EUVD-2026-16783Severity by source
AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Primary rating from GitHub Advisory · only source for this CVE.
CVSS VectorGitHub Advisory
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Lifecycle Timeline
4DescriptionGitHub Advisory
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.
AnalysisAI
Mastodon prior to versions 4.5.8, 4.4.15, and 4.3.21 contains an unauthenticated Open Redirect vulnerability in the /web/* route that allows remote attackers to redirect users to arbitrary external domains via specially URL-encoded path segments. An attacker can exploit this to conduct phishing attacks or steal OAuth credentials by crafting malicious links that bypass Rails path normalization through URL-encoded slashes (%2F). …
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Vulnerability AssessmentAI
| Risk Assessment | While the CVSS score of 4.3 is moderate, the attack vector (Network), low complexity, and lack of authentication requirements indicate broad attack surface. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker crafts a malicious URL containing URL-encoded slashes in the `/web/*` path that, when clicked by a Mastodon user, redirects them to a phishing site mimicking the real Mastodon login page. The victim, believing they are on the legitimate domain, enters their credentials or grants OAuth permissions, which are then captured by the attacker. … |
| Remediation | Upgrade Mastodon immediately to version 4.5.8, 4.4.15, or 4.3.21 depending on your current release line. … Detailed patch versions, workarounds, and compensating controls in full report. |
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Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. …
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