Executive Summary

This vulnerability exists in the Hono web application framework prior to version 4.12.21. The issue arises because app.mount() strips the mount prefix from the incoming request path using the raw URL pathname, while route matching is done against the percent-decoded path. When the path contains percent-encoded multi-byte characters, this inconsistency causes the prefix to be stripped incorrectly, resulting in the mounted sub-application receiving an incorrect path.

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Impact Analysis

The vulnerability can cause the mounted sub-application to receive an incorrect path due to improper handling of percent-encoded multi-byte characters. This may lead to unexpected behavior in routing, potentially causing security issues such as incorrect resource access or bypassing intended route protections.

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Mitigation Strategies

To mitigate this vulnerability, upgrade the Hono framework to version 4.12.21 or later, where the issue with app.mount() handling of percent-encoded multi-byte characters in paths has been fixed.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability arises from the incorrect handling of percent-encoded multi-byte characters in request paths by the Hono web framework versions prior to 4.12.21. To detect if your system is affected, you can monitor HTTP requests for malformed or unexpected routing behavior in mounted sub-applications, especially when requests contain percent-encoded characters.

One approach is to send crafted HTTP requests with percent-encoded multi-byte characters in the path and observe if the mounted sub-application receives an incorrect or partial path, which may indicate the vulnerability.

Example commands using curl to test for this behavior could be:

• curl -v 'http://yourserver/mountprefix/%E2%82%ACtest' # where %E2%82%AC is a percent-encoded multi-byte character (Euro sign)
• curl -v 'http://yourserver/mountprefix/%F0%9F%92%A9test' # where %F0%9F%92%A9 is a percent-encoded multi-byte emoji

By analyzing server logs or application responses to these requests, you can check if the mount prefix is stripped incorrectly, leading to unexpected routing or exposure of unintended routes.

Additionally, verifying the version of the Hono framework in use and ensuring it is updated to 4.12.21 or later is a direct method to confirm if the vulnerability is present.

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