https://github.com/pygments/pygments
Pygments Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/pygments/PYSEC-2021-140.yaml
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
Pygments Security Information SECURITY
https://www.debian.org/security/2021/dsa-4889
Pygments Security Information SECURITY
https://www.oracle.com/security-alerts/cpuoct2021.html

Frequently Asked Questions

What is CVE-2021-20270?
CVE-2021-20270 is a security vulnerability in Pygments involving Infinite Loop in Pygments under CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop').
Which Pygments versions are affected?
Affected versions include releases prior to 2.7.4 across affected packages: Pygments.
What version fixes CVE-2021-20270?
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-2021-20270?
CVE-2021-20270 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-2021-20270?
CVE-2021-20270 is classified under CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.).
How can I check PyPI dependencies for CVE-2021-20270?
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-9w8r-397f-prfh",
  "modified" : "2024-10-14T16:12:14Z",
  "published" : "2021-04-20T16:35:47Z",
  "aliases" : [ "CVE-2021-20270" ],
  "summary" : "Infinite Loop in Pygments",
  "details" : "An infinite loop in SMLLexer in Pygments versions 1.5 to 2.7.3 may lead to denial of service when performing syntax highlighting of a Standard ML (SML) source file, as demonstrated by input that only contains the \"exception\" keyword.",
  "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" : "1.5"
      }, {
        "fixed" : "2.7.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-20270"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pygments/pygments/commit/f91804ff4772e3ab41f46e28d370f57898700333"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.redhat.com/show_bug.cgi?id=1922136"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pygments/pygments"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pygments/PYSEC-2021-140.yaml"
  }, {
    "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://www.debian.org/security/2021/dsa-4889"
  }, {
    "type" : "WEB",
    "url" : "https://www.oracle.com/security-alerts/cpuoct2021.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-835" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-04-12T20:58:03Z",
    "nvd_published_at" : "2021-03-23T17:15:00Z"
  }
}