markdown2 Regular Expression Denial of Service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · 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 · score source

EPSS

• CVE-2021-26813: 2.32% estimated exploitation activity in the next 30 days; percentile 83.47 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

2 Jun 2021, 21:44 UTC

Last modified by the source

21 Feb 2025, 05:30 UTC

Source last checked

26 Oct 2026, 13:21 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jr9p-r423-9m2r

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jr9p-r423-9m2r

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# Patch Next advisory brief: CVE-2021-26813

Advisory: markdown2 Regular Expression Denial of Service
IDs: CVE-2021-26813, GHSA-jr9p-r423-9m2r
Record: https://patchnext.com/advisories/cve-2021-26813 (revision 3, source checked 26 Oct 2026, 13:21 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: markdown2 2.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI markdown2: affected >= 1.0.1.18, < 2.4.0; fixed in 2.4.0

## Suggested inspection (read-only)
- `python -m pip show markdown2`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on markdown2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-jr9p-r423-9m2r (OSV record GHSA-jr9p-r423-9m2r lists affected ranges: PyPI markdown2 >=1.0.1.18 <2.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-jr9p-r423-9m2r (GitHub-reviewed advisory GHSA-jr9p-r423-9m2r lists affected ranges: PyPI markdown2 >=1.0.1.18 <2.4.0)

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