# 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.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** GHSA-cv84-9p8j-fj68, PYSEC-2026-3851

## 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.

Weakness: CWE-407 (Inefficient Algorithmic Complexity), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-55099?

3 open source packages ship an affected version, including icalendar.


| Package | Ecosystem | 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.

## Is CVE-2026-55099 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55099 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55099 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55099 exists.

Affected packages and fixed versions:
- icalendar (PyPI): affected from 7.1.0; fixed in 7.1.3
- icalendar (PyPI): affected from 7.1.0; fixed in 7.1.3
- icalendar (PyPI): affected from 7.1.0; fixed in 7.1.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-55099/ (Markdown: https://insights.sbomify.com/CVE-2026-55099/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55099
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55099
- OSV: https://osv.dev/vulnerability/CVE-2026-55099

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55099/
Summaries are generated and may contain mistakes; the official description above is the record.