CVE-2021-44686

calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular...

7.5 HighFix available

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None

What an attacker can affect

Confidentiality: None
Integrity: None
Availability: High

Exploit likelihood

5.21%Up 4.2 points since Dec 7, 2021

What each source reports

NVD Score used 7.5

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.

Record: CVE-2021-44686
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 2.0: 5.0 MediumAV:N/AC:L/Au:N/C:N/I:N/A:P
Weakness: CWE-400
Updated: 2026-11-02

CVE List No score

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.

Record: CVE-2021-44686
Updated: 2026-11-02

EUVDEUVD-2021-31502 No score

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.

Record: EUVD-2021-31502
Updated: 2026-11-02

GitHub AdvisoriesGHSA-m6m7-mf2p-99p5 7.5

calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular...

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.

Record: GHSA-m6m7-mf2p-99p5
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness: CWE-400, CWE-770
Updated: 2026-11-02

OSV 7.5

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.

Record: CVE-2021-44686
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-02

OSVDEBIAN-CVE-2021-44686 7.5

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.

Record: DEBIAN-CVE-2021-44686
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-02

OSVUBUNTU-CVE-2021-44686 7.5

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.

Record: UBUNTU-CVE-2021-44686
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-02

OSVMGASA-2021-0593 No score

Updated calibre packages fix security vulnerability

ReDoS vulnerability in html_preprocess_rules in ebooks/conversion/preprocess.py

Record: MGASA-2021-0593
Updated: 2026-11-02

Affected products

Affected packages

Affected packages
Package	Affected	Fixed
`calibre` Debian:12	before 5.33.0+dfsg-1	5.33.0+dfsg-1
`calibre` Debian:13	before 5.33.0+dfsg-1	5.33.0+dfsg-1
`calibre` Debian:14	before 5.33.0+dfsg-1	5.33.0+dfsg-1
`calibre` Debian:11 From an advisory covering 2 CVEs	before 5.12.0+dfsg-1+deb11u3, plus listed versions 5.12.0+dfsg-15.12.0+dfsg-1+deb11u15.12.0+dfsg-1+deb11u2	5.12.0+dfsg-1+deb11u3
`calibre` Mageia:8	before 4.23.0-3.1.mga8	4.23.0-3.1.mga8
`calibre` Ubuntu:16.04:LTS	All versions, plus listed versions 2.33.0+dfsg-1build12.38.0+dfsg-12.45.0+dfsg-12.45.0+dfsg-1build12.48.0+dfsg-1 Show 4 moreShow less 2.48.0+dfsg-1build12.54.0+dfsg-12.55.0+dfsg-12.55.0+dfsg-1ubuntu0.2	None known
`calibre` Ubuntu:18.04:LTS	All versions, plus listed versions 3.7.0+dfsg-23.7.0+dfsg-2build13.12.0+dfsg-13.13.0+dfsg-13.14.0+dfsg-1 Show 10 moreShow less 3.15.0.1+dfsg-13.16.0+dfsg-13.16.0+dfsg-1build13.17.0+dfsg-13.17.0+dfsg-23.18.0+dfsg-1build13.19.0+dfsg-13.20.0+dfsg-13.21.0+dfsg-13.21.0+dfsg-1build1	None known
`calibre` Ubuntu:20.04:LTS	All versions, plus listed versions 3.46.0+dfsg-14.2.0+dfsg-24.3.0+dfsg-14.3.0+dfsg-24.4.0+dfsg-1 Show 14 moreShow less 4.5.0+dfsg-14.5.0+dfsg-24.5.0+dfsg-34.6.0+dfsg-14.7.0+dfsg-14.99.3+dfsg-24.99.4+dfsg-14.99.4+dfsg-1build14.99.4+dfsg+really4.10.0+py3-24.99.4+dfsg+really4.11.2-14.99.4+dfsg+really4.11.2-1build14.99.4+dfsg+really4.12.0-14.99.4+dfsg+really4.12.0-1build14.99.4+dfsg+really4.12.0-1ubuntu1	None known
`calibre` Ubuntu:22.04:LTS	All versions, plus listed versions 5.25.0+dfsg-25.33.2+dfsg-15.34.0+dfsg-15.35.0+dfsg-1ubuntu25.37.0+dfsg-15.37.0+dfsg-1build1	None known
`calibre` Ubuntu:26.04:LTS	All versions, plus listed versions 8.8.0+ds-3build18.13.0+ds+~0.10.5-38.14.0+ds+~0.10.5-18.15.0+ds+~0.10.5-18.16.0+ds+~0.10.5-2 Show 4 moreShow less 8.16.2+ds+~0.10.5-18.16.2+ds+~0.10.5-28.16.2+ds+~0.10.5-39.2.1+ds+~0.10.5-2build1	None known

History

History
Recorded	Changed	Sources
Nov 6, 2026	affected, facts
Nov 2, 2026	first indexed	CVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-44686 is a high vulnerability (CVSS 7.5) in Calibre-ebook Calibre, published December 6, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 5.21%, the 92nd percentile. Fixed in calibre 5.33.0+dfsg-1.

References

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