CVE-2026-123272
Description
A flaw was found in mcp-go. A malicious Model Context Protocol (MCP) server can declare arbitrary protected resources within its metadata due to improper validation in the OAuth metadata handler. This flaw allows an attacker to cause the authorization server to issue tokens scoped for unintended services, enabling unauthorized access when the attacker replays those tokens against the target resources.
Statement
This vulnerability is rated as Important because an adversarial Model Context Protocol (MCP) server can manipulate client authentication workflows to acquire authorization tokens for unrelated protected resources across trust boundaries. Successful exploitation requires an application or client session to establish a connection with an untrusted or rogue endpoint. In typical Red Hat deployments, clients connect exclusively to vetted, trusted infrastructure, significantly mitigating exposure to unsolicited server responses.
Mitigation
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
--- CME Mitigation Guidance (2026-11-06) ---
Disclaimer: The following mitigation solutions are provided as a courtesy, pending the application of an official patch. This content was generated utilizing AI tools in conjunction with data from https://cmetaxonomy.org. Customers are advised to independently evaluate and test these temporary mitigations within their own infrastructure.
The highest-value action is to enforce RFC 9728 resource binding in the mcp-go OAuth handler so client metadata flows reject arbitrary RFC 8707 target resource declarations from well-known endpoints: upgrade to mcp-go 1.2.1 (or the corresponding fixed Red Hat builds), which validate that the declared resource URL binds to the addressed MCP server's base URL and reject metadata on mismatch, closing all 2 injection vectors (crafted well-known protected resource metadata, RFC 8707 resource parameter injection). Until patched, this is a concern on client sessions across all platforms connecting to Model Context Protocol (MCP) servers, so Restrict client sessions to strictly vetted and trusted internal MCP endpoints, preventing applications from connecting to untrusted or external rogue servers. As a compensating measure, Enforce an explicit deny-by-default allowlist check on the resource indicator in OAuth metadata to restrict declared target resource URLs. These controls limit blast radius and contain exploitation, but they do not fix the underlying improper origin validation defect — apply the upstream patch to fully remediate.
Highest-impact CME: CME-1315 Allowlist-Preferred Input Validation (Deny-by-Default Filtering) (medium)
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 8 | N/A | 8 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | Required | N/A | Required |
| Scope | Changed | N/A | Changed |
| Confidentiality | High | N/A | High |
| Integrity Impact | High | N/A | High |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N
Understanding the Weakness (CWE)
Access Control,Other
Technical Impact: Gain Privileges or Assume Identity; Varies by Context
An attacker can access any functionality that is inadvertently accessible to the source.
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