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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

EPSS

• CVE-2026-63127: 0.19% estimated exploitation activity in the next 30 days; percentile 9.31 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The rmcp library does not validate the resource parameter in OAuth Protected Resource metadata (RFC 9728), allowing a malicious MCP server to redirect OAuth flows to a legitimate authorization server and steal the resulting access tokens. Details RFC 9728 specifies two MUST requirements for resource parameter validation: - Section 7.3: the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the resource value in the returned metadata document. - Section 3.3: if the resource value returned is not identical to the URL the client used, the data MUST NOT be used. In the current implementation (crates/rmcp/src/transport/auth.rs), the ResourceServerMetadata struct (lines 390–394) does not include a resource field: And discoveroauthserverviaresourcemetadata() (lines 1446–1465) proceeds without any resource URL validation. Recommended fix 1. Add the resource field to the struct: 2. Add validation logic after fetching…

Published by the source

16 Sep 2026, 22:13 UTC

Last modified by the source

16 Sep 2026, 22:30 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-33f5-2c5q-wgwj

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-33f5-2c5q-wgwj

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# Patch Next advisory brief: CVE-2026-63127

Advisory: RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery
IDs: CVE-2026-63127, GHSA-33f5-2c5q-wgwj
Record: https://patchnext.com/advisories/cve-2026-63127 (revision 2, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rmcp 2.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected < 2.0.0; fixed in 2.0.0

## Suggested inspection (read-only)
- crates.io rmcp: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rmcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-33f5-2c5q-wgwj (OSV record GHSA-33f5-2c5q-wgwj lists affected ranges: crates.io rmcp >=0 <2.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-33f5-2c5q-wgwj (GitHub-reviewed advisory GHSA-33f5-2c5q-wgwj lists affected ranges: crates.io rmcp >=0 <2.0.0)

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