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HIGH severity

CVE-2021-26813 — markdown2

HIGHFix: trentm/python-markdown2#387

CVE-2021-26813 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in markdown2. 1 public exploit reference exists, so weaponization risk is real. A fix is available for markdown2 — see the affected versions and patch details below.

markdown2 Regular Expression Denial of Service

Also known asGHSA-jr9p-r423-9m2rPYSEC-2021-20
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
1 known
Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
2.3%probability of exploitation in next 30 days
Lower Risk+0.09%
Lower risk than most CVEs83th percentile — riskier than 83% of all scored CVEsHighest risk
0.00%0.98%1.97%2.95%0.5%2.3%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-26813 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍markdown2

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImarkdown2≥ 1.0.1.18&&< 2.4.02.4.0pip install --upgrade 'markdown2==2.4.0'

Affected Products

2 products · 4 configurations
OS
fedorafedoraproject
3 versions
323334
Application
markdown2markdown2_project
≥ 1.0.1.18 && < 2.4.0
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

markdown2 >=1.0.1.18, fixed in 2.4.0, is affected by a regular expressio…

github.comNVDMay 2021

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for markdown2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update markdown2 to 2.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26813 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

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.
O3 Security · Impact-Aware SCA

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CVE-2021-26813: markdown2 DoS — Fixed in 2.4.0