CVE-2020-28279 is a critical-severity (CVSS 9.8) remote code execution vulnerability in flattenizer. 1 public exploit reference exists, so weaponization risk is real. A fix is available for flattenizer — see the affected versions and patch details below.
flattenizer vulnerable to prototype pollution
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2020-28279 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 376,715 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
flattenizernpmDescription
Overview
Prototype pollution vulnerability in ‘flattenizer’ versions 0.0.5 through 1.0.5 allows an attacker to cause a denial of service and may lead to remote code execution.
Details
The NPM module 'flattenizer' can be abused by Prototype Pollution vulnerability since the function 'unflatten()' did not check for the type of object before assigning value to the property. Due to this flaw an attacker could create a non-existent property or able to manipulate the property which leads to Denial of Service or potentially Remote code execution.
PoC Details
There is no validation before assigning the property to check whether the assigned argument is the Object's own property or not, the property polluted will be directly be assigned thereby polluting the Object prototype. Later in the code, if there is a check to validate polluted the valued would be substituted as "true" as it had been polluted.
var flattenizer = require("flattenizer")
flattenizer.unflatten({'__proto__.polluted': true});
console.log(polluted);
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | flattenizer | ≥ 0.0.5&&< 1.1.1 | 1.1.1npm install flattenizer@1.1.1 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flattenizer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update flattenizer to 1.1.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-28279 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2020-28279 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2020-28279. 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-2020-28279 in your dependencies?
O3 Security finds CVE-2020-28279 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.