icalendar has Algorithmic Complexity in Equality

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-55099: 0.63% estimated exploitation activity in the next 30 days; percentile 50.24 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Component.eq compares subcomponents in O(2^n) time relative to nesting depth. Because the parser accepts arbitrarily nested components, a sub-kilobyte .ics file is enough to make a single equality check run for minutes or hang indefinitely. Any application that compares parsed components (==, !=, in, set/dict membership, deduplication, test assertions) against attacker-supplied calendar data is exposed to denial of service. Details Component subclasses dict and stores children in a separate subcomponents list. eq (src/icalendar/cal/component.py:642-665) checks set-equivalence of children with two membership loops: Each ... not in ... test invokes eq on the children. For a nested chain, both loops descend the full subtree, so each level spawns two recursive comparisons: T(n) = 2·T(n-1) → O(2^n). Parsing does not gate this. Component.fromical builds the structure iteratively and imposes no depth limit, so BEGIN:VEVENT blocks can be nested to any depth (parsing the payload below…

Published by the source

25 Aug 2026, 19:27 UTC

Last modified by the source

25 Aug 2026, 19:27 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-cv84-9p8j-fj68

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# Patch Next advisory brief: CVE-2026-55099

Advisory: icalendar has Algorithmic Complexity in Equality
IDs: CVE-2026-55099, GHSA-cv84-9p8j-fj68
Record: https://patchnext.com/advisories/cve-2026-55099 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: icalendar 7.1.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI icalendar: affected >= 7.1.0, < 7.1.3; fixed in 7.1.3

## Suggested inspection (read-only)
- `python -m pip show icalendar`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on icalendar.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-cv84-9p8j-fj68 (GitHub-reviewed advisory GHSA-cv84-9p8j-fj68 lists affected ranges: PyPI icalendar >=7.1.0 <7.1.3)

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