8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2025-53365

CVE-2025-53365: MCP Python SDK has Unhandled Exception in Streamable HTTP Transport ,Leading to Denial of Service

If a client deliberately triggers an exception after establishing a streamable HTTP session, this can lead to an uncaught ClosedResourceError on the server side, causing the server to crash and requiring a restart to restore service. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.

Thank you to Rich Harang for reporting this issue.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2025-53365

CVE-2025-53365:

8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
mcp	pip	< 1.10.0	1.10.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability analysis identified a critical flaw in the MCP Python SDK's server-side session management. The root cause of the Denial of Service is an unhandled anyio.ClosedResourceError exception within the mcp.shared.session.Session._receive_loop function. This function is responsible for processing incoming messages over a streamable HTTP connection.

When a client establishes a connection and then disconnects abruptly (e.g., by crashing or closing the connection without proper termination), the underlying anyio stream raises a ClosedResourceError. The vulnerable version of the _receive_loop function did not have an exception handler for this specific error. As a result, the exception would go uncaught, leading to the termination of the server's execution task and effectively crashing the server process.

The provided patch directly addresses this issue by wrapping the message-reading loop (async for message in self._read_stream:) inside a try...except block. The newly added except anyio.ClosedResourceError: block catches the specific exception, logs a debug message, and allows the server to continue running without crashing. This ensures that a misbehaving or crashing client cannot bring down the entire server, thus mitigating the Denial of Service vulnerability.

Vulnerable functions

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WAF Rule

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Reasoning

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