CVE-2026-56762 Overview
CVE-2026-56762 affects the Hono web framework in versions prior to 4.12.12. The flaw exists in the setCookie(), serialize(), and serializeSigned() functions, which do not validate cookie names on the write path. When an application passes a user-controlled cookie name containing invalid characters such as control characters (\r or \n), Hono produces malformed Set-Cookie header values. Modern runtimes including Node.js and Cloudflare Workers reject these invalid header values and raise runtime errors before the response is sent. The issue is tracked under CWE-20: Improper Input Validation.
Critical Impact
User-controlled cookie names can trigger runtime errors that break response delivery, primarily affecting application availability and correctness.
Affected Products
- Hono web framework versions before 4.12.12
- Applications using setCookie() with user-controlled cookie names
- Applications calling serialize() or serializeSigned() on untrusted input
Discovery Timeline
- 2026-06-23 - CVE-2026-56762 published to NVD
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-56762
Vulnerability Analysis
Hono is a lightweight web framework that ships cookie helpers for setting and serializing HTTP cookies. The framework constructs Set-Cookie header values by concatenating the provided name with delimiters and the encoded value. Before version 4.12.12, the cookie name argument passes through without character validation. An application that accepts a cookie name from user input therefore allows control characters and other invalid token characters into the header value. The resulting Set-Cookie header violates RFC 6265 and HTTP header grammar.
In Node.js and Cloudflare Workers, the runtime validates outbound header values and throws an error when control characters are present. This terminates request processing and prevents the response from being delivered. The advisory confirms that header injection and HTTP response splitting could not be reproduced because runtimes intercept the malformed output. The practical impact is reduced to availability and correctness rather than confidentiality or integrity loss.
Root Cause
The cookie helper functions in Hono accept the name parameter and serialize it directly into the Set-Cookie value. The code path lacks a check against the cookie-name token grammar defined by RFC 6265, which excludes control characters, whitespace, and separators. Trusting caller-supplied names without validation is the root cause.
Attack Vector
A remote unauthenticated attacker submits crafted input that reaches a Hono cookie helper as the cookie name. Typical entry points include query parameters, form fields, or JSON body values that an application maps to a cookie name. When the response is built, the runtime rejects the malformed header and the request fails. Repeated requests against such endpoints can disrupt service availability.
The vulnerability does not produce confirmed header injection. Refer to the Hono GitHub Security Advisory GHSA-26pp-8wgv-hjvm and the VulnCheck Advisory for additional technical detail.
Detection Methods for CVE-2026-56762
Indicators of Compromise
- Application logs containing ERR_INVALID_CHAR or ERR_HTTP_INVALID_HEADER_VALUE errors originating from Hono request handlers
- Unhandled exceptions thrown from setCookie, serialize, or serializeSigned call sites
- Spikes in HTTP 500 responses correlated with requests targeting endpoints that set cookies
Detection Strategies
- Audit source code for calls to setCookie(), serialize(), and serializeSigned() where the name argument originates from request input
- Enable structured logging on the framework error handler to capture stack traces tied to cookie helper failures
- Inspect HTTP access logs for repeated failed responses from endpoints that emit Set-Cookie headers
Monitoring Recommendations
- Track error rates on routes that issue cookies and alert on sudden increases
- Forward Node.js and Cloudflare Workers runtime errors to a centralized log platform for correlation
- Add synthetic monitoring that exercises authentication and session endpoints to detect availability regressions
How to Mitigate CVE-2026-56762
Immediate Actions Required
- Upgrade Hono to version 4.12.12 or later across all production and staging deployments
- Identify every code path that derives a cookie name from user input and remove or sanitize that flow
- Add input validation that restricts cookie names to the RFC 6265 token character set before calling Hono helpers
Patch Information
The Hono maintainers fixed the issue in version 4.12.12 by adding cookie-name validation inside setCookie(), serialize(), and serializeSigned(). Applications should pin the resolved version in package.json and rebuild dependency lockfiles. See the GitHub Security Advisory GHSA-26pp-8wgv-hjvm for the release notes and patch commit.
Workarounds
- Reject any request input that is destined to become a cookie name and contains characters outside [A-Za-z0-9!#$%&'*+\-.^_|~]`
- Hardcode cookie names in application code instead of computing them from request data
- Wrap cookie-setting logic in a try/catch block and return a controlled error response to preserve availability until the upgrade is applied
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
