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CVE Record

CVE-2026-27477: Mastodon has SSRF via unvalidated FASP Provider base_url

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.

MediumCVSS 4.6Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2026-27477 affects Mastodon servers that enabled the experimental FASP feature. An unauthenticated attacker can provide a FASP base URL that points to internal or local addresses, causing the Mastodon server to make requests there. This is not relevant to typical servers unless the experimental fasp feature flag is enabled.

Executive priority

Treat as a targeted configuration-dependent update. It is not broadly urgent for Mastodon servers using default features, but organizations testing FASP should patch promptly because the issue can reach internal services from the server.

Technical view

This is a server-side request forgery issue in Mastodon FASP registration base_url validation. Affected versions are 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, only when EXPERIMENTAL_FEATURES includes fasp. The attacker controls only the URL prefix and cannot see responses, but requests may trigger behavior in internal systems.

Likely exposure

Exposure appears limited to Mastodon instances actively testing experimental FASP. Servers without the fasp experimental feature flag are stated as not affected. Internet-facing Mastodon servers with the flag enabled and affected versions should be prioritized.

Exploitation context

The CVE is not listed in KEV, and the provided sources do not report active exploitation. The source describes unauthenticated network attack potential, but with constraints: partial URL control, no response visibility, and impact dependent on reachable internal services.

Researcher notes

Key constraints matter: affected only with experimental fasp enabled, attacker controls only base_url prefix, and response data is not exposed. Validation should focus on version range, feature flag state, and vendor patch adoption. Do not assume default Mastodon deployments are vulnerable.

Mitigation direction

  • Upgrade Mastodon 4.4.x to 4.4.14 or later.
  • Upgrade Mastodon 4.5.x to 4.5.7 or later.
  • If not actively testing FASP, ensure EXPERIMENTAL_FEATURES does not include fasp.
  • Review the GitHub advisory for vendor guidance before making changes.

Validation and detection

  • Inventory Mastodon instances and versions.
  • Check whether EXPERIMENTAL_FEATURES includes fasp on each instance.
  • Confirm affected instances are upgraded to 4.4.14 or 4.5.7 or later.
  • Review logs/configuration for unexpected FASP registrations, if FASP was enabled.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · medium confidence lookup

CWE-918: Information exposure and cloud metadata lookup

Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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description · low confidence lookup

Cloud metadata behavior lookup

The CVE wording references SSRF or metadata access, so cloud discovery and credential material review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2026-27477 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
4.6 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
4.6CVSS 4.0MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U——Primary CVE score

Vulnerability scoring details

Base CVSS 4.0 score

4.6Medium
CVSS 4.0 vector shape for CVE-2026-27477Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
mastodonmastodon>= 4.4.0, < 4.4.14, >= 4.5.0, < 4.5.7Listed
Weakness

CWE details

CWE-918

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-918 · source CWE mapping

Server-Side Request Forgery (SSRF)

Server-Side Request Forgery (SSRF) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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