{"id":"CVE-2025-53366","aliases":["EUVD-2025-20093","GHSA-3qhf-m339-9g5v","OESA-2026-1151","PYSEC-2026-1616","ROOT-APP-PYPI-CVE-2025-53366"],"status":"published","title":"MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service","description":"The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.","severity":{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.09754,"percentile":0.9556,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-248"],"published":"2025-07-04T22:05:02Z","modified":"2026-10-28T02:31:07Z","fixes":[{"ecosystem":"PyPI","package":"mcp","fixed_in":["1.9.4"],"sources":["ghsa","osv"]},{"ecosystem":"Root:PyPI","package":"mcp","fixed_in":["1.1.2+aikido.4","1.0.0+aikido.3","1.1.0+aikido.2","1.2.0+aikido.4","1.8.1+aikido.2","1.6.0+aikido.4","1.9.1+aikido.2","1.8.1+aikido.3"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.9.4"}]}]},{"source":"osv","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.9.4"}]}],"versions":["0.9.1","1.0.0","1.1.0","1.1.1","1.1.2","1.1.3","1.2.0","1.2.0rc1","1.2.1","1.3.0","1.3.0rc1","1.4.0","1.4.1","1.5.0","1.6.0","1.7.0","1.7.1","1.8.0","1.8.1","1.9.0","1.9.1","1.9.2","1.9.3"]},{"source":"osv","ecosystem":"openEuler:24.03-LTS-SP3","package":"python-mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.12.3-1.oe2403sp3"}]}],"bundled":2},{"source":"osv","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.9.4"}]}],"versions":["0.9.1","1.0.0","1.1.0","1.1.1","1.1.2","1.1.3","1.2.0","1.2.0rc1","1.2.1","1.3.0","1.3.0rc1","1.4.0","1.4.1","1.5.0","1.6.0","1.7.0","1.7.1","1.8.0","1.8.1","1.9.0","1.9.1","1.9.2","1.9.3"]},{"source":"osv","ecosystem":"Root:PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.1.2+aikido.4"},{"fixed":"1.0.0+aikido.3"},{"fixed":"1.1.0+aikido.2"},{"fixed":"1.2.0+aikido.4"},{"fixed":"1.8.1+aikido.2"},{"fixed":"1.6.0+aikido.4"},{"fixed":"1.9.1+aikido.2"},{"fixed":"1.8.1+aikido.3"}]}]}],"products":[{"vendor":"modelcontextprotocol","product":"python-sdk"}],"references":[{"url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v","tags":["x_refsource_CONFIRM","advisory","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/pull/822","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4","tags":["web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2025-53366","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-3qhf-m339-9g5v","tags":["advisory"]},{"url":"https://github.com/modelcontextprotocol/python-sdk","tags":["package"]},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml","tags":["web"]},{"url":"https://pypi.org/project/mcp","tags":["web","package"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/53xxx/CVE-2025-53366.json","tags":["advisory"]},{"url":"https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-1151","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2025-53365","tags":["advisory"]}],"sources":{"cve":[{"id":"CVE-2025-53366","title":"MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service","description":"The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.","status":"published","published":"2025-07-04T22:05:02.975Z","modified":"2025-07-08T13:52:28.447Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high","assessor":"cna"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:32:49.503917Z"}],"euvd":[{"id":"EUVD-2025-20093","description":"The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.","status":"published","published":"2025-07-04T22:05:02Z","modified":"2025-07-08T13:52:28Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"updated_at":"2026-11-02T20:32:13.001759Z"}],"ghsa":[{"id":"GHSA-3qhf-m339-9g5v","title":"MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS","description":"A validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.\n\nThank you to Rich Harang for reporting this issue.","status":"published","published":"2025-07-04T22:06:49Z","modified":"2026-07-16T18:30:57Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:40:44.915484Z"}],"nvd":[{"id":"CVE-2025-53366","description":"The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.","status":"published","published":"2025-07-04T22:15:22.3Z","modified":"2026-06-17T09:38:06.047Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/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":"high","assessor":"secondary"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:11:16.997062Z"}],"osv":[{"id":"CVE-2025-53366","title":"MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service","description":"The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.","status":"published","published":"2025-07-04T22:05:02.975Z","modified":"2026-10-28T02:31:07.999367381Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T15:44:50.45326Z"},{"id":"GHSA-3qhf-m339-9g5v","title":"MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS","description":"A validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.\n\nThank you to Rich Harang for reporting this issue.","status":"published","published":"2025-07-04T22:06:49Z","modified":"2026-07-16T18:45:48.199122861Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T15:46:36.092962Z"},{"id":"OESA-2026-1151","title":"python-mcp security update","description":"The official Python SDK for Model Context Protocol servers and clients\r\n\r\nSecurity Fix(es):\n\nThe MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.10.0, if a client deliberately triggers an exception after establishing a streamable HTTP session, this can lead to an uncaught ClosedResourceError on the server side, causing the server to crash and requiring a restart to restore service. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.10.0 contains a patch for the issue.(CVE-2025-53365)\n\nThe MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, a validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.9.4 contains a patch for the issue.(CVE-2025-53366)","status":"published","published":"2026-01-16T11:10:11Z","modified":"2026-08-18T01:19:28.042260865Z","cvss":[{"version":"3.1","score":5.5,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:L/I:L/A:L","level":"medium"}],"updated_at":"2026-11-02T15:52:45.339366Z"},{"id":"PYSEC-2026-1616","title":"MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS","description":"A validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.\n\nThank you to Rich Harang for reporting this issue.","status":"published","published":"2026-07-07T16:02:56.172263Z","modified":"2026-07-07T17:47:03.420621263Z","cvss":[{"version":"4.0","score":8.7,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"updated_at":"2026-11-02T15:52:55.06457Z"},{"id":"ROOT-APP-PYPI-CVE-2025-53366","title":"CVE-2025-53366 in mcp - Patched by Root","description":"Root has patched CVE-2025-53366 in the mcp package for Root:PyPI. Multiple fixed versions available.","status":"published","published":"2026-10-04T14:12:41Z","modified":"2026-10-04T15:15:03.022727751Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"updated_at":"2026-11-02T15:53:27.940619Z"}]}}