Summary
i18next-fs-backend vulnerable to prototype pollution via crafted missing-key string
Affected configuration
Applications are directly affected only if all of the following hold:
i18next-fs-backend≤ 2.6.5 is configured as the backend.i18next-http-middleware'smissingKeyHandler(or another route that forwards untrusted request bodies toi18next.t(..., { ... })withsaveMissing: true) is reachable by untrusted users.- The default behaviour of splitting missing-key strings on
keySeparatoris in use (i.e.keySeparatoris notfalse).
Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path.
Workarounds
If users cannot upgrade immediately:
- Do not expose
i18next-http-middleware'smissingKeyHandlerto untrusted users (mount it behind authentication, or remove the route). - Disable missing-key persistence (
saveMissing: false, or nobackend.createimplementation) when accepting writes from untrusted input. - Set
keySeparator: falsein the i18next options to disable backend key splitting (note: this also disables nested translation keys).
Resources
- Original report by @codeswhite.
- Companion advisory in
i18next-http-middleware: GHSA-f49m-vf83-692w. - Previous
i18next-fs-backendsecurity release: GHSA-8847-338w-5hcj (path traversal vialng/ns, fixed in 2.6.4).
Impact
i18next-fs-backend ≤ 2.6.5, when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input), is vulnerable to prototype pollution via crafted missing-key strings.
Backend.writeFile() splits each queued missing-key string on the configured keySeparator (default .) before calling the internal setPath() walker. The walker (getLastOfPath in lib/utils.js) did not guard against unsafe segments, so a key like "__proto__.polluted" was split into ["__proto__", "polluted"] and walked straight into Object.prototype, allowing an attacker to write arbitrary properties onto the global object prototype.
Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.
CVE-2026-48713 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 (2.6.6); 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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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixed in i18next-fs-backend 2.6.6. The traversal helper now refuses to descend through __proto__, constructor, or prototype segments and drops the offending write silently. Legitimate dotted keys (e.g. "header.title") are unaffected.
A matching defence-in-depth fix has been shipped in i18next-http-middleware 3.9.7, see the companion advisory.
Frequently Asked Questions
- What is CVE-2026-48713? CVE-2026-48713 is a critical-severity security vulnerability in i18next-fs-backend (npm), affecting versions < 2.6.6. It is fixed in 2.6.6.
- How severe is CVE-2026-48713? CVE-2026-48713 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-fs-backend are affected by CVE-2026-48713? i18next-fs-backend (npm) versions < 2.6.6 is affected.
- Is there a fix for CVE-2026-48713? Yes. CVE-2026-48713 is fixed in 2.6.6. Upgrade to this version or later.
- Is CVE-2026-48713 exploitable, and should I be worried? Whether CVE-2026-48713 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-2026-48713 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-2026-48713? Upgrade
i18next-fs-backendto 2.6.6 or later.