CVE-2017-16030 in Useragent
Summary
by MITRE
Useragent is used to parse useragent headers. It uses several regular expressions to accomplish this. An attacker could edit their own headers, creating an arbitrarily long useragent string, causing the event loop and server to block. This affects Useragent 2.1.12 and earlier.
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Analysis
by VulDB Data Team • 02/14/2020
The vulnerability identified as CVE-2017-16030 resides within the useragent library version 2.1.12 and earlier, which is commonly employed for parsing user agent strings in web applications and server-side environments. This library utilizes multiple regular expressions to identify and extract information from user agent headers sent by web browsers and other HTTP clients. The flaw manifests when an attacker crafts a maliciously long user agent string that exploits the library's parsing mechanism, causing the regular expression engine to enter a computationally expensive state known as catastrophic backtracking.
The technical implementation of this vulnerability stems from the library's reliance on regular expressions that are susceptible to exponential time complexity when processing malformed input. When an attacker sends an extended user agent string containing carefully constructed patterns, the regular expression matching process can spiral into an infinite loop or consume excessive CPU resources, effectively blocking the event loop of Node.js applications that utilize this library. This behavior directly violates the principle of resource exhaustion protection and represents a classic example of a denial of service attack vector through input manipulation.
The operational impact of CVE-2017-16030 extends beyond simple service disruption, as it can potentially bring entire web applications to a halt. In high-traffic environments, a single malicious request can cause cascading failures, affecting multiple concurrent users and potentially leading to complete system unavailability. The vulnerability affects applications that rely on useragent for user identification, browser detection, or feature flagging, making it particularly dangerous in production systems where such parsing is performed on every request. This issue directly maps to CWE-400, which addresses unspecified denial of service vulnerabilities, and aligns with ATT&CK technique T1499.004 for resource exhaustion attacks.
Mitigation strategies for this vulnerability require immediate library updates to versions that address the regular expression performance issues. Organizations should implement input validation measures that limit the maximum length of user agent strings before they are processed by the useragent library. Additionally, deploying rate limiting mechanisms and implementing circuit breaker patterns can help prevent a single malicious request from causing widespread disruption. The fix typically involves optimizing the regular expressions used in the parsing logic to eliminate catastrophic backtracking scenarios and ensure predictable performance characteristics regardless of input size. Security teams should also consider implementing monitoring and alerting for unusual CPU utilization patterns that might indicate exploitation attempts.