Summary

Hono's query parsing does not stop at the URL fragment: a ? appearing after a # is treated as the start of a query string. As a result, the application can read request parameters that no other component involved in handling the request can see.

Details

A fragment is never part of the query, and every standard URL consumer — browsers, new URL(), reverse proxies — ignores everything from the first # onward. Hono's routing followed that rule; its query helpers did not.

For one and the same request, this produces an interpretation differential:

• A component in front of the application that inspects the query string — filtering rules, parameter allow/deny lists, access logging — observes no parameters, while the application reads and acts on them.
• The cache middleware removed the fragment when building its cache key, so a response influenced by parameters carried inside the fragment could be stored under a key that did not reflect them and later returned to other users.

The same divergence reaches request validation and any middleware that reads query parameters.

This requires a request target containing a literal # to reach the application. Deployments on runtimes that normalise such a target — including Cloudflare Workers — are not affected, and neither are those behind an intermediary that strips the fragment.

Impact

An attacker can cause the application to act on parameters that components in front of it never observe.

This may lead to:

• filtering rules, allow/deny lists, and audit logging being blind to parameters the application still processes
• a cached response being stored under a key that does not reflect the parameters used to produce it, and served to other users
• stored cross-site scripting, where such a parameter is reflected into a cached HTML response without escaping

This issue affects applications that read query parameters and run on a runtime that passes a literal # through to the request URL.