CVE-2026-27477
5.9 · MEDIUMOverview
CVE-2026-27477 is a medium-severity vulnerability affecting joinmastodon mastodon. It was published on February 24, 2026 and has a CVSS 3.1 base score of 5.9 (MEDIUM).
This vulnerability has a CVSS 3.1 base score of 5.9, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
Technical 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
Remediation
Check the references section for vendor advisories and patches from joinmastodon. Update mastodon to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
Affected Products
| Vendor | Product | Versions | Status |
|---|---|---|---|
| joinmastodon | mastodon | >= 4.4.0, < 4.4.14 | Affected |
Frequently Asked Questions
What is CVE-2026-27477?
CVE-2026-27477 is a medium-severity vulnerability affecting joinmastodon mastodon. It was published on February 24, 2026 and has a CVSS 3.1 base score of 5.9 (MEDIUM).
How severe is CVE-2026-27477?
This vulnerability has a CVSS 3.1 base score of 5.9, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
How do I fix or remediate CVE-2026-27477?
Check the references section for vendor advisories and patches from joinmastodon. Update mastodon to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
How can CyberStrike help with CVE-2026-27477?
CyberStrike's AI-powered security agents can automatically detect CVE-2026-27477 across your infrastructure using autonomous pentesting, DAST scanning, and HackBrowser. The platform continuously monitors for known vulnerabilities and provides actionable remediation guidance prioritized by real-world exploitability.
How CyberStrike Helps
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