CVE-2017-16086 in ua-parser
Summary
by MITRE
ua-parser is a port of Browserscope's user agent parser. ua-parser is vulnerable to a ReDoS (Regular Expression Denial of Service) attack when given a specially crafted UserAgent header.
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Analysis
by VulDB Data Team • 02/15/2020
The ua-parser library represents a widely adopted component in web applications and server-side environments that processes user agent strings to identify browser types, versions, and operating systems. This library serves as a critical element in web analytics, content delivery optimization, and security monitoring systems. When applications rely on ua-parser for user agent parsing, they typically integrate it into their request handling pipelines to extract client information for various operational purposes. The vulnerability identified as CVE-2017-16086 specifically targets the regular expression patterns used within this library to parse user agent strings, creating a potential attack vector that can significantly impact system availability and performance.
The technical flaw manifests through a Regular Expression Denial of Service (ReDoS) vulnerability classified under CWE-400, where the library's parsing logic contains a regular expression susceptible to catastrophic backtracking. When processing specially crafted user agent headers, the regular expression engine enters into an exponential time complexity scenario, causing the parsing operation to consume excessive computational resources. This occurs because the regular expression pattern allows for multiple matching paths that create nested backtracking, where the parser attempts to match various combinations of characters in a way that grows exponentially with input length. The vulnerability specifically affects the version of ua-parser that was released prior to the fix, making it a classic example of how seemingly benign parsing operations can become critical security concerns.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially enabling denial of service attacks against web applications that utilize ua-parser. Attackers can craft malicious user agent strings that, when processed by vulnerable applications, cause the parsing operations to consume excessive CPU cycles and memory resources. This can lead to service unavailability for legitimate users, particularly in high-traffic environments where multiple concurrent requests are being processed. The vulnerability affects various web frameworks and applications that depend on ua-parser, including but not limited to Node.js applications, web servers, and analytics platforms. The attack can be executed with minimal resources and can be particularly devastating when combined with other attack vectors or when targeting applications with limited computational resources.
Mitigation strategies for CVE-2017-16086 involve immediate patching of the ua-parser library to versions that address the regular expression vulnerability. Organizations should implement input validation and sanitization measures to filter out potentially malicious user agent strings before they reach the parsing logic. Network-level protections such as rate limiting and request filtering can provide additional defense-in-depth measures to prevent exploitation. The remediation process should include thorough testing to ensure that patched versions maintain compatibility with existing application functionality. Security monitoring should be enhanced to detect unusual parsing patterns or resource consumption that might indicate exploitation attempts. This vulnerability aligns with ATT&CK technique T1499.004 which covers network denial of service attacks, and demonstrates how seemingly simple parsing operations can become critical attack surfaces requiring careful attention to regular expression design and input validation practices.