GHSA-g8m3-5g58-fq7m
LOWundici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
undicinpmDescription
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=NoneOfYourBusinessis parsed asNone, the most permissive setting.SameSite=StrictLaxis parsed asLax, a downgrade fromStrict.
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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | undici | all versions | 6.27.0 |
| 📦npm | undici | ≥ 7.0.0&&< 7.28.0 | 7.28.0 |
| 📦npm | undici | ≥ 8.0.0&&< 8.5.0 | 8.5.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for undici. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update undici to 6.27.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g8m3-5g58-fq7m is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-g8m3-5g58-fq7m is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-g8m3-5g58-fq7m. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-g8m3-5g58-fq7m in your dependencies?
O3 detects GHSA-g8m3-5g58-fq7m across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.