PatchSiren cyber security CVE debrief
CVE-2026-52869 modelcontextprotocol CVE debrief
The MCP Python SDK, called mcp on PyPI, is vulnerable to session hijacking prior to version 1.27.2. The SSE and stateful Streamable HTTP transports 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. This allows a different bearer-token-authenticated client with a known session ID to inject JSON-RPC messages into that session. The vulnerability affects versions prior to 1.27.2 and is fixed in version 1.27.2.
- Vendor
- modelcontextprotocol
- Product
- python-sdk
- CVSS
- HIGH 7.1
- CISA KEV
- Not listed in stored evidence
- Original CVE published
- 2026-07-15
- Original CVE updated
- 2026-07-16
- Advisory published
- 2026-07-15
- Advisory updated
- 2026-07-16
Who should care
Users of the MCP Python SDK, particularly those using versions prior to 1.27.2, should be aware of this vulnerability and take steps to upgrade or mitigate the risk. This includes operators, platform administrators, vulnerability management teams, and security teams who may be impacted by the vulnerability. Affected deployments should be identified, and owners should be assigned for follow-up.
Technical summary
The MCP Python SDK's SSE and stateful Streamable HTTP transports are vulnerable to session hijacking. The transports use only the session_id query parameter or Mcp-Session-Id header to route requests to existing sessions, without verifying the authenticated principal that created the session. This allows an attacker with a known session ID to inject JSON-RPC messages into the session. The vulnerability affects versions prior to 1.27.2 and is fixed in version 1.27.2. Users of the MCP Python SDK should be aware of this vulnerability and take steps to upgrade or mitigate the risk.
Defensive priority
High
Recommended defensive actions
- Upgrade to version 1.27.2 or later
- Verify and validate session IDs
- Implement additional authentication mechanisms
- Monitor for suspicious activity
- Review compensating controls for exposed systems while remediation is scheduled and verified
- Check relevant monitoring, detection, and logs for exposed assets that need extra review
- Track exceptions, retest remediated assets, and close the item only after evidence is documented
Evidence notes
The CVE record was published on 2026-07-15T20:17:38.427Z and was last modified on 2026-07-16T14:16:53.550Z. The NVD entry is currently Undergoing Analysis. The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). The CVE record provides limited information about the vulnerability, and defenders should verify the affected scope, severity, and vendor guidance. The NVD entry is currently Undergoing Analysis, and additional information may be available in the future.
Official resources
AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-15T20:17:38.427Z and has not been modified since then. The NVD entry is currently Undergoing Analysis.