PYSEC-2026-3751

    Dashboard / Vulnerabilities / PYSEC-2026-3751

    PYSEC-2026-3751

    Published: 26 Aug 2026Last Modified: 2 Sept 2026

    Summary:

    Details: 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.

    References: https://www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-via-tgrep, https://github.com/nltk/nltk/security/advisories/GHSA-w3v8-gmh9-3wv7

    Affected packages

    Package

    Name: nltk

    Purl: pkg:pypi/nltk

    Affected ranges

    Type: ECOSYSTEM

    Events:

    Introduced- 0
    Fixed -3.10.3

    Affected versions

    0.8

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    PYSEC-2026-3751 | CVE-DB