{"id":"CVE-2026-123272","aliases":["EUVD-2026-117725","GHSA-h3x9-8c36-m89w","GHSA-xf6j-693p-wqxx"],"status":"published","title":"mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata","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.","severity":{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.00196,"percentile":0.09433,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"cwes":["CWE-346"],"published":"2026-11-05T18:45:36Z","modified":"2026-11-06T10:46:35Z","products":[{"vendor":"mark3labs","product":"mcp-go"}],"references":[{"url":"https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx","tags":["vendor-advisory","advisory"]},{"url":"https://github.com/mark3labs/mcp-go/pull/1053","tags":["issue-tracking","patch","fix"]},{"url":"https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443","tags":["patch","fix"]},{"url":"https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1","tags":["release-notes","advisory"]},{"url":"https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821","tags":["technical-description","article"]},{"url":"https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata","tags":["third-party-advisory","advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-123272","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-h3x9-8c36-m89w"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/123xxx/CVE-2026-123272.json","tags":["advisory"]},{"url":"https://github.com/mark3labs/mcp-go","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-123272","title":"mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata","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.","status":"published","published":"2026-11-05T18:45:36.197Z","modified":"2026-11-08T14:01:06.693Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","level":"medium","assessor":"cna"},{"version":"3.1","score":8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N","level":"high","assessor":"cna"}],"cwes":["CWE-346"],"updated_at":"2026-11-08T15:43:32.737211Z"}],"euvd":[{"id":"EUVD-2026-117725","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.","status":"published","published":"2026-11-05T18:45:36Z","modified":"2026-11-08T14:01:06Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","level":"medium"}],"updated_at":"2026-11-08T15:13:09.785497Z"}],"ghsa":[{"id":"GHSA-h3x9-8c36-m89w","title":"Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler...","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.","status":"published","published":"2026-11-05T21:34:51Z","modified":"2026-11-05T21:35:00Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium"},{"version":"3.1","score":8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N","level":"high"}],"cwes":["CWE-346"],"updated_at":"2026-11-05T21:56:38.142385Z"}],"nvd":[{"id":"CVE-2026-123272","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.","status":"published","published":"2026-11-05T19:17:20.713Z","modified":"2026-11-08T14:18:07.493Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium","assessor":"secondary"},{"version":"3.1","score":8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N","level":"high","assessor":"secondary"}],"cwes":["CWE-346"],"updated_at":"2026-11-08T16:10:21.004208Z"}],"osv":[{"id":"CVE-2026-123272","title":"mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata","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.","status":"published","published":"2026-11-05T18:45:36.197Z","modified":"2026-11-10T02:30:32.475446627Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","level":"medium"}],"cwes":["CWE-346"],"updated_at":"2026-11-07T03:08:01.726202Z"}]}}