CVE-2026-56762 Hono CVE debrief

Who should care

Developers and administrators using Hono before version 4.12.12 should be aware of this vulnerability. The vulnerability could potentially cause runtime errors, impacting the availability of applications. Although the CVSS score is medium (6.9), the issue's impact on correctness and robustness makes it significant for applications relying on Hono for cookie handling.

Technical summary

The vulnerability in Hono before 4.12.12 stems from the lack of validation for cookie names in the setCookie(), serialize(), and serializeSigned() functions. This oversight allows for the inclusion of invalid characters, such as control characters, in cookie names when provided by user-controlled input. The primary impact of this vulnerability is on the correctness and robustness of applications, potentially leading to runtime errors due to malformed Set-Cookie header values. In modern runtimes like Node.js and Cloudflare Workers, these invalid header values are rejected, causing runtime errors before the response is sent. This prevents potential header injection or response splitting attacks but affects application availability.

Defensive priority

Apply the patch: Upgrade to Hono version 4.12.12 or later to fix the vulnerability. Validate cookie names: Implement additional validation for cookie names to ensure they do not contain invalid characters, enhancing the security and robustness of cookie handling.

Recommended defensive actions

• Upgrade to Hono version 4.12.12 or later.
• Implement additional validation for cookie names to prevent invalid characters.
• Review and update application code to handle user-controlled cookie names securely.
• Monitor applications for runtime errors related to cookie handling.
• Consider implementing compensating controls, such as Web Application Firewalls (WAFs), to detect and prevent potential attacks.

Evidence notes

The CVE-2026-56762 vulnerability was made public on June 23, 2026, with an update on June 24, 2026. The issue is related to Hono's handling of cookie names, specifically the lack of validation for user-controlled input. The vulnerability primarily affects the availability of applications due to potential runtime errors caused by malformed Set-Cookie header values.

Official resources