CVE-2026-56762: Hono Cookie Name Validation Vulnerability – MEDIUM Severity

Technical summary

Hono versions prior to 4.12.12 lack input validation on the cookie name parameter across three functions: setCookie(), serialize(), and serializeSigned(). When untrusted cookie names containing control characters (\r, \n, or other invalid characters) are passed to these functions, the resulting Set-Cookie header becomes malformed. Although HTTP header injection or response splitting attacks were theoretically possible, they are not practically exploitable in current runtimes because Node.js and Cloudflare Workers validate header values before transmission and throw errors. This converts a potential integrity issue into an availability concern—the application fails to send the response rather than sending an injected one.

Business impact

Applications using Hono risk unexpected crashes when cookie operations are performed with unsanitized user input. For production systems, this translates to service unavailability during the window of failure. The impact is limited to availability rather than confidentiality or integrity, since modern runtimes prevent the header from being transmitted. However, if an attacker can trigger these crashes repeatedly, they can cause denial of service. Teams relying on Hono for API gateways, web servers, or middleware should prioritize patching to prevent user-facing downtime.

Affected systems

Any application using Hono framework versions before 4.12.12 is affected, particularly those that accept cookie names from external sources (query parameters, request headers, user input). Cloudflare Workers, Node.js applications, and other modern JavaScript runtimes that validate headers are affected in terms of runtime errors. Older or more permissive runtimes might behave differently and should be assessed independently.

Exploitability

Exploitation requires an attacker to control or influence the cookie name parameter passed to setCookie(), serialize(), or serializeSigned(). This is a low-barrier attack since no authentication or special privileges are typically needed—a public API endpoint accepting user input in cookie logic is sufficient. However, the practical outcome in modern runtimes is limited to denial of service via runtime error rather than header injection. No active exploitation in the wild has been confirmed, and the vulnerability is not listed on CISA's Known Exploited Vulnerabilities catalog.

Remediation

Upgrade Hono to version 4.12.12 or later. This release includes input validation on cookie names to reject control characters and other invalid characters before they reach the header generation functions. For organizations unable to upgrade immediately, implement application-level validation of all user-controlled cookie names before passing them to Hono functions, rejecting names containing whitespace, control characters, or other non-standard characters.

Patch guidance

Check your package.json or lock file to confirm your current Hono version. If it is below 4.12.12, update via npm (npm install hono@latest) or your preferred package manager. Verify in your changelog or release notes that the update includes the cookie name validation fix. Test your application thoroughly in a staging environment before deploying to production, especially any code paths that construct cookie names from user input. No breaking changes are anticipated with this patch.

Detection guidance

Monitor application logs for runtime errors originating from cookie operations, particularly TypeErrors or malformed header errors when Set-Cookie headers are being constructed. Search your codebase for calls to setCookie(), serialize(), and serializeSigned() where the cookie name parameter is derived from user input without prior validation. In runtime environments, enable detailed error logging to catch and alert on header validation failures. Automated dependency scanning tools should flag Hono versions below 4.12.12.

Why prioritize this

Although the CVSS score of 5.3 reflects a medium-severity, low-complexity vulnerability, prioritization should focus on the attack surface in your environment. If your applications pass user input directly to Hono cookie functions, this should be addressed soon to prevent availability disruptions. The lack of active exploitation and the mitigating effect of modern runtimes mean this is not an emergency, but it should not be deferred indefinitely. Organizations with strict uptime requirements should treat this as higher priority.

Risk score, explained

The CVSS 3.1 score of 5.3 (MEDIUM) reflects network-accessible attack surface (AV:N), low attack complexity (AC:L), no privilege requirements (PR:N), no user interaction (UI:N), and impact limited to availability (A:L). No confidentiality or integrity impact is scored because modern runtimes prevent the malformed header from leaving the application. The score appropriately reflects a correctness and robustness issue rather than a security boundary breach.

Frequently asked questions