CVE-2026-16729

undici vulnerable to cookie attribute injection via unsanitized domain and unparsed setCookie fields

Workarounds

• Sanitize domain values against the RFC 1034 letter-digit-hyphen set before passing to setCookie.
• Do not pass user-controlled data to the unparsed field.

The setCookie function has two attribute injection paths. validateCookieDomain does not reject semicolons (validateCookiePath already does at 0x3B), so a domain value like example.com; SameSite=None lands verbatim as Domain=example.com; SameSite=None. The unparsed array's loop only checks each entry contains = and does not sanitize values, so an entry like X-Custom=val; HttpOnly lands unchanged, injecting HttpOnly without the caller setting cookie.httpOnly = true.

Applications that pass user-controlled input to these fields, typically multi-tenant or reverse-proxy servers that scope session cookies to a tenant-supplied domain, can have SameSite CSRF protections bypassed, Secure or HttpOnly forced or stripped, or the intended SameSite tier overridden.

CVE-2026-16729 has a CVSS score of 4.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. 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 (6.28.0, 7.29.0, 8.9.0); upgrading removes the vulnerable code path.

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Patched in undici v6.28.0, v7.29.0, and v8.9.0.

1. What is CVE-2026-16729? CVE-2026-16729 is a medium-severity security vulnerability in undici (npm), affecting versions < 6.28.0. It is fixed in 6.28.0, 7.29.0, 8.9.0.
2. How severe is CVE-2026-16729? CVE-2026-16729 has a CVSS score of 4.8 (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 undici are affected by CVE-2026-16729? undici (npm) versions < 6.28.0 is affected.
4. Is there a fix for CVE-2026-16729? Yes. CVE-2026-16729 is fixed in 6.28.0, 7.29.0, 8.9.0. Upgrade to this version or later.
5. Is CVE-2026-16729 exploitable, and should I be worried? Whether CVE-2026-16729 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-2026-16729 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-2026-16729?
   • Upgrade undici to 6.28.0 or later
   • Upgrade undici to 7.29.0 or later
   • Upgrade undici to 8.9.0 or later

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