{"id":"CVE-2026-33252","aliases":["GHSA-89xv-2j6f-qhc8","GO-2026-4773"],"url":"https://o3.security/vulnerability/CVE-2026-33252","summary":"MCP Go SDK Allows Cross-Site Tool Execution for HTTP Servers without Authorizatrion","details":"The Go SDK's Streamable HTTP transport accepted browser-generated cross-site `POST` requests without validating the `Origin` header and without requiring `Content-Type: application/json`. In deployments without Authorization, especially stateless or sessionless configurations, this allows an arbitrary website to send MCP requests to a local server and potentially trigger tool execution.\n\n#### Impact:\n\nA malicious website may have been able to send cross-site POST requests with `Content-Type: text/plain`, which due to CORS-safelisted properties would reach the MCP message handling without any CORS preflight barrier.\n\n#### Fix:\n\nThe SDK was modified to perform `Content-Type` header validation for POST requests and introduced a configurable protection for verifying the origin of the request in commit a433a83. Users are advised to update to v1.4.1 to use this additional protection.\n\nNote: v1.4.1 requires Go 1.25 or later.\n\n#### Credits:\n\nThank you to Lê Minh Quân for reporting the issue.","published":"2026-03-23T23:44:16.106Z","modified":"2026-08-12T03:51:12.350505025Z","cvss":{"score":7.1,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:L"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/modelcontextprotocol/go-sdk","fixedVersion":"1.4.1"}],"fix":{"url":"https://github.com/modelcontextprotocol/go-sdk/commit/a433a831d6e5d5ac3b9e625a8095aa8eaa040dfc","label":"modelcontextprotocol/go-sdk@a433a83"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33252.json"},{"type":"ADVISORY","url":"https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-89xv-2j6f-qhc8"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33252"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/go-sdk/commit/a433a831d6e5d5ac3b9e625a8095aa8eaa040dfc"},{"type":"PACKAGE","url":"https://github.com/modelcontextprotocol/go-sdk"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:12.350505025Z"}}