rmcp OAuth client fetches server-controlled resource_metadata URLs — GHSA-C9XM-49CP-XCR9

Summary

## Summary The `rmcp` OAuth client accepts a server-controlled `resource_metadata=` URL from the `WWW-Authenticate` header and fetches it without same-origin or private-network validation. An attacker-controlled MCP server can return a `401 WWW-Authenticate: Bearer resource_metadata="..."` header pointing at an internal URL, including localhost, RFC 1918 addresses, or cloud metadata endpoints. The client then…

Timeline

Published by source

2026-10-02

Risk context

• Reported severity or threat level: medium.
• Reported CVSS score: 6.3 out of 10.
• Review exposure for the affected product or family: rust: rmcp.

Product

rust: rmcp

What to do

General, cautious steps (verify details in the official source):

• Review exposure and plan remediation based on risk and environment.
• Identify affected product versions in your inventory and verify whether you are impacted.
• Apply vendor patches/updates or recommended mitigations as soon as available.
• Read the official advisory for exact affected versions and remediation steps.

Official advisory

• Read the official source

Evidence and provenance

Canonical advisory ID

GHSA-C9XM-49CP-XCR9

Source

GitHub

Vendor

GitHub

Published by source

2026-10-02

Last source activity

2026-10-02

Curated by

hlukh.ch

Related advisories and connections

These advisories are connected by shared CVE, product, vendor, source, or time signals.