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CVE-2026-63127
Published:September 16, 2026
Updated:September 22, 2026
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.
Affected Packages
https://github.com/modelcontextprotocol/rust-sdk.git (GITHUB):
Affected version(s) >=rmcp-v0.2.0 <rmcp-v2.0.0
Fix Suggestion:
Update to version rmcp-v2.0.0
rmcp (RUST):
Affected version(s) >=0.1.0 <2.0.0
Fix Suggestion:
Update to version 2.0.0
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CVSS v4
Base Score:
8.4
Attack Vector
NETWORK
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
NONE
User Interaction
PASSIVE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
LOW
Vulnerable System Availability
NONE
Subsequent System Confidentiality
HIGH
Subsequent System Integrity
LOW
Subsequent System Availability
NONE
CVSS v3
Base Score:
8.2
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
REQUIRED
Scope
CHANGED
Confidentiality
HIGH
Integrity
LOW
Availability
NONE
Weakness Type (CWE)
Insufficient Verification of Data Authenticity
EPSS
Base Score:
0.19