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AVID-2026-R0924
Description
Inefficient Regular Expression Complexity in nltk (CVE-2021-43854)
Details
NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. Versions prior to 3.6.5 are vulnerable to regular expression denial of service (ReDoS) attacks. The vulnerability is present in PunktSentenceTokenizer, sent_tokenize and word_tokenize. Any users of this class, or these two functions, are vulnerable to the ReDoS attack. In short, a specifically crafted long input to any of these vulnerable functions will cause them to take a significant amount of execution time. If your program relies on any of the vulnerable functions for tokenizing unpredictable user input, then we would strongly recommend upgrading to a version of NLTK without the vulnerability. For users unable to upgrade the execution time can be bounded by limiting the maximum length of an input to any of the vulnerable functions. Our recommendation is to implement such a limit.
Reason for inclusion in AVID: CVE-2021-43854 describes a ReDoS vulnerability in Python’s NLTK tokenizers. NLTK is a widely-used NLP toolkit, and the vulnerability affects functions commonly used in AI/NLP pipelines (tokenization). This is a software supply-chain issue because it resides in a software package (NLTK) that could be pulled as a dependency in general-purpose AI systems, potentially enabling resource exhaustion attacks. The CVE is a security vulnerability (DoS) with clear evidence and references.
References
- NVD entry
- https://github.com/nltk/nltk/security/advisories/GHSA-f8m6-h2c7-8h9x
- https://github.com/nltk/nltk/issues/2866
- https://github.com/nltk/nltk/pull/2869
- https://github.com/nltk/nltk/commit/1405aad979c6b8080dbbc8e0858f89b2e3690341
Affected or Relevant Artifacts
- Developer: nltk
- Deployer: nltk
- Artifact Details:
| Type | Name |
|---|---|
| System | nltk |
Impact
AVID Taxonomy Categorization
- Risk domains: Security
- SEP subcategories: S0100: Software Vulnerability
- Lifecycle stages: L06: Deployment
CVSS
| Version | 3.1 |
| Vector String | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| Base Score | 7.5 |
| Base Severity | 🔴 High |
| Attack Vector | NETWORK |
| Attack Complexity | 🟢 Low |
| Privileges Required | NONE |
| User Interaction | NONE |
| Scope | UNCHANGED |
| Confidentiality Impact | NONE |
| Integrity Impact | NONE |
| Availability Impact | 🔴 High |
CWE
| ID | Description |
|---|---|
| CWE-400 | CWE-400: Uncontrolled Resource Consumption |
Other information
- Report Type: Advisory
- Credits:
- Date Reported: 2021-12-23
- Version: 0.3.3
- AVID Entry