CVE-2026-55099

HighCVSS 7.5

Published: 25/08/2026(UTC)Updated: 09/09/2026(UTC)Translated: 26/08/2026(UTC)NVD NIST

Exploitation Probability (EPSS)

Low risk

0.38%

32th percentile - higher than 32% of all known CVEs

Summary

icalendar versions 7.1.0 through 7.1.3 contain a vulnerability in the Component equality method that performs O(2^n) work relative to nesting depth. An attacker can submit a small .ics file with deeply nested BEGIN:VEVENT blocks, potentially pinning a CPU core for a long time, causing denial of service.

Risk Assessment

An attacker could exploit this vulnerability to overload the server, potentially leading to unavailability of calendar sync, import, and invite processing services.

Recommendation

Update the icalendar library to version 7.1.3 or later, which contains a fix for this vulnerability.

Other vulnerabilities in icalendar

CVE-2026-77399Medium
icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 6.1.0 until 7.2.2, vInt.from_ical accepts an attacker-controlled VALARM REPEAT value and applications that request alarm times can eagerly expand it without an application-level limit. Alarms.times and Alarms.active reach the unbounded expansion in versions starting with 6.1.0, while Alarm.triggers adds a second affected path starting with 7.0.0. Parsing alone does not trigger the issue, but accessing these properties can consume excessive CPU time and heap memory and terminate or stall a service. This issue is fixed in version 7.2.2.

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS