{"id":"CVE-2026-63127","published":"2026-09-16T15:17:39.817","lastModified":"2026-09-16T16:17:14.323","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.","cvssScore":8.2,"cvssSeverity":"HIGH","cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N","cwes":["CWE-345"],"vendors":[],"products":[],"references":[{"url":"https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/pull/937","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj","tags":[]}],"exploitRefs":[{"url":"https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/pull/937","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj","tags":[]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj","tags":[]}],"hasPoc":true,"ai":{"summary":"The flaw in the rmcp crate's OAuth implementation allows a malicious MCP server to trick a victim into obtaining a legitimate access token, which can then be captured and used by the attacker to impersonate the victim.","exploitability":"Exploitation requires a victim to connect to the malicious server and complete the authorization flow. The attacker must publish metadata for a different legitimate MCP resource.","blast_radius":"If exploited, the attacker can impersonate the victim within the token's granted scopes, potentially leading to data breaches or unauthorized access to resources.","remediation":"Upgrade to version 2.0.0 or later.","detection":"No reliable host or network indicator is derivable from the published description.","tags":["auth-bypass","oauth","rust","rmcp"],"model":"qwen2.5:7b-instruct","analyzedAt":"2026-10-03T08:58:31.270Z"}}