CVE-2026-27477 Mastodon has SSRF via unvalidated FASP Provider base_url
An attacker can force the Mastodon server to make requests to internal systems via unvalidated FASP base_url, requiring the experimental feature to be enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-27477?
CVE-2026-27477 is a medium-severity server-side request forgery vulnerability in mastodon, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-918
Which versions are affected by CVE-2026-27477?
1 open source package ship an affected version, including mastodon. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mastodon Bitnami | From 4.4.0 | 4.4.14, 4.5.7 |
Products include Joinmastodon Mastodon.
How do I fix CVE-2026-27477?
Upgrade mastodon to a fixed version for your release line: 4.4.14 or 4.5.7.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-27477 being exploited?
There is no known exploitation of CVE-2026-27477 in the wild and no public exploit code.
Other vulnerabilities in mastodon
- CVE-2023-36460 Mastodon vulnerable to arbitrary file creation through media attachments
- CVE-2022-2166 Improper Restriction of Excessive Authentication Attempts in mastodon/mastodon
- CVE-2022-24307 Missing authorization in n/a
- CVE-2018-21018 Auth bypass in n/a
- CVE-2024-23832 Mastodon Remote user impersonation and takeover
Sources
Also known as BIT-mastodon-2026-27477, GHSA-46w6-g98f-wxqm.