undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching (CVE-2026-11525) | HOL Guard CVE

Low · CVSS 3.7CVE-2026-11525GHSA-G8M3-5G58-FQ7M

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

Answer in brief

CVE-2026-11525 records a Low severity (CVSS 3.7) vulnerability in undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching. The current sources do not mark it as known exploited. The current feed maps undici (npm), undici (npm), undici (npm), undici (npm) and additional mapped packages. Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Jun 17, 2026Updated Sep 10, 2026Source checked Sep 17, 2026First seen by HOL Jun 19, 2026Material review Sep 10, 2026

Upstream Advisory

Key facts

Risk: Low · CVSS 3.7
Exploitation: Not marked as known exploited
Affected software: 6 mapped packages or products
Fix availability: Available

Why this deserves its current priority

CVSS is 3.7. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps undici (npm), undici (npm), undici (npm), undici (npm) and additional mapped packages. Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
undicinpm	<6.27.0	6.27.0
undicinpm	>=7.0.0,<7.28.0	7.28.0
undicinpm	>=8.0.0,<8.5.0	8.5.0
undicinpm	>=0 <6.27.0	6.27.0
undicinpm	>=7.0.0 <7.28.0	7.28.0
undicinpm	>=8.0.0 <8.5.0	8.5.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for undici, undici, undici.
Review the reported fix. Update undici to 6.27.0; undici to 7.28.0; undici to 8.5.0; undici to 6.27.0; undici to 7.28.0; undici to 8.5.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Jun 17, 2026
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Jun 19, 2026

Sources and claim methodology

Upstream source 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: - `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. ## Patches Upgrade to undici v6.27.0, 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.

Quoted source text, attributed separately from HOL analysis.