CVE-2026-77399 icalendar: Denial of service via unbounded VALARM REPEAT expansion
Attackers can cause denial of service in applications using the icalendar library by triggering unbounded expansion of VALARM REPEAT values.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-77399?
CVE-2026-77399 is a medium-severity denial of service vulnerability in collective icalendar, published on 22 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-77399?
How do I fix CVE-2026-77399?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-77399 being exploited?
There is no known exploitation of CVE-2026-77399 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-77399, GHSA-qjcq-q7h7-r74v, UBUNTU-CVE-2026-77399.