CVE-2026-27477
MEDIUMCVSS Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
Lifecycle Timeline
3Description
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.
Analysis
Mastodon servers with the experimental FASP feature enabled are vulnerable to Server-Side Request Forgery (SSRF) attacks, allowing unauthenticated attackers to register accounts with arbitrary base URLs that force the server to make requests to internal or local addresses. While attackers cannot control the full request path or view responses, this exposure of internal systems to external manipulation could facilitate reconnaissance or attacks on backend infrastructure. …
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Remediation
Within 30 days: Identify affected systems running versions 4.4.0 and apply vendor patches as part of regular patch cycle. Vendor patch is available.
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