CVE-2021-3765 in validator.js
Summary
by MITRE • 11/02/2021
validator.js is vulnerable to Inefficient Regular Expression Complexity
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Analysis
by VulDB Data Team • 11/05/2021
The validator.js library presents a critical vulnerability classified as inefficient regular expression complexity, which can lead to significant performance degradation and potential denial of service conditions. This vulnerability stems from the library's handling of certain input patterns that cause regular expressions to exhibit exponential time complexity during validation operations. The flaw affects the library's ability to process user inputs efficiently, particularly when dealing with malformed or specially crafted strings that trigger catastrophic backtracking in the underlying regex engines.
This vulnerability falls under the CWE-1333 category, which specifically addresses inefficient regular expression complexity that can lead to denial of service attacks through resource exhaustion. The technical implementation of validator.js relies on complex regular expressions to validate various data formats including email addresses, URLs, and other structured inputs. When maliciously crafted inputs are processed, these regular expressions can enter into infinite loops or require exponentially increasing computational resources, causing the application to become unresponsive or crash entirely. The vulnerability is particularly dangerous because it can be exploited through user-controlled inputs that flow through validation functions, making it applicable across numerous attack vectors within web applications.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially compromise entire application availability. Attackers can construct specific input strings that cause the regular expression engine to perform an enormous number of operations, leading to resource exhaustion and system instability. This creates a denial of service scenario where legitimate users cannot access services due to the validation processes consuming excessive computational resources. The vulnerability is particularly concerning in high-traffic applications where multiple validation operations occur simultaneously, as the cumulative effect can overwhelm system resources and result in complete service disruption.
Mitigation strategies for this vulnerability involve multiple layers of defensive measures that address both the immediate technical flaw and broader security practices. Organizations should implement input validation rate limiting and timeouts to prevent excessive resource consumption during validation operations. The recommended approach includes upgrading to patched versions of validator.js that address the specific regex complexity issues, implementing proper regex engine configuration to prevent catastrophic backtracking, and applying input sanitization techniques that reduce the complexity of validation patterns. Additionally, implementing proper monitoring and alerting mechanisms can help detect abnormal validation behavior and respond quickly to potential exploitation attempts. Security teams should also consider implementing web application firewalls and input filtering rules that can identify and block suspicious validation patterns before they reach the vulnerable code paths, aligning with defensive strategies outlined in the attack mitigation frameworks such as those referenced in the mitre attack framework for preventing resource exhaustion attacks.