CVE-2021-44686: Calibre-Ebook Calibre

High severity, CVSS 7.5. EPSS: 5.2% chance of exploitation in the next 30 days.

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.

Affected products

• Calibre-Ebook Calibre: before 5.32.0 (fixed in 5.32.0)
• Fedoraproject Fedora: version 34 only

Published 2021-12-07. Last modified 2026-06-17.