Gogs vulnerable to Stored XSS via Mermaid diagrams

Summary

### 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.](https://github.com/gogs/gogs/releases/tag/v0.13.0)

Currently used version of the library [mermaid 11.9.0](https://github.com/gogs/gogs/tree/main/public/plugins/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](https://github.com/mermaid-js/mermaid/security/advisories/GHSA-8gwm-58g9-j8pw)) ``` 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