CVE-2018-20164 in UAP-Core
Summary
by MITRE
An issue was discovered in regex.yaml (aka regexes.yaml) in UA-Parser UAP-Core before 0.6.0. A Regular Expression Denial of Service (ReDoS) issue allows remote attackers to overload a server by setting the User-Agent header in an HTTP(S) request to a value containing a long digit string. (The UAP-Core project contains the vulnerability, propagating to all implementations.)
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Analysis
by VulDB Data Team • 07/10/2023
The vulnerability identified as CVE-2018-20164 represents a critical Regular Expression Denial of Service (ReDoS) flaw within the UA-Parser UAP-Core library version 0.5.0 and earlier. This issue resides in the regex.yaml configuration file, also known as regexes.yaml, which serves as the core parsing mechanism for User-Agent string analysis across numerous web applications and security systems. The vulnerability stems from poorly constructed regular expressions that are susceptible to catastrophic backtracking when processing specific malicious input patterns. The flaw affects all implementations that rely on the UAP-Core project, making it particularly widespread and dangerous in production environments where User-Agent parsing is a common security control.
The technical implementation of this vulnerability occurs when an attacker crafts a malicious User-Agent header containing a specially formatted digit string that triggers exponential backtracking in the regular expression engine. When the vulnerable system processes this input, the regular expression parser enters a state where it repeatedly attempts different matching paths, causing CPU consumption to spike exponentially with input length. This behavior is characteristic of ReDoS attacks and maps directly to CWE-400, which specifically addresses Uncontrolled Resource Consumption. The attack vector is particularly insidious because it requires no authentication and can be executed through simple HTTP requests, making it accessible to any attacker with network access to the target system. The vulnerability demonstrates the classic ReDoS pattern described in the ATT&CK framework under T1496 for Resource Exhaustion, where attackers leverage application weaknesses to consume system resources.
The operational impact of CVE-2018-20164 extends far beyond simple performance degradation, as it can effectively render affected systems unavailable to legitimate users through sustained resource exhaustion. When exploited, the vulnerability allows attackers to consume excessive CPU cycles and memory resources, potentially leading to complete service disruption or denial of service conditions. This is particularly concerning for web applications, load balancers, and security appliances that rely on User-Agent parsing for threat detection, access control, or traffic analysis. The vulnerability affects not only the direct parsing functionality but also cascading effects throughout the system architecture, as the resource exhaustion can impact other services running on the same infrastructure. Organizations using vulnerable versions of the UAP-Core library face significant risk of service interruption, especially during high-traffic periods when multiple concurrent attacks can compound the resource consumption effects.
Mitigation strategies for CVE-2018-20164 primarily focus on upgrading to UAP-Core version 0.6.0 or later, which includes fixed regular expressions that eliminate the vulnerable patterns. Organizations should also implement input validation and sanitization measures at the network perimeter, including rate limiting and User-Agent header length restrictions to prevent exploitation attempts. Additionally, implementing proper monitoring and alerting for unusual CPU usage patterns can help detect exploitation attempts before they cause significant service disruption. The fix addresses the root cause by replacing the vulnerable regular expressions with more efficient patterns that do not exhibit catastrophic backtracking behavior, aligning with security best practices outlined in NIST SP 800-160 and OWASP Top Ten security guidelines. Organizations should also conduct comprehensive testing to ensure that all systems using the UAP-Core library are properly updated and that existing security controls adequately address this vulnerability.