CVE-2026-52869Network attack vector

Cross-session message injection via unbound session identifiers

Published Jul 15, 2026 · Updated Jul 15, 2026

Authorization bypass in MCP Python SDK before 1.27.2 allows remote authenticated users to inject JSON-RPC messages into other sessions. The SSE and stateful Streamable HTTP transports select a session solely from session_id or Mcp-Session-Id and never compare its owner with the request's bearer-token principal. Exploitation requires bearer-token authentication plus a known random session ID; it permits cross-client message injection and, on Streamable HTTP, reading the result.

CVSS severity7.1
High
EPSS probability0.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authorization bypass in MCP Python SDK before 1.27.2 allows remote authenticated users to inject JSON-RPC messages into other sessions. The SSE and stateful Streamable HTTP transports select a session solely from session_id or Mcp-Session-Id and never compare its owner with the request's bearer-token principal. Exploitation requires bearer-token authentication plus a known random session ID; it permits cross-client message injection and, on Streamable HTTP, reading the result.

The record

CVE
CVE-2026-52869
Published
Jul 15, 2026
Updated
Jul 15, 2026
Vendor
Model Context Protocol
Product
MCP Python SDK
Classifications
CWE-639, T1078
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.27.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Model Context Protocol · MCP Python SDKversion=< 1.27.2

Published CVSS scores

7.1GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-639Authorization Bypass Through User-Controlled Key
T1078Valid Accounts

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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