CVE-2026-55099

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.

• CWE-400
• CWE-407

• https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49
• https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79
• https://github.com/collective/icalendar/releases/tag/v7.1.3
• https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68
• https://www.openwall.com/lists/oss-security/2026/06/23/8
• https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68

Published

Aug 25, 2026

Last Modified

Sep 9, 2026

Status

Deferred

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