CVE-2026-63127 (CVSS 8.2)

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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CVSS v3.1 Score

8.2

HIGH

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

Weakness Type (CWE)

CWE-345 CWE-345

References

Other References

https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec https://github.com/modelcontextprotocol/rust-sdk/pull/937 https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0 https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj

Frequently Asked Questions

What is CVE-2026-63127? +

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 has a CVSS v3.1 base score of 8.2 (HIGH).

How severe is CVE-2026-63127? +

CVE-2026-63127 has a CVSS v3.1 score of 8.2 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

How do I check if I'm vulnerable to CVE-2026-63127? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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