CVE-2026-123272 mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata
- Exploited in the wild
- Not known
- Fix
- None listed
What is CVE-2026-123272?
CVE-2026-123272 is a medium-severity vulnerability in mark3labs mcp-go, published on 5 Nov 2026.
Official description
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.
Weakness: CWE-346
Which versions are affected by CVE-2026-123272?
How do I fix CVE-2026-123272?
No fix has been published for CVE-2026-123272 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-123272 being exploited?
There is no known exploitation of CVE-2026-123272 in the wild and no public exploit code.
Sources
Also known as GHSA-h3x9-8c36-m89w, GHSA-xf6j-693p-wqxx.