XSS in Nextcloud Text application

Vulnerability Description

Nextcloud Text is a collaborative document editing application that uses Markdown. A cross-site scripting vulnerability is present in versions prior to 19.0.13, 20.0.11, and 21.0.3. The Nextcloud Text application shipped with Nextcloud server used a `text/html` Content-Type when serving files to users. Due the strict Content-Security-Policy shipped with Nextcloud, this issue is not exploitable on modern browsers supporting Content-Security-Policy. The issue was fixed in versions 19.0.13, 20.0.11, and 21.0.3. As a workaround, use a browser that has support for Content-Security-Policy.

References

• https://github.com/nextcloud/security-advisories/security/advisories/GHSA-x4w3-jhcr-57pq
• https://github.com/nextcloud/text/pull/1689
• https://hackerone.com/reports/1241460