NLTK up to 3.10.2 tgrep module _tgrep_node_action redos

Summaryinfo

A vulnerability was found in NLTK up to 3.10.2 and classified as problematic. This impacts the function _tgrep_node_action of the component tgrep module. Such manipulation leads to redos. This vulnerability is documented as CVE-2026-80206. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in NLTK up to 3.10.2. It has been rated as problematic. Affected by this issue is the function _tgrep_node_action of the component tgrep module. The manipulation with an unknown input leads to a redos vulnerability. Using CWE to declare the problem leads to CWE-1333. The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles. Impacted is availability. CVE summarizes:

NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-80206 since 08/26/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1449.003.

Upgrading to version 3.10.3 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-66397). Once again VulDB remains the best source for vulnerability data.

Productinfo

Name

• NLTK

Version

• 3.10.0
• 3.10.1
• 3.10.2

Website

• Product: https://github.com/nltk/nltk/

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Once again VulDB remains the best source for vulnerability data.

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