Mastodon vulnerable to Stored XSS through the translation feature

Vulnerability Description

Mastodon is a free, open-source social network server based on ActivityPub. In versions on the 4.x branch prior to versions 4.0.10, 4.2.8, and 4.2.0-rc2, under certain conditions, attackers can abuse the translation feature to bypass the server-side HTML sanitization, allowing unescaped HTML to execute in the browser. The impact is limited thanks to Mastodon's strict Content Security Policy, blocking inline scripts, etc. However a CSP bypass or loophole could be exploited to execute malicious XSS. Furthermore, it requires user interaction, as this can only occur upon clicking the “Translate” button on a malicious post. Versions 4.0.10, 4.2.8, and 4.2.0-rc2 contain a patch for this issue.

References

• https://github.com/mastodon/mastodon/security/advisories/GHSA-2693-xr3m-jhqr
• https://github.com/mastodon/mastodon/commit/ff32475f5f4a84ebf9619e7eef5bf8b4c075d0e2