MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-52869
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-15 20:17:38 UTC
Updated2026-07-17 18:06:50 UTC
DescriptionThe 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.

Risk And Classification

Primary CVSS: v3.1 7.1 HIGH from [email protected]

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

EPSS: 0.002500000 probability, percentile 0.163690000 (date 2026-07-20)

Problem Types: CWE-639 | CWE-639 CWE-639: Authorization Bypass Through User-Controlled Key

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.1HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
3.1CNADECLARED7.1HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
Low

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Lfprojects Mcp Python Sdk All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Modelcontextprotocol Python-sdk affected < 1.27.2 Not specified

References

ReferenceSourceLinkTags
github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9... [email protected] github.com Vendor Advisory
github.com/modelcontextprotocol/python-sdk/pull/2719 [email protected] github.com Issue Tracking, Patch
github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec60... [email protected] github.com Patch
github.com/modelcontextprotocol/python-sdk/pull/2690 [email protected] github.com Issue Tracking, Patch
github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc7... [email protected] github.com Patch
github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2 [email protected] github.com Release Notes
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

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