---
id: CVE-2026-77399
title: >-
  icalendar is an RFC 5545 compatible parser and generator of iCalendar files
  for Python
summary: >-
  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 expan…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-400
  - CWE-834
vendor: collective
product: icalendar
affected:
  - 'icalendar >= 6.1.0, < 7.2.2'
published: '2026-09-22'
updated: '2026-09-23'
sourceUpdated: '2026-09-23T18:12:04.247'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77399'
references:
  - url: >-
      https://github.com/collective/icalendar/commit/157f85137c65565a308bbcf4fdcc7d8ebce01267
    label: security-advisories@github.com
  - url: >-
      https://github.com/collective/icalendar/commit/cddd37226adbc52cca52a399f61d38a378221219
    label: security-advisories@github.com
  - url: 'https://github.com/collective/icalendar/pull/1571'
    label: security-advisories@github.com
  - url: 'https://github.com/collective/icalendar/releases/tag/v7.2.2'
    label: security-advisories@github.com
  - url: >-
      https://github.com/collective/icalendar/security/advisories/GHSA-qjcq-q7h7-r74v
    label: security-advisories@github.com
tags:
  - nvd
  - cve.org
epss: 0.00312
epssPercentile: 0.21456
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-09-23T14:10:14.416349Z'
ingestedAt: '2026-09-22T20:10:15.089Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.