# CVE-2025-66511: Nextcloud Calendar app used predictable proposal participant tokens

> Attackers can predict meeting proposal tokens to view details and submit dates in Nextcloud Calendar.

- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Dec 2025
- **Updated:** 5 Dec 2025
- **Also known as:** GHSA-whm3-vv55-gf27

## What is CVE-2025-66511?

CVE-2025-66511 is a medium-severity vulnerability in nextcloud security-advisories, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The tokens are not purely random generated. This vulnerability is fixed in 6.0.3.

Weakness: CWE-330 (Use of Insufficiently Random Values)

## Which versions are affected by CVE-2025-66511?

It affects 1 product.


Products: Nextcloud Calendar.

## How do I fix CVE-2025-66511?

No fix has been published for CVE-2025-66511 yet. Limit exposure of the affected software until one is available.

## Is CVE-2025-66511 being exploited?

There is no known exploitation of CVE-2025-66511 in the wild and no public exploit code.

## Other vulnerabilities in nextcloud security-advisories

- [CVE-2025-66550](https://insights.sbomify.com/CVE-2025-66550/): Nextcloud Calendar attachments of local files are offered to downloaded (medium 5.7)
- [CVE-2022-24838](https://insights.sbomify.com/CVE-2022-24838/): Command Injection in Appointment Emails for Nextcloud Calendar (medium 5.3)
- [CVE-2018-3763](https://insights.sbomify.com/CVE-2018-3763/): Cross-site scripting in Nextcloud Calendar application (medium 4.8)
- [CVE-2024-37316](https://insights.sbomify.com/CVE-2024-37316/): Nextcloud Calendar's event create can create attachments that link to other websites (medium 4.6)
- [CVE-2026-45286](https://insights.sbomify.com/CVE-2026-45286/): Nextcloud: Calendar app leaked user identifiers via attendee suggestion endpoint (medium 4.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-66511 and fix it if it is affected.

What it is: CVE-2025-66511 is a medium-severity vulnerability in nextcloud security-advisories, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-66511 in the wild and no public exploit code.

Affected product: nextcloud security-advisories. 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-2025-66511/ (Markdown: https://insights.sbomify.com/CVE-2025-66511/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66511
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66511
- OSV: https://osv.dev/vulnerability/CVE-2025-66511

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-66511/
Summaries are generated and may contain mistakes; the official description above is the record.