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CVE-2026-72818

NLTK TweetTokenizer URL Pattern Backtracks Catastrophically on Naked-Domain-Like Input

CVSS 4.0 8.7 HIGHCVSS 3.1 7.5 HIGHCheck source for fixed versionsKEV status unknown

Original advisory

cve-program · CVE-2026-72818

Published

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.

Read the publisher's advisory

Affected versions, risk and source history

cve-program · CVE-2026-72818

Published

nltk

https://pypi.org

Publisher's version information

Shown as published. These rules are not interpreted as NuGet package version ranges.

{
                    "vendor": "nltk",
                    "product": "nltk",
                    "defaultStatus": "unaffected",
                    "packageURL": "pkg:pypi/nltk",
                    "collectionURL": "https://pypi.org",
                    "packageName": "nltk",
                    "repo": "https://github.com/nltk/nltk",
                    "versions": [
                        {
                            "version": "0",
                            "lessThan": "3.10.1",
                            "status": "affected",
                            "versionType": "semver"
                        },
                        {
                            "version": "3.10.1",
                            "status": "unaffected",
                            "versionType": "semver"
                        }
                    ],
                    "programFiles": [
                        "nltk/tokenize/casual.py"
                    ]
                }

Original source record · Cite this exact observation · Complete original publisher JSON