ZDF
Sign inSign up

Vulnerabilities

Security incident · GHSA-vqqp-x5qm-5qp4

FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

Published Oct 23, 2026 · catalog updated Oct 23, 2026 · FastMCP MCP client wrappers

Last checked Oct 24, 2026, 12 days ago.

Open the advisory

Exploitation likelihood is not on this record for GHSA-vqqp-x5qm-5qp4. A missing EPSS score is not a low risk. Compare FastMCP MCP client wrappers 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 MCP client wrappers).
  • 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
Medium5.9
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 credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assertion. Signing keys themselves remain local; assertion replayability depends on the assertion source and audience policy. Affected stable versions are 4.0.0–4.1.1 of and . The wrappers were introduced in 4.0.0a1; 3.x does not include them. The 4.1.2 fix adds to and and requires MCP SDK 2.2.0. It also discards refresh credentials from reloaded tokens: without persisted authorization-server metadata, inherited refresh could otherwise transmit credentials before issuer discovery, including when a cached token was valid at startup and expired later. Cached tokens are isolated by configured issuer, so configuring or changing the issuer acquires a new token. Valid cached access tokens for the same issuer and their expiry remain intact. **Configuration required for protection:** configure with the issuer identifier of the authorization server that issued the credentials. Upgrading alone is insufficient: omission remains supported for compatibility and emits . Rotate client secrets or other reusable credentials that may already have been sent to an untrusted endpoint. The fix is available in FastMCP 4.1.2. ## Original report ### Summary fastmcp's ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider do not expose or validate OAuth issuer binding when connecting to MCP servers. A client secret configured for a legitimate authorization server can be transmitted to an attacker-controlled authorization server if the MCP server URL is attacker-supplied or compromised. This is the same vulnerability class as GHSA-qx49-fqc8-xw99 (Python MCP SDK) and GHSA-6qxp-vccf-f47h (TypeScript M

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>= 4.0.0a1, < 4.1.2
  • pip fastmcp-slim>= 4.0.0a1, < 4.1.2

How to fix GHSA-vqqp-x5qm-5qp4

  • Upgrade

    GHSA-vqqp-x5qm-5qp4 lists a fix boundary at 4.1.2. Compare the installed build with that boundary before you change production.

  • Version check

    Affected < 4.1.2

  • 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-vqqp-x5qm-5qp4. A missing EPSS score is not a low risk. Compare FastMCP MCP client wrappers with the linked advisory before you change production.

Which versions are affected?

Compare what you have installed with >= 4.0.0a1, < 4.1.2. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-287: Improper Authentication; CWE-522: Insufficiently Protected Credentials. 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 credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assert

  2. Oct 23, 2026 · GitHub Advisory Database

    Updated

    ## Maintainer assessment and remediation This report is accepted as a FastMCP credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assert

Sources

  • GitHub Advisory DatabasePackage advisoryGHSA-vqqp-x5qm-5qp4

    FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

    github.com

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