CVE-2026-55099 icalendar: Algorithmic Complexity in Equality
Attackers can cause denial of service by submitting deeply nested iCalendar files that trigger exponential complexity during equality comparisons.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55099?
CVE-2026-55099 is a high-severity denial of service vulnerability in collective icalendar, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-55099?
3 open source packages ship an affected version, including icalendar.
| Package | Affected | Fixed in |
|---|---|---|
icalendar PyPI | From 7.1.0 | 7.1.3 |
icalendar PyPI | From 7.1.0 | 7.1.3 |
icalendar PyPI | From 7.1.0 | 7.1.3 |
How do I fix CVE-2026-55099?
Upgrade icalendar to 7.1.3 or later.
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-2026-55099 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55099 exists.
Sources
Also known as GHSA-cv84-9p8j-fj68, PYSEC-2026-3851.