Remote Information Disclosure in geelen mcp-remote

Executive Summary

This vulnerability involves the mcp-remote library versions 0.1.18 through 0.1.38 where the SSE transport eventSourceInit fetch wrapper adds an Authorization: Bearer header to requests without verifying the origin. This could allow tokens to be forwarded to unintended origins if the wrapper lacks proper origin checks.

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Detection Guidance

The vulnerability involves the SSE transport wrapper in mcp-remote versions 0.1.18 through 0.1.38 adding Authorization headers without origin verification. To detect, inspect network traffic for unauthorized Authorization headers in SSE requests or check if the mcp-remote package version falls within the affected range.

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Impact Analysis

An attacker could potentially intercept or misuse sensitive tokens if the SSE transport wrapper does not enforce origin checks, leading to unauthorized access to protected resources or data.

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Compliance Impact

This vulnerability could lead to unauthorized data exposure, potentially violating GDPR's data protection principles or HIPAA's safeguards for protected health information if tokens are mishandled.

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Mitigation Strategies

Update mcp-remote to a version beyond 0.1.38 or implement origin checks in the SSE transport wrapper. Ensure tokens are bound to issuer, audience, and expected destination origin. Verify that existing controls like @modelcontextprotocol/sdk@1.25.3 and undici@7.12.0 are enforced for origin matching.

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