NLTK Symlink Escape Vulnerability Before 3.9.4

Executive Summary

This vulnerability affects NLTK versions before 3.9.4. It allows local attackers to read arbitrary files outside the intended corpus root directory through a symlink escape in the CorpusReader.open() function. The issue occurs because path validation is lexical and does not consider symlink resolution, enabling attackers to place symlinks inside the corpus root that point to files outside the allowed boundary.

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Detection Guidance

To detect this vulnerability, check the installed NLTK version using pip show nltk. If the version is less than 3.9.4, the system is vulnerable. Additionally, review any custom NLTK corpus readers for improper symlink handling in CorpusReader.open().

Commands: pip show nltk | grep Version; find / -type l -name '*.txt' 2>/dev/null | xargs ls -l 2>/dev/null

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Impact Analysis

An attacker could exploit this to read sensitive files such as application secrets, configuration files, or system files accessible by the application user. This could lead to unauthorized data exposure, potential privilege escalation, or further attacks if sensitive information is compromised.

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Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, violating compliance requirements for data protection such as GDPR or HIPAA. Exposure of personal or health information may result in legal penalties, reputational damage, and loss of trust due to non-compliance with regulatory standards.

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Mitigation Strategies

Upgrade NLTK to version 3.9.4 or later immediately using pip install --upgrade nltk. Review and restrict write permissions to NLTK corpus directories to prevent symlink creation. Audit applications using NLTK for exposed sensitive files.

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