CVE-2020-7767 in express-validators
Summary
by MITRE • 11/11/2020
All versions of package express-validators are vulnerable to Regular Expression Denial of Service (ReDoS) when validating specifically-crafted invalid urls.
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Analysis
by VulDB Data Team • 12/05/2020
The vulnerability identified as CVE-2020-7767 affects the express-validators package, which is widely used in node.js applications for input validation purposes. This particular vulnerability manifests as a Regular Expression Denial of Service (ReDoS) flaw that specifically targets URL validation functionality within the package. The issue arises when the package processes malformed or specially crafted URLs that trigger catastrophic backtracking in its regular expression patterns. The vulnerability is particularly concerning because it can be exploited by attackers who craft malicious input to cause significant performance degradation or complete service unavailability. The ReDoS attack exploits the exponential time complexity inherent in certain regular expression implementations, where the validation process becomes increasingly computationally expensive as input length increases, ultimately leading to resource exhaustion.
The technical flaw within express-validators stems from the implementation of regular expressions used for URL validation that are susceptible to catastrophic backtracking patterns. When an attacker submits a URL containing carefully constructed malicious sequences, the regular expression engine enters into an exponential time complexity state where it repeatedly tries different matching paths. This behavior is classified under CWE-400 as an Uncontrolled Resource Consumption vulnerability, specifically manifesting as a denial of service condition. The vulnerability is particularly dangerous because URL validation is a common operation in web applications, making this flaw exploitable in numerous contexts including web forms, API endpoints, and request parameter validation. The attack vector is straightforward as it requires only sending a specially crafted URL to any application using the vulnerable package.
The operational impact of CVE-2020-7767 extends beyond simple service disruption to potentially compromise entire application availability and system resources. When exploited, the vulnerability can cause applications to become unresponsive or crash entirely, as the validation process consumes excessive CPU cycles and memory resources. This makes it particularly dangerous in high-traffic environments where a single malicious request can bring down services or create significant performance degradation for legitimate users. The vulnerability affects all versions of the express-validators package, meaning that organizations using any version of this library are at risk. The attack can be executed remotely without requiring authentication or special privileges, making it a critical security concern for web applications that process user input through URL validation mechanisms.
Mitigation strategies for this vulnerability should prioritize immediate remediation through package updates to versions that address the ReDoS flaw. Organizations should conduct comprehensive vulnerability assessments to identify all applications and systems using the express-validators package and ensure they are updated to patched versions. Additionally, implementing input validation rate limiting and resource monitoring can help detect and prevent exploitation attempts. The solution aligns with ATT&CK technique T1499.004 which involves resource exhaustion attacks, and organizations should consider implementing defensive measures such as request throttling and circuit breaker patterns. Network-level protections including web application firewalls and intrusion detection systems can also provide additional layers of defense. Long-term mitigation should include adopting more secure regular expression patterns that avoid catastrophic backtracking, implementing proper input sanitization, and conducting regular security audits of third-party dependencies to identify and address similar vulnerabilities before they can be exploited in production environments.