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Vulnerabilities

Vulnerability · CVE-2026-27477

Mastodon is a free, open-source social network server based on ActivityPub.

Published Feb 24, 2026 · catalog updated Jun 17, 2026 · Mastodon is a free,

Check if you run it

No confirmed exploitation is on record for CVE-2026-27477. EPSS is 0.37%. Act only if a listed version of Mastodon is a free, is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Mastodon is a free,).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium4.6
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.37%
Remediation
Advisory link
Affected products
cpe joinmastodon/mastodon

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 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 to a value including . 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 are not affected.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe joinmastodon/mastodonversion unspecified

How to fix CVE-2026-27477

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-27477. EPSS is 0.37%. Act only if a listed version of Mastodon is a free, is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-918: Server-Side Request Forgery (SSRF). EPSS 0.37%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Feb 24, 2026 · NIST NVD

    Detected

    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 that includes or resolves to a local / internal address, leading to the Mastodon server making requests to that address. This only

  2. Jun 17, 2026 · NIST NVD

    Updated

    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 that includes or resolves to a local / internal address, leading to the Mastodon server making requests to that address. This only

Sources

  • CVE.orgVulnerability recordCVE-2026-27477

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-27477

    30-day exploitation probability 0.4%

    first.org
  • NIST NVDVulnerability recordCVE-2026-27477

    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

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.