Gptline: NLTK: FileSystemPathPointer.open() sandbox check is dead code — arbitrary file read via file:// protocol (CVE-2026-65915)

The vulnerability (CVE-2026-65915) arises from a dead code sandbox check in nltk/data.py's FileSystemPathPointer.open() method. The check compares a normalized path against itself, which is always true, so the intended ValueError to block absolute file access never triggers. Since the path is already absolute-normalized during initialization, the check is ineffective. Consequently, any file accessible to the process can be read by passing a file:// URL to nltk.data.load() or nltk.data.find(). The recommended fix replaces the faulty check with a validation against a whitelist of allowed data directories using os.path.commonpath to ensure the requested file path is within permitted locations.

Applications that allow user input to influence the argument to nltk.data.load() or nltk.data.find() are vulnerable to arbitrary file read attacks. An attacker can read sensitive files such as /etc/passwd, environment files, private keys, or credentials accessible to the process user. This can lead to information disclosure but does not allow code execution or data modification. The CVSS v3.1 score is 6.5 (medium severity) reflecting network attack vector, low complexity, low privileges required, no user interaction, and high confidentiality impact.

A patch is available that corrects the sandbox check in FileSystemPathPointer.open() by validating file paths against an allowed list of nltk data directories. Users should update to a fixed version (>=3.10.0 for NLTK) or apply the patch to ensure the sandbox check properly restricts file access. Until patched, avoid passing untrusted input to nltk.data.load() or nltk.data.find().