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Vulnerabilities

Vulnerability · GHSA-7xgj-8392-5q4r

FastMCP: Post-transform component authorization is skipped on direct calls

Published Oct 23, 2026 · catalog updated Oct 23, 2026 · FastMCP: Post-transform component authorization

Last checked Oct 24, 2026, 12 days ago.

Open the advisory

Exploitation likelihood is not on this record for GHSA-7xgj-8392-5q4r. A missing EPSS score is not a low risk. Compare FastMCP: Post-transform component authorization with the linked advisory before you change production.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (FastMCP: Post-transform component authorization).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
  • EPSS is missing. A missing score is not a low risk.
Severity
Medium6.8
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
Awaiting synchronization with FIRST
Remediation
Version ranges on file
Affected products
pip fastmcp · pip fastmcp-slim

## Maintainer assessment and remediation This report is accepted as a FastMCP authorization vulnerability. In supported configurations, a server transform can change the metadata used by a component's authorization policy. Direct lookups previously checked the original component but returned the transformed component without rechecking its policy. A client that knows the component identifier can invoke a tool or retrieve a resource or prompt that the configured policy denies. The fixes authorize the final transformed tools, resources, templates, and prompts. A denied concrete resource also shadows a matching resource template at its URI, preventing proxy templates from retrieving the same protected resource upstream. Authorized callers, authorized component versions, and templates for genuinely absent concrete resources remain supported. Affected stable release lines: FastMCP 3.0.0–3.4.9 and 4.0.0–4.1.1. The missing final authorization check is present in v3.0.0 history and was reproduced on v3.4.9 and v4.1.0. Both and installations are affected when using component authorization with transforms or protected concrete resources overlapping proxy templates. Fixes are available in FastMCP 3.4.10 and 4.1.2. Until upgrading, avoid overlapping proxy templates for protected resources and enforce the required authorization at the handler or server boundary rather than relying on a policy whose inputs are changed by a transform. ## Original report # Post-transform component authorization is skipped on direct calls ## Summary FastMCP's direct component paths can return a transformed component before its applicable , allowing low-privilege callers to execute hidden protected behavior. I reproduced this against FastMCP 4.1.0 and commit using only an in-process FastMCP server. The attacker does not have server-side access and does not control FastMCP configuration. ## Details ### Attacker prerequisites A server uses a supported provider, server, or session transform and a metada

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.

  • pip fastmcp>= 3.0.0, < 3.4.10
  • pip fastmcp>= 4.0.0, < 4.1.2
  • pip fastmcp-slim>= 3.0.0, < 3.4.10
  • pip fastmcp-slim>= 4.0.0, < 4.1.2

How to fix GHSA-7xgj-8392-5q4r

  • Upgrade

    GHSA-7xgj-8392-5q4r lists a fix boundary at 3.4.10. Compare the installed build with that boundary before you change production.

  • Version check

    Affected < 3.4.10

  • Official advisories

What to do

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

Should you patch?

Exploitation likelihood is not on this record for GHSA-7xgj-8392-5q4r. A missing EPSS score is not a low risk. Compare FastMCP: Post-transform component authorization with the linked advisory before you change production.

Which versions are affected?

Compare what you have installed with >= 3.0.0, < 3.4.10, >= 4.0.0, < 4.1.2. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-862: Missing Authorization. EPSS is not on this record yet. 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. Oct 23, 2026 · GitHub Advisory Database

    Detected

    ## Maintainer assessment and remediation This report is accepted as a FastMCP authorization vulnerability. In supported configurations, a server transform can change the metadata used by a component's authorization policy. Direct lookups previously checked the original component but returned the transformed component without rechecking its policy. A client that knows the component identifier can i

  2. Oct 23, 2026 · GitHub Advisory Database

    Updated

    ## Maintainer assessment and remediation This report is accepted as a FastMCP authorization vulnerability. In supported configurations, a server transform can change the metadata used by a component's authorization policy. Direct lookups previously checked the original component but returned the transformed component without rechecking its policy. A client that knows the component identifier can i

Sources

References

Freshness

Last verified in this catalog Oct 23, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • GitHub Advisory DatabaseOct 24, 2026