Vulnerability in i18next-fs-backend (CVE-2026-48713)
Summary
Vulnerability in i18next-fs-backend (CVE-2026-48713). Data can be tampered with by attackers. Exploitable via missingKeyHandler. Mitigation: upgrade to 2.6.4 or later.
Original description
The description as published by the source.
Versions prior to 2.6.6 are vulnerable to prototype pollution via crafted missing-key strings when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input). 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. Applications are affected only if the missingKeyHandler (or another route that forwards untrusted request bodies to i18next.t(..., { ... }) with saveMissing: true) is reachable by untrusted users and the default behaviour of splitting missing-key strings on keySeparator is in use (i.e. keySeparator is not false). Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path. This issue has been fixed in version 2.6.6. If developers using the library are unable to upgrade immediately, they should take the following precautions: do not expose i18next-http-middleware's missingKeyHandler to untru…
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-48713 has been found in i18next-fs-backend.
Attackers can target a specific entry point like missingKeyHandler over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 9.1/10.
What to do: upgrade i18next-fs-backend to 2.6.4 or later.
If unsure, ask your IT team or search "i18next-fs-backend CVE-2026-48713" on the vendor's site.
CVE-2026-48713 (i18next-fs-backend) — CWE-1321 / CVSS v3 9.1
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: missingKeyHandler / keySeparator / getLastOfPath / Object.prototype
Patched: 2.6.4 — apply immediately
Workaround: set keySeparator: false
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-1321) exists in i18next-fs-backend. Attackers reach the vulnerable code path via missingKeyHandler without authentication.
Check the vendor advisory for the affected versions of i18next-fs-backend. Attack surface: missingKeyHandler / keySeparator / getLastOfPath / Object.prototype.
Severity: Critical (CVSS 9.1/10). Data can be tampered with by attackers.
Update to 2.6.4.
Workaround: set keySeparator: false
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'i18next-fs-backend' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
grep -r 'i18next-fs-backend' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
i18next-fs-backendを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply temporary workaround mitigate
set keySeparator: falseパッチが適用されるまでの応急処置として、set keySeparator: false を実施。回避策の副作用 (機能低下) を確認した上で。
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3Apply patch patch
Upgrade i18next-fs-backend to 2.6.4ステージング環境で 2.6.4 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
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4Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory github.com
- advisory nvd.nist.gov
- advisory github.com
- package github.com
- patch github.com
- web github.com