Bug 2535503 (CVE-2026-63127)

Summary:	CVE-2026-63127 rmcp: RMCP: Token impersonation via missing OAuth resource field validation
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security <prodsec-ir-bot>
Status:	NEW ---	QA Contact:
Severity:	high	Docs Contact:
Priority:	high
Version:	unspecified	CC:	rhel-process-autobot, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
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Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in rmcp, a Rust SDK for the Model Context Protocol. The OAuth implementation in rmcp omits the RFC 9728 resource field from ResourceServerMetadata. This allows a malicious Model Context Protocol (MCP) server to publish metadata for a different legitimate MCP resource. Consequently, a victim connecting to the malicious server may obtain a legitimate access token, which the client then sends to the malicious server. This enables the attacker to capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes, leading to unauthorized access and information disclosure.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2535525, 2535526
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Description OSIDB Bzimport 2026-09-16 15:05:10 UTC

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.