High severity · published Jul 15, 2026
CVE-2026-52869
MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal
7.1
CVSS
0.5%
EPSS
No
On KEV
Description
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.27.2, the SSE and stateful Streamable HTTP transports mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying the authenticated principal that created the session, allowing a different bearer-token-authenticated client with a known session ID to inject JSON-RPC messages into that session. This issue is fixed in version 1.27.2.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
Attack vector
Network
Complexity
High
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
High
Availability
Low
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 45% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
PY
python-sdk · modelcontextprotocol
< 1.27.2 · source: vendor
MP
Mcp Python Sdk · Lfprojects
before 1.27.2 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-52869
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-52869
Patch
https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00
Patch
https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a
Issue Tracking
https://github.com/modelcontextprotocol/python-sdk/pull/2690
Issue Tracking
https://github.com/modelcontextprotocol/python-sdk/pull/2719
Release Notes
https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2
Vendor Advisory
https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58
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