---
id: CVE-2026-123272
title: >-
  mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known
  Protected Resource Metadata
summary: >-
  Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability
  in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to
  obtain tokens for other resources by declaring arbitrary resources in
  well-known metada…
severity: high
cvss: 8
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N'
cvssSource: cna
cwe:
  - CWE-346
vendor: mark3labs
product: mcp-go
affected:
  - mcp-go >= v0.48.0 < v1.2.1
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-11-05T21:01:43.498683Z'
published: '2026-11-05'
updated: '2026-11-05'
sourceUpdated: '2026-11-05T21:01:52.297Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-123272'
references:
  - url: >-
      https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx
    label: GitHub Security Advisory (GHSA-xf6j-693p-wqxx)
  - url: 'https://github.com/mark3labs/mcp-go/pull/1053'
  - url: >-
      https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443
  - url: 'https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1'
    label: mcp-go v1.2.1 Release Notes
  - url: >-
      https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821
  - url: >-
      https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata
    label: >-
      VulnCheck Advisory: mcp-go before v1.2.1 Origin Validation Error via
      Unvalidated Well-Known Protected Resource Metadata
tags:
  - cve.org
ingestedAt: '2026-11-05T22:38:31.943Z'
---

## Overview

Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to obtain tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers can declare a different MCP server as the RFC 8707 resource indicator, causing the authorization server to mint tokens the attacker replays against that resource.

## Affected

- `mcp-go >= v0.48.0 < v1.2.1`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.