Home/CVE/Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval b
CVE

CVE-2026-27477

Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval b

Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval by an administrator. In versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, an unauthenticated attacker can register a FASP with an attacker-chosen base_url that includes or resolves to a local / internal address, leading to the Mastodon server making requests to that address.

This only affects Mastodon servers that have opted in to testing the experimental FASP feature by setting the environment variable EXPERIMENTAL_FEATURES to a value including fasp. An attacker can force the Mastodon server to make http(s) requests to internal systems. While they cannot control the full URL that is being requested (only the prefix) and cannot see the result of those requests, vulnerabilities or other undesired behavior could be triggered in those systems.

The fix is included in the 4.4.14 and 4.5.7 releases. Admins that are actively testing the experimental "fasp" feature should update their systems. Servers not using the experimental feature flag fasp are not affected.

MEDIUM · CVSS 5.9 EPSS 0.00072
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
Sigma rules0 YARA rules0
⬡

Weakness Classification

CWE-918Server-Side Request Forgery (SSRF)
▤

Affected Products & Versions

2
joinmastodon mastodon>= 4.4.0 and < 4.4.14
joinmastodon mastodon>= 4.5.0 and < 4.5.7
▣

Scoring & Timeline

5.9
MEDIUM · CVSS v3.1 · security-advisories@github.com
View on NVD
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
Published to NVD24 Feb 2026 · 08:27 PM
CVSS VectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
🔗

References & Sources

2
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://github.com/mastodon/mastodon/commit/7b85d2182361e68d51d9a02f94fb1070b5f503b1Patch
https://github.com/mastodon/mastodon/security/advisories/GHSA-46w6-g98f-wxqmPatchVendor Advisory
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