CVE-2026-71513 NLTK 3.10.0 through 3.10.2 Remote Code Execution via AllowlistUnpickler Dotted-Name Bypass

High 8.7 Published 22 Aug 2026 Updated 27 Aug 2026

Attackers can execute arbitrary code by crafting malicious transition-parser models that bypass allowlist checks in NLTK's unpickler.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: Available

What is CVE-2026-71513?

CVE-2026-71513 is a high-severity unsafe deserialization vulnerability in nltk, published on 22 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.

Weakness: CWE-502

Which versions are affected by CVE-2026-71513?

3 open source packages ship an affected version, including nltk. It affects 1 product.

Package	Affected	Fixed in
`nltk` PyPI	From `3.10.0`	`3.10.3`
`nltk` PyPI	From `3.10.0`	`3.10.3`
`nltk` PyPI	From `3.10.0`	`3.10.3`

Products include Nltk Nltk.

How do I fix CVE-2026-71513?

Upgrade nltk to 3.10.3 or later.

Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.

Is CVE-2026-71513 being exploited?

There is no known exploitation of CVE-2026-71513 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-71513, GHSA-5gh2-94qg-qppq, PYSEC-2026-3867, UBUNTU-CVE-2026-71513.