GHSA-f49m-vf83-692w is a critical-severity (CVSS 9.1) CWE-1321 vulnerability in i18next-http-middleware. A fix is available for i18next-http-middleware — see the affected versions and patch details below.
i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-f49m-vf83-692w.
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
GHSA-f49m-vf83-692w 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.
i18next-http-middlewarenpmDescription
Impact
i18next-http-middleware ≤ 3.9.6's missingKeyHandler blocked the literal request-body keys __proto__, constructor, and prototype (added in 3.9.3, see GHSA-5fgg-jcpf-8jjw), but did not reject dotted variants such as "__proto__.polluted". Downstream backends that split the missing-key string on a configured keySeparator (notably i18next-fs-backend ≤ 2.6.5) hand these keys to an unguarded setPath() walker that writes to Object.prototype.
Applications that expose missingKeyHandler to untrusted input AND use i18next-fs-backend ≤ 2.6.5 are directly exploitable for remote prototype pollution. Other downstream backends that split the missing-key string the same way may be similarly affected.
Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.
Patches
Fixed in i18next-http-middleware 3.9.7. A new utils.hasUnsafeKeySegment(key, keySeparator) helper is now used by missingKeyHandler; the configured i18next.options.keySeparator is honoured (default .; false disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. "header.title") are unaffected.
The root-cause fix has been shipped in i18next-fs-backend 2.6.6 — see the companion advisory.
Workarounds
If users cannot upgrade immediately:
- Do not expose
missingKeyHandlerto untrusted users (mount it behind authentication, or remove the route). - Add a request-body filter ahead of the handler that rejects any top-level key containing
__proto__,constructor, orprototypeafter splitting on a configuredkeySeparator. - Disable missing-key persistence (
saveMissing: false) when accepting writes from untrusted input.
Resources
- Original report by @codeswhite.
- Companion advisory in
i18next-fs-backend: GHSA-2933-q333-qg83. - Previous
i18next-http-middlewaresecurity release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | i18next-http-middleware | all versions | 3.9.7npm install i18next-http-middleware@3.9.7 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for i18next-http-middleware, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update i18next-http-middleware to 3.9.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-f49m-vf83-692w 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 GHSA-f49m-vf83-692w can be triaged on real exposure rather than presence alone.
Tailored to GHSA-f49m-vf83-692w. 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-f49m-vf83-692w in your dependencies?
O3 Security finds GHSA-f49m-vf83-692w across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.