CVE-2021-3828 in nltk
Summary
by MITRE • 09/27/2021
nltk is vulnerable to Inefficient Regular Expression Complexity
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Analysis
by VulDB Data Team • 10/02/2021
The nltk library presents a significant security vulnerability classified as inefficient regular expression complexity, which falls under the category of denial of service attacks. This vulnerability specifically affects the natural language toolkit's handling of regular expressions during text processing operations, creating a potential attack vector where malicious input can cause excessive computational resources to be consumed. The flaw manifests when the library processes regular expressions that contain patterns susceptible to catastrophic backtracking, allowing attackers to craft inputs that cause the regular expression engine to perform exponentially increasing amounts of work. This type of vulnerability is particularly dangerous in text processing applications where user input is parsed through regular expressions, as it can lead to complete system resource exhaustion and application unresponsiveness.
The technical implementation of this vulnerability stems from the library's use of regular expressions without proper complexity bounds or input validation mechanisms. When processing text containing specially crafted regular expression patterns, the nltk library can be forced into a state where the regular expression engine enters into a computationally expensive backtracking phase. This behavior is characteristic of regular expressions with nested quantifiers and alternations that can cause exponential time complexity growth. The vulnerability specifically impacts the library's text parsing and tokenization functions, where regular expressions are used to identify word boundaries, sentence structures, and other linguistic patterns. According to the CWE taxonomy, this represents a CWE-400: Uncontrolled Resource Consumption vulnerability, which is classified under the broader category of resource exhaustion attacks that can lead to system instability.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially compromise entire applications that depend on nltk for text processing tasks. Attackers can exploit this weakness by submitting carefully constructed text inputs that trigger the inefficient regular expression patterns, causing the application to consume excessive CPU cycles and memory resources. In high-traffic environments, this can result in complete service disruption and may enable attackers to perform resource exhaustion attacks against the hosting infrastructure. The vulnerability is particularly concerning in web applications and APIs that accept user input for processing through nltk, as it allows for remote code execution through resource exhaustion attacks that can be amplified through repeated requests. This aligns with the ATT&CK framework's T1499.004 technique for network denial of service attacks, where attackers leverage application weaknesses to consume system resources.
Mitigation strategies for this vulnerability require multiple layers of defense to protect against inefficient regular expression complexity attacks. The primary approach involves updating to the latest version of nltk where the regular expression patterns have been optimized and bounded to prevent catastrophic backtracking scenarios. Organizations should implement input validation and sanitization measures to filter out potentially malicious regular expression patterns before they reach the vulnerable library functions. Additionally, deploying rate limiting and resource monitoring mechanisms can help detect and prevent exploitation attempts by limiting the amount of processing time and memory that individual requests can consume. Security teams should also consider implementing regular expression complexity analysis tools that can identify and flag potentially dangerous patterns before they are processed by the application. The implementation of these defensive measures aligns with security best practices outlined in the OWASP Top Ten and follows the principle of least privilege by ensuring that regular expression processing is bounded and monitored to prevent resource exhaustion attacks.