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CVE-2026-55099 is a denial-of-service vulnerability in the Python icalendar library affecting versions 7.1.0, 7.1.1, and 7.1.2. The issue is caused by the Component.eq implementation, which compares subcomponents with O(2^n) time complexity relative to nesting depth. Because the parser accepts arbitrarily nested calendar components, an attacker can supply a very small malicious .ics file containing deeply nested components that triggers pathological comparison behavior. A single equality-related operation on parsed attacker-controlled calendar data can then take minutes to complete or hang indefinitely. The vulnerable behavior can be reached through equality and membership operations including ==, !=, in, set or dict membership, deduplication logic, and test assertions that compare parsed components.
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