Attack Chain

1. An attacker identifies an application using a vulnerable version of i18next-fs-backend (versions prior to 2.6.4) and exposes the language code to user input via query parameters (e.g., ?lng=), cookies, or request headers.
2. The attacker crafts a malicious lng value containing directory traversal sequences, such as ../../../../etc, to target sensitive files outside the intended locale directory.
3. The attacker sends a request to the application with the crafted lng parameter.
4. The application passes the unsanitized lng value to the i18next.t() function.
5. The i18next-fs-backend library interpolates the malicious lng value into the loadPath configuration option, without proper validation. For example, loadPath: '/locales/{{lng}}/{{ns}}.json' becomes /locales/../../../../etc/{{ns}}.json.
6. The backend attempts to read the file specified by the crafted path (e.g., /etc/passwd).
7. If successful, the contents of the targeted file are returned as a translation resource, potentially exposing sensitive information. If the attacker crafted the lng or ns value to point to a .js or .ts file containing malicious code, the backend will execute the file using eval(), leading to arbitrary code execution on the server.
8. Alternatively, if the application attempts to write a missing translation key using the crafted path (via addPath), the attacker could overwrite arbitrary files on the system, potentially leading to application compromise.

Impact

Successful exploitation of this vulnerability can have severe consequences. Arbitrary file read allows attackers to access sensitive data, such as configuration files, database credentials, or application source code. Arbitrary file overwrite can lead to application malfunction or complete compromise. If the application uses .js or .ts files for localization and the attacker is able to inject malicious code into those files through path traversal, arbitrary code execution can result, potentially allowing the attacker to gain full control of the server. The number of victims depends on the popularity and configuration of applications using the vulnerable i18next-fs-backend library.

Recommendation

• Upgrade to i18next-fs-backend version 2.6.4 or later to patch the path traversal vulnerability as this version introduces the isSafePathSegment and interpolatePath functions to sanitize the path.
• If upgrading is not immediately feasible, sanitize the lng and ns values at the application boundary before passing them to i18next. Reject values containing .., /, \, control characters, and limit the length to prevent path traversal as mentioned in the advisory.
• If using .js or .ts locale files, carefully review them for any suspicious or unexpected code. The advisory highlights that these files must be treated as trusted code.
• Monitor web server logs for suspicious requests containing directory traversal sequences in the lng or ns parameters. Deploy the first Sigma rule for this purpose.