CVE-2026-62384 in NLTK
Summary
by MITRE • 08/22/2026
NLTK versions before 3.10.2 contain a symlink-based sandbox bypass in FramenetCorpusReader that allows attackers to read arbitrary XML files outside the corpus root. Attackers can place symlinks with names containing no path separators inside the corpus subdirectory, which pass the path validation guard and are resolved to files outside the intended corpus root when accessed via frame_by_name(), _lu_file(), or doc() methods.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability identified in Natural Language Toolkit (NLTK) versions prior to 3.10.2 represents a critical security flaw within the FramenetCorpusReader component, specifically classified under CWE-59: Improper Link Resolution Before File Access. This issue stems from an insufficient validation of file paths when handling corpus data, allowing for a symlink-based sandbox escape that compromises the integrity and confidentiality of the system's filesystem access controls. The core technical flaw lies in how the reader processes filenames provided to methods such as frame_by_name(), _lu_file(), and doc(). These functions are designed to restrict access strictly within the boundaries of the specified corpus root directory, yet they fail to adequately resolve or validate symbolic links before performing file operations.
Attackers can exploit this weakness by placing symlinks with names that contain no path separators inside a corpus subdirectory. Because the validation logic primarily checks for explicit path traversal characters like dots and slashes within the filename string itself, these carefully crafted symlinks bypass the security guard. When an application invokes one of the affected methods to access data associated with such a symlink name, the operating system resolves the symbolic link at runtime rather than treating it as a simple file name. This resolution process allows the reader to dereference the link and read arbitrary XML files located outside the intended corpus root directory. Consequently, this mechanism enables unauthorized information disclosure by granting attackers the ability to access sensitive configuration files, credentials stored in XML formats, or other proprietary data residing on the host system but not part of the NLTK dataset.
The operational impact of this vulnerability is significant for any application relying on NLTK's FramenetCorpusReader to process linguistic data from untrusted sources. If an attacker can influence the input passed to these methods, they can leverage the symlink resolution behavior to read files that should remain inaccessible due to sandboxing restrictions. This capability effectively neutralizes the directory isolation intended by the corpus reader implementation. In environments where NLTK is used within web applications or services processing user-supplied data, this flaw could lead to severe confidentiality breaches. The ability to read arbitrary XML files may expose internal system structures, database connection strings, or other critical configuration details that aid in further exploitation of the target environment.
To mitigate this vulnerability, organizations must upgrade NLTK to version 3.10.2 or later, where the path validation logic has been hardened to properly resolve and verify symlinks against the corpus root boundary before allowing access. For systems unable to immediately patch due to dependency constraints, defensive coding practices should be implemented as a temporary countermeasure. Developers can enforce strict input sanitization by rejecting any filenames containing symbolic link indicators or by explicitly checking if resolved paths remain within the expected directory using canonical path resolution functions provided by the underlying operating system's file API. Additionally, running NLTK processes with restricted filesystem permissions and utilizing chroot jails or containerized environments can limit the blast radius of such an exploit should it be attempted against unpatched systems. This incident underscores the importance of rigorous input validation in libraries that handle external data sources, aligning with broader security standards regarding secure file handling practices outlined in CWE-59 and related ATT&CK techniques involving unauthorized access to system resources through path manipulation.