NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incr_download methods that allows attackers to overwrite files outside the install root through pre-existing hardlinks. Attackers with write access to a shared downloader directory can create hardlinks pointing to outside-root files that are then overwritten during normal package extraction, mutating files outside the intended install tree.

NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability affecting the Downloader.download and Downloader.incr_download methods. The flaw allows attackers with write access to a shared downloader directory to create hardlinks pointing to files outside the intended install root. During normal package extraction, these hardlinks cause files outside the install tree to be overwritten. This constitutes a path traversal/containment bypass that can lead to arbitrary file overwrite on the host system. The vulnerability is exploitable in shared or multi-user environments where the downloader directory is accessible to multiple users. It has been patched in NLTK version 3.10.3. Advisories have been published on GitHub Security Advisories and VulnCheck.