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Fencer Vulns
high 8.2

CVE-2026-63127 json

RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

published 2026-09-16 53 d ago modified 2026-09-30 39 d ago also GHSA-33f5-2c5q-wgwj , EUVD-2026-80824

cvss
8.2
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 9%
public exploits
0
none known

Summary

RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's OAuth implementation in crates/rmcp/src/transport/auth.rs omits the RFC 9728 resource field from ResourceServerMetadata and allows discover_oauth_server_via_resource_metadata to use protected-resource metadata without confirming that the returned resource identifier exactly matches the configured MCP server. A malicious MCP server can publish metadata for a different legitimate MCP resource and its authorization server, causing a victim who connects and completes the authorization flow to obtain a legitimate access token that the client subsequently sends to the malicious server. The attacker can capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes. This issue is fixed in version 2.0.0.

Patch / fix

  • crates.io rmcp 2.0.0
  • goose RHEL 10.2 1.45.0-1.el10_2
  • goose RHEL 9 1.45.0-1.el9_8

red hat advisories RHSA-2026:80765RHSA-2026:80764

  • openai-codex openSUSE Tumbleweed 0.155.1-1.1

opensuse advisories openSUSE-SU-2026:11849-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-63127 updated cvss 8.2

rmcp: RMCP: Token impersonation via missing OAuth resource field validation

  • Red Hat Enterprise Linux 9 goose, goose 1.45.0-1.el10_2 Red Hat Enterprise Linux 10 goose 1.45.0-1.el10_2
    affected: workaround, no fix available

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
53 d ago

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 9% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
GitHub 3.1 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Affected packages

ecosystem package affected fixed in
crates.io rmcp
<2.0.0
2.0.0

Affected products

  • modelcontextprotocol rust-sdk

Classification

Weakness
  • CWE-345 Insufficient Verification of Data Authenticity

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.