NLTK: Downloader.download follows hardlinks and overwrites outside-root files

Summary

NLTK's downloader now blocks symlink escapes during ZIP extraction, but it still treats pre-existing hardlinks inside the install tree as ordinary in-root files. A normal package install can therefore overwrite an outside-root inode through that hardlink.

Details

• Vulnerability type: Filesystem containment bypass
• Affected component: nltk.downloader.Downloader.download, nltk.downloader.Downloader.incr_download
• Affected versions: Published 3.9.4 and current source v3.10.0-rc2 both reproduced for the extraction-stage overwrite.
• Patched versions: 3.10.3
• Root cause: The downloader validates traversal and symlink conditions but does not reject pre-existing hardlink aliases inside the install tree.

The install flow correctly rejects a pre-existing symlink at an extraction target, yet it accepts a pre-existing hardlink at the same path. When the package is installed, extracted member data is written through the hardlink and mutates the outside inode.

PoC

Preconditions

• The attacker can plant files inside a writable shared downloader root on the same filesystem as the target file.

Steps

1. Prepare a downloader root and create a hardlink inside it that points to an outside target file.
2. Confirm a symlink at the same path is rejected as a negative control.
3. Run a normal Downloader.download() package install whose extracted member lands on the hardlink path.
4. Observe the outside target file is overwritten while the downloader still reports the package as installed.

Minimal reproducible excerpt

extract_hardlink_before ORIGINAL
extract_hardlink_after PWNED
extract_hardlink_status installed

Impact

A shared or attacker-influenced downloader directory can be turned into an overwrite primitive against same-filesystem files outside the intended install root.

Remediation

Treat pre-existing hardlinks as unsafe in extraction targets, verify that each write path stays within the intended install tree at the inode level, and add regression tests that pair hardlinks with existing symlink controls.