CVE-2026-72818: Nltk Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-qx2g-xrx7-vfh8
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CVSS 7.5
CWE-1333
Published
Advisory Summary

CVE-2026-72818 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-1333: Inefficient Regular Expression Complexity) affecting Nltk libraries in the PyPI ecosystem. The issue involves nltk tweettokenizer vulnerable to denial of service through catastrophic regex backtracking. Remediation is available by upgrading affected packages to version 3.10.1.

CVE-2026-72818 at a Glance

CVE Identifier CVE-2026-72818
GHSA Identifier GHSA-qx2g-xrx7-vfh8
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-1333: Inefficient Regular Expression Complexity
Affected Product Nltk
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-72818

To remediate this vulnerability, upgrade all affected Nltk dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

nltk fixed: 3.10.1 
pip install nltk>=3.10.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show nltk

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-72818, inspect your installed packages using pip:

pip show nltk

Analyze the terminal output to locate any version of Nltk in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.10.1), your application is affected.

Note: The pip show nltk command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1333: Inefficient Regular Expression Complexity, where the product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential. In Nltk, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

The URLS regular expression in nltk/tokenize/casual.py, compiled into TweetTokenizer.WORD_RE and applied by TweetTokenizer.tokenize, contains a naked-domain branch whose domain-label prefix [a-z0-9]+(?:[.\-][a-z0-9]+)* is unbounded. Input consisting of many alternating label separators can be partitioned in exponentially many ways, and because the branch also requires a trailing top-level domain that such input never supplies, the engine explores those partitions before failing at each offset. A few kilobytes of input therefore consumes seconds to minutes of single-threaded CPU, and the HANG_RE substitution performed before matching does not collapse the pattern. TweetTokenizer is intended for tokenizing untrusted social-media text, so any service that applies it, or the module-level casual_tokenize, to submitted text can be stalled per request without authentication. Version 3.10.1 bounds the label repetition.

CVE-2026-72818 Affected Nltk Versions

1 component(s)
Fixed Versions:
3.10.1
Product / Package Ecosystem Affected Versions Fixed Version
nltk
pypi/nltk
PyPI
< 3.10.1
3.10.1

References

NVD — CVE-2026-72818 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-72818
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/issues/3704
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/pull/3701
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/e092ed52eccae642304448bffc8d23cb301f85c1
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/blob/3.9.4/nltk/tokenize/casual.py
Nltk Release RELEASE
https://github.com/nltk/nltk/releases/tag/v3.10.1
www.vulncheck.com/advisories/nltk-twe... WEB
https://www.vulncheck.com/advisories/nltk-tweettokenizer-url-pattern-backtracks-catastrophically-on-naked-domain-like-input

Frequently Asked Questions

What is CVE-2026-72818?
CVE-2026-72818 is a security vulnerability in Nltk involving NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking under CWE-1333: Inefficient Regular Expression Complexity.
Which Nltk versions are affected?
Affected versions include releases prior to 3.10.1 across affected packages: nltk.
What version fixes CVE-2026-72818?
Upgrading to 3.10.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nltk affected?
Yes, nltk is affected in releases prior to 3.10.1. Upgrade your dependency to 3.10.1 or later.
What is the CVSS score of CVE-2026-72818?
CVE-2026-72818 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-72818?
CVE-2026-72818 is classified under CWE-1333: Inefficient Regular Expression Complexity (The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.).
How can I check PyPI dependencies for CVE-2026-72818?
Run the dependency tree command: 'pip show nltk'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-qx2g-xrx7-vfh8",
  "modified" : "2026-09-02T15:16:37Z",
  "published" : "2026-08-21T00:31:23Z",
  "aliases" : [ "CVE-2026-72818" ],
  "summary" : " NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking",
  "details" : "The URLS regular expression in nltk/tokenize/casual.py, compiled into TweetTokenizer.WORD_RE and applied by TweetTokenizer.tokenize, contains a naked-domain branch whose domain-label prefix [a-z0-9]+(?:[.\\-][a-z0-9]+)* is unbounded. Input consisting of many alternating label separators can be partitioned in exponentially many ways, and because the branch also requires a trailing top-level domain that such input never supplies, the engine explores those partitions before failing at each offset. A few kilobytes of input therefore consumes seconds to minutes of single-threaded CPU, and the HANG_RE substitution performed before matching does not collapse the pattern. TweetTokenizer is intended for tokenizing untrusted social-media text, so any service that applies it, or the module-level casual_tokenize, to submitted text can be stalled per request without authentication. Version 3.10.1 bounds the label repetition.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-72818"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/issues/3704"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/pull/3701"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/e092ed52eccae642304448bffc8d23cb301f85c1"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/blob/3.9.4/nltk/tokenize/casual.py"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/releases/tag/v3.10.1"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-tweettokenizer-url-pattern-backtracks-catastrophically-on-naked-domain-like-input"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:16:37Z",
    "nvd_published_at" : "2026-08-20T22:18:05Z"
  }
}