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Fencer Vulns
high 7.5

CVE-2021-26813 json

markdown2 Regular Expression Denial of Service

published 2021-03-03 5 y ago modified 2026-06-17 4 mo ago also GHSA-jr9p-r423-9m2r , PYSEC-2021-20 , EUVD-2021-0118

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
2.3%
EPSS 2.3% chance of exploitation in 30 days, percentile 83%
public exploits
0
none known

Summary

markdown2 >=1.0.1.18, fixed in 2.4.0, is affected by a regular expression denial of service vulnerability. If an attacker provides a malicious string, it can make markdown2 processing difficult or delayed for an extended period of time.

Patch / fix

  • pypi markdown2 2.4.0
  • markdown2_project markdown2 2.4.0
  • python-markdown2 Debian 14, Debian 13, Debian 12 2.3.10-1.1

debian advisories DEBIAN-CVE-2021-26813

  • python-markdown2 openSUSE Tumbleweed 2.5.5-1.3
  • python-markdown2 openSUSE Tumbleweed 2.4.13-1.2
  • python-markdown2 openSUSE Tumbleweed 2.4.0-2.4
  • python-markdown2 Package Hub 15 SP2 2.4.0-bp152.2.4.1
  • python-markdown2 openSUSE Leap 15.2 2.4.0-lp152.2.3.1

opensuse advisories openSUSE-SU-2026:12016-1openSUSE-SU-2024:14146-1openSUSE-SU-2024:11237-1openSUSE-SU-2021:0451-1openSUSE-SU-2021:0429-1

  • python-markdown2 Arch Linux 2.4.0-1

arch linux advisories AVG-1637

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-26813 updated cvss 7.5

python-markdown2: Regular expression denial of service

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

2.32% chance of exploitation in the next 30 days, higher than 83% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/trentm/python-markdown2/pull/387 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
pypi markdown2
>=1.0.1.18, <2.4.0
2.4.0

Classification

Weakness
  • CWE-1333 Inefficient Regular Expression Complexity

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.