Authorization bypass through user-controlled key in MCP Python SDK - CVE-2026-52869

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Main Vulnerability Database Vulnerabilities #VU163090

Authorization bypass through user-controlled key in MCP Python SDK - CVE-2026-52869

Published: November 9, 2026


Vulnerability identifier: #VU163090
CSH Severity: Low
CVSS v4: 7.6 [CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-52869
CWE-ID: CWE-639
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

The vulnerability allows a low-privileged remote attacker to inject JSON-RPC messages into another client's session.

The vulnerability exists due to missing principal verification when the MCP Python SDK's SSE and stateful Streamable HTTP transports route requests by session identifier with built-in bearer-token authentication enabled. A low-privileged remote attacker can use a session identifier obtained out of band with their own bearer token to bypass client isolation.


Affected software

MCP Python SDK
watsonx.data

How to mitigate CVE-2026-52869

Install security update from vendor's website.

MCP Python SDK - update to 1.27.2

External References

Related Security Bulletins