---
id: CVE-2026-11525
title: >-
  undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive
  substring matching
summary: >-
  undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive
  substring matching
severity: low
cvss: 3.7
cwe:
  - CWE-183
vendor: undici
product: undici
affected:
  - undici < 6.27.0
  - 'undici >= 7.0.0, < 7.28.0'
  - 'undici >= 8.0.0, < 8.5.0'
patched:
  - undici 6.27.0
  - undici 7.28.0
  - undici 8.5.0
published: '2026-06-19'
updated: '2026-06-19'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-g8m3-5g58-fq7m'
references:
  - url: 'https://github.com/nodejs/undici/security/advisories/GHSA-g8m3-5g58-fq7m'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-11525'
  - url: 'https://cna.openjsf.org/security-advisories.html'
  - url: 'https://github.com/advisories/GHSA-g8m3-5g58-fq7m'
tags:
  - ghsa
  - npm
epss: 0.00238
epssPercentile: 0.13294
ingestedAt: '2026-06-22T15:59:08.194Z'
ecosystem: npm
---

## Overview

## 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.

## Affected packages

- `undici < 6.27.0`
- `undici >= 7.0.0, < 7.28.0`
- `undici >= 8.0.0, < 8.5.0`

## Remediation

Upgrade to a patched release:

- `undici 6.27.0`
- `undici 7.28.0`
- `undici 8.5.0`