CVE-2026-27477: medium-severity vulnerability in mastodon
Mastodon has SSRF via unvalidated FASP Provider base_url
Published · Updated
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13Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 4.6epss 0.3%
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.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U
Affected products
mastodon · mastodonRelated CVEs — mastodon
In the same product, most dangerous first.
CVE-2023-36460CRITICALMastodon vulnerable to arbitrary file creation through media attachmentsEPSS 40.1%CVE-2024-23832CRITICALMastodon Remote user impersonation and takeoverEPSS 2.5%CVE-2023-36461HIGHMastodon vulnerable to Denial of Service through slow HTTP responsesEPSS 1.3%CVE-2023-28853HIGHMastodon's blind LDAP injection in login allows the attacker to leak arbitrary attributes from LDAP databaseEPSS 1.3%CVE-2023-36459CRITICALMastodon vulnerable to Cross-site Scripting through oEmbed preview cardsEPSS 1.2%CVE-2026-72916MEDIUMMastodon: SSRF Protection Bypass via IPv4-compatible IPv6 AddressesEPSS 1.0%