Official MCP Python SDK Flaw Can Let Malicious Servers Steal OAuth Credentials

A malicious MCP server could trick an application built on the official MCP Python SDK into handing over the OAuth credentials it uses to log in to a real service, the SDK's maintainers said in a security advisory.

Affected versions sent the client secret, the authorization code, and the PKCE proof key to a token endpoint the attacker controlled. The fix is in versions 1.30.0 and

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id: 3318
uid: mvRfy
insdate: 2026-09-29 07:10:05
title: Official MCP Python SDK Flaw Can Let Malicious Servers Steal OAuth Credentials
additional: A malicious MCP server could trick an application built on the official MCP Python SDK into handing over the OAuth credentials it uses to log in to a real service, the SDK's maintainers said in a security advisory.

Affected versions sent the client secret, the authorization code, and the PKCE proof key to a token endpoint the attacker controlled. The fix is in versions 1.30.0 and
category: Cybersecurity
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