CVE-2026-11525

undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching

Workarounds

After parsing a Set-Cookie header, validate that the resulting sameSite attribute is one of 'Strict', 'Lax', or 'None' (exact, case-insensitive) before forwarding or relying on it.

When undici parses a Set-Cookie header, it accepts any SameSite attribute value that contains Strict, Lax, or None as a substring, rather than the case-insensitive exact match specified by RFC 6265. Non-spec values are silently mapped to one of the three standard tokens:

• SameSite=NoneOfYourBusiness is parsed as None, the most permissive setting.
• SameSite=StrictLax is parsed as Lax, a downgrade from Strict.

Affected applications are those that consume Set-Cookie headers from server responses (for example via undici's fetch or proxy code paths) and then forward or rely on the parsed sameSite attribute. A malicious or non-compliant server can coerce the consumer's view of a cookie's SameSite policy to a weaker value, silently degrading the SameSite enforcement the cookie is supposed to provide.

This was introduced in undici 5.15.0 when the cookies feature was added.

CVE-2026-11525 has a CVSS score of 3.7 (Low). 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 (6.27.0, 7.28.0, 8.5.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to undici v6.27.0, v7.28.0 or v8.5.0.

1. What is CVE-2026-11525? CVE-2026-11525 is a low-severity security vulnerability in undici (npm), affecting versions < 6.27.0. It is fixed in 6.27.0, 7.28.0, 8.5.0.
2. How severe is CVE-2026-11525? CVE-2026-11525 has a CVSS score of 3.7 (Low). 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-2026-11525? undici (npm) versions < 6.27.0 is affected.
4. Is there a fix for CVE-2026-11525? Yes. CVE-2026-11525 is fixed in 6.27.0, 7.28.0, 8.5.0. Upgrade to this version or later.
5. Is CVE-2026-11525 exploitable, and should I be worried? Whether CVE-2026-11525 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-2026-11525 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-2026-11525?
   • Upgrade undici to 6.27.0 or later
   • Upgrade undici to 7.28.0 or later
   • Upgrade undici to 8.5.0 or later

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