Improper OAuth Validation in mcp-go Leading to Token Replay Attacks

Executive Summary

This vulnerability in Mark3labs mcp-go before v1.2.1 allows malicious MCP servers to obtain OAuth tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers manipulate the RFC 8707 resource indicator to trick authorization servers into minting tokens for a different resource, which the attacker can then replay against the victim resource.

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Detection Guidance

To detect this vulnerability, check if your mcp-go version is between 0.48.0 and 1.2.0. Run: mcp-go version or check package dependencies. Inspect OAuth token requests for mismatched resource indicators in metadata. Monitor logs for unexpected token issuance or access attempts.

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Impact Analysis

If you use a vulnerable version of mcp-go (0.48.0 to 1.2.0), a malicious MCP server could trick your OAuth client into requesting tokens for unintended resources. This could lead to unauthorized access to your data or services if the attacker replays the tokens against the victim resource.

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Compliance Impact

This vulnerability could lead to unauthorized data access, violating confidentiality requirements in GDPR and HIPAA. It may result in data breaches, non-compliance with access controls, and failure to protect sensitive information, potentially leading to legal penalties or reputational damage.

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Mitigation Strategies

Upgrade mcp-go to version 1.2.1 or later immediately. Update dependencies in your project to exclude vulnerable versions. Review OAuth configurations to ensure resource validation is enforced. Monitor for suspicious token requests or unauthorized access.

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