Summary
Hono: Query parser reads parameters after the URL fragment, causing cache-key and proxy interpretation differentials
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.
CVE-2026-84363 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.13.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-84363? CVE-2026-84363 is a medium-severity security vulnerability in hono (npm), affecting versions < 4.13.5. It is fixed in 4.13.5.
- How severe is CVE-2026-84363? CVE-2026-84363 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of hono are affected by CVE-2026-84363? hono (npm) versions < 4.13.5 is affected.
- Is there a fix for CVE-2026-84363? Yes. CVE-2026-84363 is fixed in 4.13.5. Upgrade to this version or later.
- Is CVE-2026-84363 exploitable, and should I be worried? Whether CVE-2026-84363 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-84363 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-84363? Upgrade
honoto 4.13.5 or later.