This vulnerability allows an unauthenticated attacker to register a FASP provider with a base_url that points to an internal or local address. The Mastodon server then makes HTTP(S) requests to that address without validation. The attacker cannot control the full URL or access the response, but the forced requests can trigger vulnerabilities or other unintended behavior on internal systems. The weakness is a server‑side request forgery (CWE‑918).

The issue affects Mastodon servers running version 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6 that have enabled the experimental FASP feature by setting the EXPERIMENTAL_FEATURES environment variable to include fasp. Servers using the official releases after 4.4.14 or 4.5.7, or those not exercising the fasp feature, are not impacted. The problem exists only for administrators who have approved the FASP registration process.

With a CVSS score of 4.6 the severity is moderate, and the EPSS score is below 1%, indicating a low likelihood of exploitation in the real world. The vulnerability is not listed in CISA’s KEV catalog. An attacker can exploit it without authentication, but must have the fasp feature enabled. The lack of response visibility limits the immediate affect, yet internal systems could be coerced into vulnerable actions. The risk is therefore modest but non‑zero for enabled systems.