CVE-2026-33868 — Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'

Description (CNA)

Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the `/web/*` route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft. The issue occurs because URL-encoded slashes (`%2F`) bypass Rails path normalization and are interpreted as host-relative redirects. Versions 4.5.8, 4.4.15, and 4.3.21 patch the issue.

Affected (CNA)

Vendor / product	Versions
mastodon / mastodon	affected >= 4.5.0, < 4.5.8; affected >= 4.4.0, < 4.4.15; affected < 4.3.21

CWE

CWE-601

References

https://github.com/mastodon/mastodon/security/advisories/GHSA-xqw8-4j56-5hj6

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.