What’s Vulnerable › Vulnerabilities

CVE-2026-77399: icalendar: Denial of service via unbounded VALARM REPEAT expansion

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-22 · updated 2026-09-23
• Weakness: CWE-400

Description

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.

Affected products

• collective icalendar: from 6.1.0, < 7.2.2

References

• https://github.com/collective/icalendar/commit/157f85137c65565a308bbcf4fdcc7d8ebce01267
• https://github.com/collective/icalendar/commit/cddd37226adbc52cca52a399f61d38a378221219
• https://github.com/collective/icalendar/pull/1571
• https://github.com/collective/icalendar/releases/tag/v7.2.2
• https://github.com/collective/icalendar/security/advisories/GHSA-qjcq-q7h7-r74v

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