CVE-2026-80206
Description
A flaw was found in NLTK, specifically within its tgrep module. This vulnerability, known as Regular Expression Denial of Service (ReDoS), allows a remote attacker to provide specially crafted regular expressions. When processed, these expressions can cause indefinite CPU saturation, leading to a complete denial of service for the Python process utilizing the NLTK library.
Statement
A Regular Expression Denial of Service (ReDoS) vulnerability was discovered in NLTK's tgrep module within the _tgrep_node_action function. When processing user-supplied regular expressions inside /regex/ pattern nodes via functions such as tgrep_positions() or tgrep_compile(), NLTK executes re.search without input validation or execution timeouts. If an application exposes these interfaces to unauthenticated or low-privileged attackers, supplying a specially crafted regular expression triggers catastrophic backtracking, resulting in CPU saturation and process-level denial of service.
Mitigation
Sanitize or restrict external input passed to NLTK's tgrep compilation functions to prevent execution of untrusted regular expressions, or enforce application-level timeouts using process isolation/multiprocessing wrappers to restrict CPU consumption.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.9 | N/A | 5.9 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU)
Attackers can create crafted inputs that intentionally cause the regular expression to use excessive backtracking in a way that causes the CPU consumption to spike.
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