CVE-2021-44686Public exploitNetwork attack vector

Conversion interruption via catastrophic regex backtracking

Published Dec 6, 2021 · Updated Nov 4, 2025

Regular expression denial of service in calibre before 5.32.0 allows remote attackers to interrupt HTML conversion with crafted input. The html_preprocess_rules expression in ebooks/conversion/preprocess.py applies an unbounded multiline match to head markup, causing catastrophic backtracking when an opening head tag is followed by many newlines without a closing tag. Exploitation requires crafted HTML to reach the conversion pipeline and consumes processing resources without a reported confidentiality or integrity impact.

CVSS severity7.5
High
EPSS probability4.99%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in calibre before 5.32.0 allows remote attackers to interrupt HTML conversion with crafted input. The html_preprocess_rules expression in ebooks/conversion/preprocess.py applies an unbounded multiline match to head markup, causing catastrophic backtracking when an opening head tag is followed by many newlines without a closing tag. Exploitation requires crafted HTML to reach the conversion pipeline and consumes processing resources without a reported confidentiality or integrity impact.

The record

CVE
CVE-2021-44686
Published
Dec 6, 2021
Updated
Nov 4, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 6, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 4, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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