NLTK before 3.10.0 contains an arbitrary local file read vulnerability in StreamBackedCorpusView that bypasses pathsec.ENFORCE by calling builtins.open() directly instead of pathsec.open(). Attackers who control the fileid argument can read arbitrary local files regardless of the ENFORCE setting, including sensitive system files and application credentials.

NLTK versions before 3.10.0 contain an arbitrary local file read vulnerability in the StreamBackedCorpusView component. The vulnerability arises because the code calls builtins.open() directly instead of using the security-aware pathsec.open(), effectively bypassing the pathsec.ENFORCE security control. Attackers who can control the fileid argument can exploit this flaw to read arbitrary local files on the system, regardless of the ENFORCE setting. This could expose sensitive system files, configuration files, and application credentials. The vulnerability has been assigned CVE-2026-63312 and is documented in both the NVD and a GitHub Security Advisory. A fix is available by upgrading to NLTK 3.10.0 or later. The issue represents a security control bypass in a widely-used Python natural language processing library, making it potentially impactful for applications that process user-supplied corpus file identifiers.