CVE-2026-35209
HIGHCVE-2026-35209 is a high-severity (CVSS 7.5) CWE-1321 vulnerability in defu. O3 Security confirms whether CVE-2026-35209 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
defu: Prototype pollution via `__proto__` key in defaults argument
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
defunpmDescription
defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to defu() are vulnerable to prototype pollution. A crafted payload containing a __proto__ key can override intended default values in the merged resul. The internal _defu function used Object.assign({}, defaults) to copy the defaults object. Object.assign invokes the __proto__ setter, which replaces the resulting object's [[Prototype]] with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing __proto__ key guard in the for...in loop and land in the final result. Version 6.1.5 replaces Object.assign({}, defaults) with object spread ({ ...defaults }), which uses [[DefineOwnProperty]] and does not invoke the __proto__ setter.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | defu | all versions | 6.1.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for defu. 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 defu to 6.1.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-35209 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 CVE-2026-35209 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 CVE-2026-35209. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-35209 in your dependencies?
O3 detects CVE-2026-35209 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.