Summary
i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names
Full technical description
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).
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.
CVE-2026-48714 has a CVSS score of 9.1 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.9.7); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2026-48714? CVE-2026-48714 is a critical-severity security vulnerability in i18next-http-middleware (npm), affecting versions < 3.9.7. It is fixed in 3.9.7.
- How severe is CVE-2026-48714? CVE-2026-48714 has a CVSS score of 9.1 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of i18next-http-middleware are affected by CVE-2026-48714? i18next-http-middleware (npm) versions < 3.9.7 is affected.
- Is there a fix for CVE-2026-48714? Yes. CVE-2026-48714 is fixed in 3.9.7. Upgrade to this version or later.
- Is CVE-2026-48714 exploitable, and should I be worried? Whether CVE-2026-48714 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48714 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48714? Upgrade
i18next-http-middlewareto 3.9.7 or later.