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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
2026-07-17
Advisory published
2026-07-15
Advisory updated
2026-07-17

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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-52869 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-52869

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-52869 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-52869

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.