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.
The product does not properly control the allocation and maintenance of a limited resource.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Nltk | Nltk | * | 3.6.5 (excluding) |
| Nltk | Ubuntu | bionic | * |
| Nltk | Ubuntu | esm-apps/bionic | * |
| Nltk | Ubuntu | esm-apps/focal | * |
| Nltk | Ubuntu | esm-apps/xenial | * |
| Nltk | Ubuntu | esm-infra-legacy/trusty | * |
| Nltk | Ubuntu | focal | * |
| Nltk | Ubuntu | hirsute | * |
| Nltk | Ubuntu | impish | * |
| Nltk | Ubuntu | kinetic | * |
| Nltk | Ubuntu | trusty | * |
| Nltk | Ubuntu | trusty/esm | * |
| Nltk | Ubuntu | upstream | * |
| Nltk | Ubuntu | xenial | * |
Mitigation of resource exhaustion attacks requires that the target system either:
The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.
The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.