CVE-2026-123272
Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in…
- Risk score
- 53/100
- CVSS
- 8.0 v3.1
- Published
- 5 Nov 2026
Open in the live forecast Today's forecast
What to know
- What it is
- 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 indicato…
- Affected
- See advisory
- Fix
- Upgrade the product to 1.2.1 or later.
- Exploitation
- No known exploitation
Why it is Yellow
- CVSS 8.0 high
- Remotely exploitable without logging in
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
- https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata
Original 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.
Last activity 5 Nov 2026. Patch Forecast updates every hour.