Arbitrary File Write via Archive Extraction (Zip Slip) Affecting nltk package, versions [,3.4.5)

nltk is a Natural Language Toolkit (NLTK) is a Python package for natural language processing.

Affected versions of this package are vulnerable to Arbitrary File Write via Archive Extraction (Zip Slip). It allows attackers to write arbitrary files via a ../ (dot dot slash) in an NLTK package (ZIP archive) that is mishandled during extraction.

It is exploited using a specially crafted zip archive, that holds path traversal filenames. When exploited, a filename in a malicious archive is concatenated to the target extraction directory, which results in the final path ending up outside of the target folder. For instance, a zip may hold a file with a "../../file.exe" location and thus break out of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicous file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

+2018-04-15 22:04:29 ..... 19 19 good.txt

+2018-04-15 22:04:42 ..... 20 20 ../../../../../../root/.ssh/authorized_keys

• GitHub Commit

• Mike Salvatore's Article on Zip-slip

• NLTK Changelog

• Proof of Concept (Mike Salvatore)