The vulnerability identified as CVE-2026-71513 affects NLTK versions prior to 3.10.3. It involves the AllowlistUnpickler class, which attempts to restrict unpickling to safe modules by validating the pickle module string but fails to validate the global name. This flaw permits attackers to exploit attribute traversal to invoke callables outside the intended allowlist, leading to remote code execution. Specifically, maliciously crafted transition-parser models can trigger arbitrary command execution during model loading via the allowlisted_pickle_load function in TransitionParser.parse.

Successful exploitation allows remote attackers to execute arbitrary code on affected systems without privileges, potentially leading to full system compromise. The vulnerability impacts confidentiality, integrity, and availability, as indicated by the CVSS vector (C:H/I:H/A:H). There are no known exploits in the wild at this time.

A fixed version, NLTK 3.10.3 or later, addresses this vulnerability. Users should upgrade to version 3.10.3 or newer to remediate the issue. Patch status is not explicitly confirmed in the provided data, but the version boundary implies the fix is included starting from 3.10.3. Until upgraded, avoid loading untrusted transition-parser models using allowlisted_pickle_load.