Compliance Impact

The vulnerability in the googleapis/mcp-toolbox allows authentication bypass due to improper handling of OAuth token validation, potentially granting unauthorized access to protected tools and underlying data sources.

Such unauthorized access risks exposure of sensitive information, which can lead to non-compliance with common standards and regulations like GDPR and HIPAA that mandate strict access controls and protection of personal and sensitive data.

By allowing tokens missing the mandatory "active" field to be accepted, the system fails to enforce proper authentication, increasing the risk of data breaches and unauthorized data processing, which are critical compliance concerns under these regulations.

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Executive Summary

This vulnerability is an authentication bypass in the googleapis/mcp-toolbox's generic opaque token validation process. When the toolbox verifies an OAuth 2.0 token via an introspection endpoint, it expects a mandatory "active" field in the response to determine if the token is valid. However, if the introspection response omits this "active" field entirely, the toolbox mistakenly treats the token as valid, granting unauthorized access.

The root cause is that the code only rejects tokens explicitly marked as inactive (active field set to false), but does not handle the case where the active field is missing, leading to acceptance of potentially invalid tokens.

The vulnerability was fixed by separating Google and generic MCP OAuth verification, enforcing stricter validation of the "active" field, and improving overall token validation logic.

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Impact Analysis

This vulnerability can allow attackers to bypass authentication controls by presenting tokens that lack the mandatory "active" field, which the system incorrectly accepts as valid.

As a result, unauthorized users could gain access to protected tools and sensitive underlying data sources within the googleapis/mcp-toolbox environment.

This could lead to exposure of sensitive information, unauthorized actions, and compromise of system integrity.

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Detection Guidance

Detection of this vulnerability involves checking whether the OAuth 2.0 introspection endpoint responses omit the mandatory "active" field in the token validation response. Specifically, you should monitor and analyze introspection responses to see if tokens without the "active" field are being accepted.

You can use network traffic inspection tools like curl or tcpdump to capture and inspect OAuth introspection responses. For example, using curl to query the introspection endpoint and check the response payload:

• curl -X POST -d 'token=YOUR_TOKEN' https://your-introspection-endpoint/oauth2/introspect

Then verify if the JSON response contains the "active" field. If the field is missing or null, and the system accepts the token, it indicates the vulnerability.

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Mitigation Strategies

Immediate mitigation involves updating the googleapis/mcp-toolbox to version 1.4.0 or later, which includes the fix for this vulnerability.

The fix separates Google and generic MCP OAuth verification processes, enforces stricter validation of the "active" field in OAuth tokens, and improves issuer validation. Applying this update will prevent acceptance of tokens missing the "active" field.

Additionally, review your authentication logs and token validation logic to ensure tokens without the "active" field are rejected.

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