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CVE Record

CVE-2021-44686: calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular Expression Denial...

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-44686 is a denial-of-service issue in calibre before 5.32.0. A crafted ebook or HTML conversion input may cause excessive processing through a vulnerable regular expression, potentially slowing or hanging conversion workflows. The provided sources do not indicate data theft, code execution, or active exploitation.

Executive priority

Prioritize remediation where calibre is used in shared or automated conversion workflows. For isolated desktop installations, handle through normal patching. No source in the bundle supports emergency response for active exploitation.

Technical view

The issue is a Regular Expression Denial of Service in calibre's html_preprocess_rules inside ebooks/conversion/preprocess.py. It affects calibre versions before 5.32.0. The impact is availability during preprocessing/conversion, especially where untrusted content is converted automatically or at scale.

Likely exposure

Exposure is most likely in systems running calibre before 5.32.0 that process untrusted ebooks or HTML-derived content. Desktop-only use is lower urgency; automated conversion services, shared workers, or batch pipelines have higher operational risk.

Exploitation context

The source bundle does not show KEV listing or confirmed active exploitation. Public references identify the vulnerable component and the 5.32.0 comparison, but do not provide evidence of widespread exploitation. Treat this as a resource-exhaustion risk rather than compromise evidence.

Researcher notes

Key evidence is limited to the CVE description, Launchpad issue, third-party advisory issue, calibre 5.31.1-to-5.32.0 comparison, and distro advisories. No CVSS, CWE, exploit status, or detailed downstream impact is supplied in the bundle.

Mitigation direction

  • Upgrade calibre to 5.32.0 or later.
  • Use fixed distro packages where Fedora or Debian guidance applies.
  • Avoid processing untrusted ebook or HTML content on outdated calibre versions.
  • Apply resource limits to automated conversion workers where feasible.
  • Check vendor and distribution advisories for environment-specific fixes.

Validation and detection

  • Inventory hosts and containers running calibre.
  • Confirm installed calibre versions are 5.32.0 or later.
  • Review automated workflows that convert untrusted ebooks or HTML.
  • Check package manager advisories for Fedora or Debian fixed builds.
  • Monitor conversion jobs for hangs or abnormal CPU duration.
Prepared
Confidence
medium
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
6Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.