Deserialization of Untrusted Data Affecting nltk package, versions [3.10.0, 3.10.3)
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Start learning- Snyk IDSNYK-PYTHON-NLTK-19233537
- published23 Aug 2026
- disclosed22 Aug 2026
- creditTohirov Fazliddin Mahmud o'g'li
Introduced: 22 Aug 2026
NewCVE-2026-71513 (opens in a new tab)How to fix?
Upgrade nltk to version 3.10.3 or higher.
Overview
nltk is a Natural Language Toolkit (NLTK) is a Python package for natural language processing.
Affected versions of this package are vulnerable to Deserialization of Untrusted Data via unsafe deserialization in AllowlistUnpickler. An attacker can execute arbitrary commands by supplying a crafted transition-parser model containing malicious pickle data. Because AllowlistUnpickler validates only the pickle module string and does not validate the referenced global name, an attacker can use dotted-name attribute traversal to resolve callables outside the intended allowlisted namespace. When TransitionParser.parse() loads the malicious model through allowlisted_pickle_load(), the attacker-controlled callable can be executed.
Details
Serialization is a process of converting an object into a sequence of bytes which can be persisted to a disk or database or can be sent through streams. The reverse process of creating object from sequence of bytes is called deserialization. Serialization is commonly used for communication (sharing objects between multiple hosts) and persistence (store the object state in a file or a database). It is an integral part of popular protocols like Remote Method Invocation (RMI), Java Management Extension (JMX), Java Messaging System (JMS), Action Message Format (AMF), Java Server Faces (JSF) ViewState, etc.
Deserialization of untrusted data (CWE-502) is when the application deserializes untrusted data without sufficiently verifying that the resulting data will be valid, thus allowing the attacker to control the state or the flow of the execution.