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Low, CVSS 3.7amended

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

CVE-2026-11525GHSA-g8m3-5g58-fq7m

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (undici 8.5.0, 7.28.0, 6.27.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm undici

    Affected
    < 6.27.0>= 7.0.0, < 7.28.0>= 8.0.0, < 8.5.0
    Fixed in
    6.27.0, 7.28.0, 8.5.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS
  • CVE-2026-11525: 0.24% estimated exploitation activity in the next 30 days; percentile 14.99 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-g8m3-5g58-fq7m

  2. Revision 1 · created ·

    Published from osv record GHSA-g8m3-5g58-fq7m

Agent brief

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# Patch Next advisory brief: CVE-2026-11525

Advisory: undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching
IDs: CVE-2026-11525, GHSA-g8m3-5g58-fq7m
Record: https://patchnext.com/advisories/cve-2026-11525 (revision 2, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: undici 8.5.0, 7.28.0, 6.27.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm undici: affected < 6.27.0; >= 7.0.0, < 7.28.0; >= 8.0.0, < 8.5.0; fixed in 6.27.0, 7.28.0, 8.5.0

## Suggested inspection (read-only)
- `npm ls undici --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on undici.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-g8m3-5g58-fq7m (OSV record GHSA-g8m3-5g58-fq7m lists affected ranges: npm undici >=0 =7.0.0 =8.0.0 <8.5.0 (fixed))
- Verified: https://github.com/advisories/GHSA-g8m3-5g58-fq7m (GitHub-reviewed advisory GHSA-g8m3-5g58-fq7m lists affected ranges: npm undici >=0 =7.0.0 =8.0.0 <8.5.0)

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