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Vulnerability · Published Mar 27, 2026 · Assigned by GitHub_M

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

Medium 4.3

In plain English

A medium-severity open redirect in mastodon from mastodon. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Ready-made attack tooling exists: Nuclei.

Severity (CVSS 3.1)4.3Medium
Exploit odds (30 days)0.5%top 58.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack tooling1Nuclei

Official 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.

Affected products (as reported)

VendorProductVersions
mastodon mastodon >= 4.5.0, < 4.5.8>= 4.4.0, < 4.4.15< 4.3.21

Weakness: CWE-601 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

References