CVE-2026-123272
HIGH
NVD
CVSS Score
8
Severity
HIGH
Published
Nov 05, 2026
Vendor
unknown
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.
References
- https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821
- https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443
- https://github.com/mark3labs/mcp-go/pull/1053
- https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1
- https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx