{"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","slug":"CVE-2026-123272","body":"## Overview\n\nMark3labs 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.\n\n## Affected\n\n- `mcp-go >= v0.48.0 < v1.2.1`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"twilight","depthScore":44,"depthScoreParts":{"impact":44,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}