CVE-2026-84363 in Hono
Summary
by MITRE • 09/02/2026
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.
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Analysis
by VulDB Data Team • 09/02/2026
The vulnerability identified in Hono versions prior to 4.13.5 represents a significant security flaw rooted in how the framework parses URL components, specifically regarding the handling of hash fragments and query strings. In standard web architecture, the fragment identifier, denoted by a literal hash character, is intended for client-side use only and should not be transmitted to the server as part of the request URI. However, Hono's internal query helpers incorrectly interpret a question mark appearing after this hash symbol as the beginning of a valid query string. This parsing logic diverges from the behavior of standard browser implementations, the native URL API in JavaScript runtimes, reverse proxies, and most security middleware layers that adhere to RFC 3986 standards by treating everything following the hash as opaque data ignored during server-side processing.
This discrepancy creates a severe bypass mechanism for various security controls designed to filter or validate incoming request parameters. Because Hono processes these hidden query parameters while other components in the infrastructure do not, an attacker can inject malicious payloads into parts of the URL that are invisible to access logging systems, parameter allow and deny lists, and input validation middleware. This effectively allows unauthorized data manipulation that bypasses auditing trails and security filters, leading to a state where the application behaves inconsistently with respect to its own security policies. The flaw is categorized under CWE-20: Improper Input Validation, as the framework fails to correctly validate or ignore non-standard URL structures that should be discarded by compliant parsers.
The operational impact of this vulnerability extends beyond simple parameter injection due to the behavior of Hono's Cache Middleware. When caching responses, the middleware constructs cache keys based on the request path and standard query parameters but explicitly removes the fragment identifier from the key generation process. Consequently, if an attacker crafts a URL with malicious parameters hidden within the hash-fragment section, the server processes these parameters and generates a response influenced by them. This poisoned response is then stored in the cache under a clean key that does not reflect the presence of those harmful parameters. Subsequent users requesting the same base path receive this cached, compromised content without their requests containing any malicious input themselves.
This mechanism facilitates Stored Cross-Site Scripting (XSS) attacks when the affected parameter values are reflected into HTML responses and served from cache without proper escaping or sanitization. Since the response is stored once and delivered to multiple users, a single exploitation attempt can compromise numerous victims who never directly interact with the malicious payload in their own browser sessions. This scenario aligns with ATT&CK technique T1059: Command and Scripting Interpreter if script execution occurs via reflected XSS, or more broadly under injection techniques that leverage caching mechanisms for persistence. The attack vector requires a runtime environment and intermediary path capable of passing the literal hash character through to the application server; environments such as Cloudflare Workers or intermediaries configured to strip fragments before reaching the origin are not susceptible to this specific exploitation chain.
To mitigate this vulnerability, organizations must upgrade Hono to version 4.13.5 or later, where the parsing logic has been corrected to align with standard URL specifications and ignore query-like structures within hash fragments. For systems that cannot immediately update, implementing strict input validation at the reverse proxy level is critical to ensure that any request containing a hash character followed by query parameters is either rejected or sanitized before reaching the application layer. Additionally, ensuring that cache keys incorporate all relevant parts of the URL structure, including those typically ignored, can prevent cache poisoning attacks derived from this parsing inconsistency. Regular auditing of middleware configurations and validation rules against edge-case URL structures remains essential to maintain robust security posture in web applications utilizing dynamic routing frameworks.