CVE-2026-41693 — i18next-fs-backend
HIGHCVE-2026-41693 is a high-severity (CVSS 8.2) Path Traversal vulnerability in i18next-fs-backend. A fix is available for i18next-fs-backend — see the affected versions and patch details below.
i18next-fs-backend: Path traversal via unsanitised lng/ns allows arbitrary file read/overwrite
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 CVE-2026-41693.
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-2026-41693 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 378,567 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-fs-backendnpmDescription
Summary
Versions of i18next-fs-backend prior to 2.6.4 interpolate the caller-supplied lng and ns values directly into the configured loadPath and addPath templates with no path-component validation and no sanitisation. When an application exposes the resolved language code to user-controlled input (?lng= query parameter, cookie, request header), a crafted value can break out of the intended locale directory.
Affected call sites in lib/index.js:
read(line 38 pre-patch):const filename = interpolate(loadPath, { lng: language, ns: namespace })removeFile(line 101 pre-patch): same pattern againstaddPathwriteFile(line 127 pre-patch): same pattern againstaddPathfor queued missing-key writes
The helper interpolate in lib/utils.js substitutes raw values with no encoding — unlike the addQueryString helper in i18next-http-backend, there is no equivalent safety for path interpolation.
Impact
- Arbitrary file read. With a
loadPathlike/locales/{{lng}}/{{ns}}.json, an attacker-controlledlng = '../../etc'(and matchingns) causes the backend to read a file outside the locale directory. For parsers that tolerate arbitrary content (YAML's freeform text), the file contents surface as a translation resource. - Arbitrary file overwrite.
addPathis interpolated the same way for missing-key writes (thecreate()code path and the debounced writer inwriteFile). A traversinglng/nscombination can cause the process to write JSON structures to an unintended filesystem location, potentially overwriting application files if the process user has write access. - Chain with
.js/.tseval.i18next-fs-backendsupports loading.jsand.tslocale files byeval-ing their content (intentional feature, documented as requiring trusted sources). Combining traversal with that path — for examplelng = '../../../app/config'againstloadPath: '/locales/{{lng}}/{{ns}}.js'— would cause the backend to execute a server-side file as JavaScript, exfiltrating whatever it can touch (process.env, connected services).
Exploitation requires the application to pass an untrusted lng/ns value through to i18next.t() without its own validation. Many i18next setups do exactly this via i18next-browser-languagedetector (query string / cookie detection).
Affected versions
All versions of i18next-fs-backend prior to 2.6.4.
Patch
Fixed in 2.6.4. lib/utils.js now exports:
isSafePathSegment(v)— returnstrueonly ifvis a non-empty string of ≤ 128 chars that does not contain..,/,\, control characters, or a prototype key (__proto__,constructor,prototype). Legitimate i18next language-code shapes (BCP-47,en_US,zh-Hant-HK,pirate-speak,my-custom.ns,+-joined multi-language values) all pass.interpolatePath(template, data)— substitutes variables like the existinginterpolatebut refuses the whole result if any segment failsisSafePathSegment. Callers bail out with an error (read) or silently drop the queued write (writeFile,removeFile).
The .js / .ts eval behaviour is intentionally retained — dynamic expressions in locale files are a documented feature of this backend, and safe replacements like dynamic import() are async-only and incompatible with this backend's sync-capable code path. The README has a new "Security considerations" section that spells out the trust model: .js/.ts locale files must be treated as code.
Workarounds
No workaround short of upgrading. If you cannot upgrade immediately, sanitise lng / ns at your application boundary before passing them to i18next — reject values containing .., /, \, control characters, and cap the length.
Credits
Discovered via an internal security audit of the i18next ecosystem.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | i18next-fs-backend | all versions | 2.6.4npm install i18next-fs-backend@2.6.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for i18next-fs-backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update i18next-fs-backend to 2.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-41693 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-2026-41693 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-41693. 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-2026-41693 in your dependencies?
O3 Security finds CVE-2026-41693 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.