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Official MCP Python SDK OAuth credential theft security flaw

Vulnerability
First reported
Last updated
Happening score
H score 32
1 unique sources, 1 articles

Summary

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Official MCP Python SDK clients were found vulnerable to an OAuth credential theft flaw that let a malicious MCP server redirect login handling and capture secrets. The issue affected 1.9.1 through 1.29.1 on the 1.x line and 2.0.0 through 2.1.1 on the 2.x line, exposing the client secret, authorization code, and PKCE proof key. Fixed versions 1.30.0 and 2.2.0 stop the redirect abuse, and no attacks have been reported.

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Timeline

  1. 29.09.2026 09:08 1 articles · 1h ago

    Official MCP Python SDK release notes add issuer checks to block OAuth credential redirection

    Mitigation Patch Update

    Release notes for the official MCP Python SDK's 1.30.0 and 2.2.0 versions added issuer checks so a client verifies the login service it expects before accepting authorization details, closing the redirect path that could send OAuth credentials to a different endpoint.

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  2. 29.09.2026 09:08 2 articles · 1h ago

    Security advisory details OAuth credential theft in the official MCP Python SDK

    Initial Disclosure

    On September 28, the security advisory and Cycode's writeup described a malicious MCP server tricking applications built on the official MCP Python SDK into sending the client secret, authorization code, and PKCE proof key to an attacker-controlled token endpoint, and neither source reported any attacks using the flaw.

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