VulnSocial

CVE-2026-63127

LOWCVSS 8.2 · HIGH

Signal is active with 1 mentions in latest observed window

Immediate actions

  • Track advisory updates for patch or workaround availability

Recommended action window: Monitor and triage in normal cycle

NVD description

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.

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0.0/ 10 priority

Sources & remediation

Other references
Weakness type (CWE)
CWE-345

Priority

LOW

Exploitation

NONE

PoC

NONE

Patch

NONE

Momentum

NONE

Threat summary

  • 1 mentions across 1 observed day

What's happening

  • 1 total mentions across 1 day

Deep dive

Activity timeline1 mentions / 1d
00111Mentions · 2026-10-04: 110-04
Full discourse1 post
  • Sagar Tanur 🇮🇳@Sagarvd01

    WorkOS pulled three MCP auth bugs into one root cause (Oct 2 writeup): trusting what the other side said. 1) MCP Python SDK GHSA-qx49: client followed attacker token endpoint after discovery fallback (fix 1.30.0/2.2.0 + issuer= for M2M) 2) rmcp CVE-2026-63127: client skipped resource match in protected-resource metadata (fix 2.0.0) 3) LiteLLM CVE-2026-59822: gateway accepted fabricated Bearer into empty auth object (fix 1.84.0, CISA KEV) two directions, same miss: verify on every path, treat missing fields as reject, never substitute a default identity. scanners that only key off CVEs will miss GHSA-qx49. inventory SDKs and gateways, not just CVEs.

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