CVE-2026-79674 in NLTK
Summary
by MITRE • 08/25/2026
NLTK versions before 3.10.3 contain a path sandbox bypass vulnerability in corpus-reader constructors that allows attackers to read files outside the intended data root. Attackers can supply arbitrary corpus root paths to LinThesaurusCorpusReader and PanLexLiteCorpusReader constructors to access filesystem content and SQLite databases outside the pathsec sandbox boundary.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in Natural Language Toolkit (NLTK) versions prior to 3.10.3 represents a critical failure in input validation within corpus-reader constructors, specifically affecting LinThesaurusCorpusReader and PanLexLiteCorpusReader classes. This flaw constitutes a path sandbox bypass that undermines the security boundary designed to restrict file access to designated data directories. In typical usage scenarios, NLTK applications are expected to operate within a confined environment where they can only read from specific corpus roots defined by the developer or administrator. However, due to insufficient sanitization of user-supplied paths passed during object instantiation, an attacker who controls these inputs can manipulate the underlying file system operations to traverse outside the intended sandbox boundary. This lack of strict path validation allows for arbitrary file access beyond the scope of the application's security policy.
From a technical perspective, the core issue lies in how the constructors handle relative and absolute paths without adequately resolving or restricting them against the configured root directory. By supplying crafted arguments that include directory traversal sequences such as dot-dot-slash patterns or by leveraging absolute path specifications, an attacker can redirect file read operations to sensitive locations on the host system. This capability extends beyond simple text files; it includes access to SQLite databases and other structured data stores that may reside outside the corpus root. The exploitation of this flaw does not require complex binary manipulation but rather relies on precise string construction within the application's API calls, making it relatively straightforward for an attacker with any level of interaction with these specific NLTK components.
The operational impact of this vulnerability is significant, particularly in environments where NLTK processes untrusted or semi-trusted data sources. If a web service or automated pipeline utilizes vulnerable versions of NLTK to process user-provided corpus configurations, it becomes susceptible to unauthorized information disclosure. Attackers can exfiltrate sensitive configuration files, database credentials stored within SQLite databases, internal logs, or other proprietary data residing on the server's file system. This breach of confidentiality compromises the integrity and security posture of the hosting environment, potentially leading to further exploitation if the accessed data contains additional secrets or keys that facilitate lateral movement or privilege escalation within the network infrastructure.
This vulnerability aligns with CWE-22 Improper Limitation of a Pathname to a Restricted Directory, as it involves failing to restrict file access to an intended directory structure. Furthermore, in the context of attack tactics, this behavior corresponds to ATT&CK technique T1083 File and Directory Discovery, where adversaries explore the system to gather information for subsequent stages of an intrusion. The ability to read arbitrary files is often a precursor to more severe attacks such as credential theft or remote code execution if combined with other vulnerabilities in the application stack.
To mitigate this risk, organizations must immediately upgrade NLTK to version 3.10.3 or later, where these path validation mechanisms have been hardened. For applications that cannot be updated instantly due to dependency constraints, implementing strict input validation at the application layer is essential. Developers should enforce allow-listing of permitted corpus roots and reject any paths containing traversal characters or absolute references outside the expected directory tree. Additionally, running NLTK processes with minimal file system permissions using principles of least privilege can limit the damage even if a bypass occurs. Regular security audits focusing on third-party library usage and path handling logic are recommended to prevent similar sandbox escape vulnerabilities in future deployments.