Gogs vulnerable to Stored XSS via Mermaid diagrams

Summary

Stored XSS via mermaid diagrams due to usage of vulnerable renderer library

Details

Gogs introduced support for rendering mermaid diagrams in version 0.13.0.

Currently used version of the library mermaid 11.9.0 is vulnerable to at least two XSS scenarios with publicly available payloads

Resources: https://github.com/mermaid-js/mermaid/security/advisories/GHSA-7rqq-prvp-x9jh https://github.com/mermaid-js/mermaid/security/advisories/GHSA-8gwm-58g9-j8pw

PoC

1. Create a markdown file eg. README.md containing following malicious mermaid diagram (payload based on CVE-2025-54880)

architecture-beta
    group api(cloud)[API]
    service db "<img src=x onerror=\"alert(document.domain)\">" [Database] in api

2. The XSS should pop whenever either repository or file is viewed

Demo

https://github.com/user-attachments/assets/98320f62-6c1c-4254-aa61-95598c725235

Impact

The attacker can potentially achieve account takeover In a worst case scenario if the victim were an instance admin this could lead to a compromise of the entire deployment

Proposed remediation steps

1. Upgrade to a patched version of the third party library https://github.com/mermaid-js/mermaid/releases/tag/v10.9.5
2. Consider running mermaid using sandbox level which would mitigate impact of future potential cross-site scripting issues https://mermaid.js.org/config/usage.html#securitylevel