What is CVE-2021-20270?

CVE-2021-20270 is a high-severity vulnerability in Pygments, classified under Loop with Unreachable Exit Condition (Infinite Loop). CVSS score: 7.5/10. Published 2021-03-23.

How severe is CVE-2021-20270?

High severity. CVSS v3 base score is 7.5 out of 10.

Is CVE-2021-20270 known to be exploited?

3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Pygments

CVE-2021-20270

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.

Published 2021-03-23 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.028 (86.3th percentile), scored 2026-11-01.

Low probability of exploitation in the next 30 days, but still tracked. 86th percentile — 86.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.028 · newest: 0.028 · change: 0.000

Affected products

Pygments
Debian Linux — versions 9.0, 10.0
Fedoraproject Fedora — versions 33
Red Hat Enterprise Linux — versions 7.0, 8.0
Red Hat Openshift Container Platform — versions 3.11, 4.0
Red Hat OpenStack Platform — versions 10.0
Red Hat Software Collections
Python-Pygments — versions python-pygments 2.7.4

Weakness classification (CWE)

CWE-835 — Loop with Unreachable Exit Condition (Infinite Loop)

Public proof-of-concept exploits

References

bugzilla.redhat.com/show_bug.cgi (Patch, Third Party Advisory, Issue Tracking)
www.debian.org/security/2021/dsa-4889 (vendor-advisory, Third Party Advisory)
lists.debian.org/debian-lts-announce/2021/05/msg00003.html (mailing-list, Mailing List, Third Party Advisory)
lists.debian.org/debian-lts-announce/2021/05/msg00006.html (mailing-list, Mailing List, Third Party Advisory)
www.oracle.com/security-alerts/cpuoct2021.html (Third Party Advisory, Not Applicable)

Frequently asked questions

What is CVE-2021-20270?: CVE-2021-20270 is a high-severity vulnerability in Pygments, classified under Loop with Unreachable Exit Condition (Infinite Loop). CVSS score: 7.5/10. Published 2021-03-23.
How severe is CVE-2021-20270?: High severity. CVSS v3 base score is 7.5 out of 10.
Is CVE-2021-20270 known to be exploited?: 3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.