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CVE-2026-27477

Mastodon has SSRF via unvalidated FASP Provider base_url

MEDIUM
5.9
CVSS Score
Published: February 24, 2026 at 07:00 PM UTC
Updated: June 17, 2026 at 10:27 AM UTC
2 products affected across 2 vendorsMastodonMastodon

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Severity
MEDIUM5.9
Exploit prediction
EPSS 0.3%
SSVC
No known exploit

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.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mastodon· Joinmastodon
95%
4.4.0 ≤ v < 4.4.144.5.0 ≤ v < 4.5.7
Mastodon· Mastodon
60%
v < 4.4.14v < 4.5.7
Affected range stops below 4.4.14, so it looks safe — our inference, not a vendor statement
Affected range stops below 4.5.7, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (2)

x_refsource_CONFIRMPatchVendor Advisory
Source: cverecord
https://github.com/mastodon/mastodon/security/advisories/GHSA-46w6-g98f-wxqm
x_refsource_MISCPatch
Source: cverecord
https://github.com/mastodon/mastodon/commit/7b85d2182361e68d51d9a02f94fb1070b5f503b1

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

5.9MEDIUM[email protected]
Attack Vector
NETWORK
Complexity
HIGH
Privileges
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
HIGH
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-918

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