# 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.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Sep 2026
- **Updated:** 23 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-77399, GHSA-qjcq-q7h7-r74v, UBUNTU-CVE-2026-77399

## 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.

Weakness: CWE-834 (Excessive Iteration), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-77399?


## How do I fix CVE-2026-77399?

Upgrade to a fixed version.

## Is CVE-2026-77399 being exploited?

There is no known exploitation of CVE-2026-77399 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77399 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-77399 in the wild and no public exploit code.

Affected product: collective icalendar. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-77399/ (Markdown: https://insights.sbomify.com/CVE-2026-77399/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77399
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77399
- OSV: https://osv.dev/vulnerability/CVE-2026-77399

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77399/
Summaries are generated and may contain mistakes; the official description above is the record.