Hono vulnerable to Prototype Pollution possible through __proto__ key allowed in parseBody({ dot: true })

Summary

When using parseBody({ dot: true }) in HonoRequest, specially crafted form field names such as __proto__.x could create objects containing a __proto__ property.

If the parsed result is later merged into regular JavaScript objects using unsafe merge patterns, this may lead to prototype pollution in the target object.

Details

The parseBody({ dot: true }) feature supports dot notation to construct nested objects from form field names.

In previous versions, the __proto__ path segment was not filtered. As a result, specially crafted keys such as __proto__.x could produce objects containing __proto__ properties.

While this behavior does not directly modify Object.prototype within Hono itself, it may become exploitable if the parsed result is later merged into regular JavaScript objects using unsafe merge patterns.

Impact

Applications that merge parsed form data into regular objects using unsafe patterns (for example recursive deep merge utilities) may become vulnerable to prototype pollution.