CVE-2026-47675

The serialize() function in hono/cookie validates domain and path options against characters that corrupt Set-Cookie header syntax (;, \r, \n), but does not apply the same validation to sameSite and priority. An application that passes user-controlled input into either option may produce a Set-Cookie response header containing attacker-chosen additional attributes.

Details

When constructing a Set-Cookie header value, serialize() appends the sameSite and priority option values directly into the output string after a presentation-only transformation (capitalizing the first character). Although the TypeScript type signature constrains these options to specific string literals, that constraint is not enforced at runtime; any string value, including one containing ; or line-feed characters, passes through unchanged.

The validation guard that rejects ;, \r, and \n from domain and path is not applied to sameSite or priority. An application that passes a request-derived value to either option therefore provides an injection point into the header line.

This issue arises when an application passes user-controlled input to the sameSite or priority option of setCookie() or serialize().

An attacker who can control the sameSite or priority option value may inject additional attributes into a Set-Cookie response header.

This may lead to:

• Cookie attribute injection, overriding Domain, Path, HttpOnly, Secure, or Max-Age for the affected cookie
• HTTP response header injection on runtimes that do not strictly validate header values, enabling a second attacker-controlled Set-Cookie header in the same response

This issue affects applications that pass user-derived input into the sameSite or priority option of hono/cookie serialization functions.

CVE-2026-47675 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. 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 (4.12.21); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-47675? CVE-2026-47675 is a medium-severity security vulnerability in hono (npm), affecting versions < 4.12.21. It is fixed in 4.12.21.
2. How severe is CVE-2026-47675? CVE-2026-47675 has a CVSS score of 4.3 (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.
3. Which versions of hono are affected by CVE-2026-47675? hono (npm) versions < 4.12.21 is affected.
4. Is there a fix for CVE-2026-47675? Yes. CVE-2026-47675 is fixed in 4.12.21. Upgrade to this version or later.
5. Is CVE-2026-47675 exploitable, and should I be worried? Whether CVE-2026-47675 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
6. What actually determines whether CVE-2026-47675 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.
7. How do I fix CVE-2026-47675? Upgrade hono to 4.12.21 or later.