CVE-2026-84363: Hono: Query parser reads parameters after the URL fragment, causing cache-key and proxy interpretation differentials
Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.13.5, Hono's query helpers treat a question mark after a literal hash fragment as the start of a query string, so the application can read request parameters that browsers, new URL(), reverse proxies, filtering rules, parameter allow and deny lists, access logging, request validation, and other middleware do not observe. The Cache Middleware removes the fragment when building its cache key, allowing a response influenced by parameters inside the fragment to be stored under a key that omits those parameters and later served to other users. This can bypass filtering and auditing, poison cached responses, and enable stored cross-site scripting when an affected parameter is reflected into cached HTML without escaping. Exploitation requires a runtime and intermediary path that passes a literal hash character through to the request URL; Cloudflare Workers and intermediaries that strip fragments are not affected. This issue is fixed in version 4.13.5.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Honoto a version that resolves this vulnerability.Fixed in 4.13.5
Event History
Frequently Asked Questions
Which deployments are realistically exposed?
Deployments using Hono before 4.13.5 are exposed only when the runtime and intermediary path pass a literal hash character through in the request URL. Cloudflare Workers and intermediaries that strip fragments are not affected.
What does an attacker need to exploit this issue?
An attacker must be able to send a request whose literal hash fragment reaches Hono and includes a question mark followed by parameters. Exploitation is more consequential where those parameters affect application behavior, filtering, logging, validation, or cacheable responses.
When can this lead to cache poisoning or stored cross-site scripting?
Cache poisoning can occur when Hono Cache Middleware stores a response influenced by parameters after the fragment under a cache key that omits those parameters. Stored cross-site scripting is possible if an affected parameter is reflected into cached HTML without escaping.
What should be done if upgrading cannot happen immediately?
Ensure the runtime or intermediary removes literal URL fragments before requests reach Hono. Review routes using query helpers and Cache Middleware, especially where query-controlled values are reflected into HTML or used for access controls, filtering, or validation.