CVE-2026-11525 undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching
Attackers can downgrade cookie SameSite protections by exploiting permissive substring matching in undici's Set-Cookie parser, weakening CSRF defenses.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-11525?
CVE-2026-11525 is a low-severity vulnerability in Nodejs Undici, published on 17 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Impact: 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. For example, SameSite=NoneOfYourBusiness is parsed as None (the most permissive setting), and 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.
Patches: Upgrade to undici v6.26.0, v7.28.0 or v8.5.0.
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.
Weakness: CWE-183
Which versions are affected by CVE-2026-11525?
6 open source packages ship an affected version, including undici. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
undici npm | All versions | 6.27.0 |
undici npm | All versions | 6.27.0 |
undici npm | From 7.0.0 | 7.28.0 |
undici npm | From 7.0.0 | 7.28.0 |
undici npm | From 8.0.0 | 8.5.0 |
undici npm | From 8.0.0 | 8.5.0 |
Products include Nodejs Undici.
How do I fix CVE-2026-11525?
Upgrade undici to a fixed version for your release line: 6.27.0, 7.28.0 or 8.5.0.
Is CVE-2026-11525 being exploited?
There is no known exploitation of CVE-2026-11525 in the wild and no public exploit code.
Other vulnerabilities in Nodejs Undici
- CVE-2026-109855 undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake
- CVE-2026-109869 undici vulnerable to Denial of Service via unhandled error on excessive content-encodings
- CVE-2026-87018 undici vulnerable to Denial of Service via TLS 1.2 certificate-changing renegotiation
- CVE-2026-19534 undici vulnerable to Denial of Service via unrequested WebSocket subprotocol
- CVE-2026-12151 undici WebSocket client vulnerable to denial of service via fragment count bypass
Sources
Also known as AZL-91194, CLEANSTART-2026-BZ34895, CLEANSTART-2026-RS05836, DEBIAN-CVE-2026-11525, ECHO-e368-9a50-56c5, GHSA-g8m3-5g58-fq7m and 18 more.