CVE-2026-33868CWE-601

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

Medium · published March 27, 2026

CVSS v3.1
4.3
EPSS
1%
Percentile
45.3
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v3.1
4.3 · MEDIUM
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
CVSS v4.0
Not supplied
EPSS
0.00552 · 45.3th percentile
Weakness
CWE-601 · URL Redirection to Untrusted Site ('Open Redirect')
Published
2026-03-27T19:50Z
EPSS history
Timeline
  • 27 MAR 19:50Z
    Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'
    cvelistv5