Model Context Protocol RMCP SDK up to 1.x OAuth Implementation auth.rs discover_oauth_server_via_resource_metadata improper authorization

Summaryinfo

A vulnerability, which was classified as critical, has been found in Model Context Protocol RMCP SDK up to 1.x. Affected is the function discover_oauth_server_via_resource_metadata of the file crates/rmcp/src/transport/auth.rs of the component OAuth Implementation. Performing a manipulation results in improper authorization. This vulnerability is reported as CVE-2026-63127. The attack is possible to be carried out remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in Model Context Protocol RMCP SDK up to 1.x. It has been classified as critical. This affects the function discover_oauth_server_via_resource_metadata of the file crates/rmcp/src/transport/auth.rs of the component OAuth Implementation. The manipulation with an unknown input leads to a improper authorization vulnerability. CWE is classifying the issue as CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-63127 since 07/15/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1548.002 according to MITRE ATT&CK.

Upgrading to version 2.0.0 eliminates this vulnerability.

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Productinfo

Vendor

• Model Context Protocol

Name

• RMCP SDK

Version

• 1.x

CPE 2.3info

• 🔒

CPE 2.2info

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