CVE-2026-33868

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

State: Published
Severity: 4.3 Medium
Published: Mar 27, 2026
Updated: Mar 31, 2026
Reserved: Mar 24, 2026

Description

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.

Scores

CVSS	Score	Vector	Source
3.1	4.3 Medium	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N	GitHub_M

Weaknesses

CWE-601 URL Redirection to Untrusted Site ('Open Redirect')

Affected Products

Product	Versions
mastodonmastodon	>= 4.5.0, < 4.5.8Affected >= 4.4.0, < 4.4.15Affected < 4.3.21Affected

References

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

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