CVE-2026-27477Network attack vector

SSRF via unvalidated FASP provider base URL

Published Feb 24, 2026 · Updated Feb 24, 2026

SSRF in FASP requests in Mastodon 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6 allows remote attackers to reach internal systems. Fasp::Request sends URLs derived from an attacker-chosen provider base URL without the custom Request::Socket validation that rejects private resolved addresses. Only servers with EXPERIMENTAL_FEATURES containing fasp are exposed; attackers control only the URL prefix and cannot read responses.

CVSS severity5.9
Medium
EPSS probability0.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in FASP requests in Mastodon 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6 allows remote attackers to reach internal systems. Fasp::Request sends URLs derived from an attacker-chosen provider base URL without the custom Request::Socket validation that rejects private resolved addresses. Only servers with EXPERIMENTAL_FEATURES containing fasp are exposed; attackers control only the URL prefix and cannot read responses.

The record

CVE
CVE-2026-27477
Published
Feb 24, 2026
Updated
Feb 24, 2026
Vendor
Mastodon
Product
Mastodon
Classifications
CWE-918, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Feb 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 4.4.0, < 4.4.14
  2. Affected versionversion=>= 4.5.0, < 4.5.7

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mastodon · Mastodonversion=>= 4.4.0, < 4.4.14; version=>= 4.5.0, < 4.5.7

Published CVSS scores

4.6GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
5.9NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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