---
id: "CVE-2026-123272"
kind: "vulnerability"
aliases: ["CVE-2026-123272"]
title: "mcp-go: OAuth metadata check lets malicious MCP servers obtain other servers' tokens"
severity: "high"
cvss: 8.0
epss: null
exploited: false
status: "open"
category: "ai-tooling"
products: []
ecosystems: []
tags: ["ai-tooling", "go", "mcp", "mcp-go", "oauth"]
not_affected_if: ["local-only-servers", "no-oauth-client", "trusted-servers-only"]
published_at: "2026-11-05T19:17:20+00:00"
first_seen_at: "2026-11-05T19:40:36+00:00"
updated_at: "2026-11-05T19:42:57+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-123272", "first_seen_at": "2026-11-05T19:40:36+00:00"}]
exploit_sources: []
---

# mcp-go: OAuth metadata check lets malicious MCP servers obtain other servers' tokens

mcp-go is a Go library for building Model Context Protocol clients and servers. Before v1.2.1, its OAuth handler accepts a resource indicator declared in a server's well-known metadata without checking it, so a malicious MCP server can get tokens minted for a different MCP server and replay them against that server.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Find the mcp-go version in `go.mod` or with `go list -m github.com/mark3labs/mcp-go`
- Check whether the code uses the OAuth client flow (OAuthHandler) to connect to remote MCP servers
- Confirm whether the application connects to MCP servers that are not fully trusted

## Fix

- mcp-go: upgrade to `v1.2.1` or later

## Timeline

- 2026-11-05T19:40:36+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-123272

## Documents

- [advisory] Vendor advisory: github.com https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata
- [fix] Patch: github.com https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443
- [fix] Release notes: github.com https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1

## From the sources

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.

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