CVE-2026-48125 in ua-parser-js
Summary
by MITRE • 07/15/2026
UAParser.js is a JavaScript library to detect browsers, operating systems, CPUs, and devices from user-agent data. From 2.0.1 until 2.0.10, a regular expression denial-of-service vulnerability exists when using the Client Hints API. By sending a crafted Sec-CH-UA-Model header to an application that calls UAParser(headers).withClientHints(), an attacker can cause excessive CPU time due to catastrophic backtracking in the device regex because Client Hints values are copied without the UA_MAX_LENGTH limit used for User-Agent values. This issue is fixed in version 2.0.10.
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Analysis
by VulDB Data Team • 07/15/2026
The UAParser.js library represents a critical vulnerability in web application security through its handling of client hints data, specifically affecting versions between 2.0.1 and 2.0.10. This JavaScript library serves as a fundamental component for user-agent string parsing, enabling applications to identify browser types, operating systems, CPU architectures, and device information from HTTP headers. The vulnerability manifests when applications utilize the Client Hints API through the withClientHints() method, creating an attack surface where malicious actors can exploit malformed input data.
The technical flaw stems from inconsistent input validation between traditional user-agent strings and client hints data processing within the library's regular expression patterns. While user-agent values are properly constrained by UA_MAX_LENGTH limits that prevent excessive processing time, client hint values including the Sec-CH-UA-Model header bypass these protective measures entirely. This discrepancy creates a scenario where attacker-controlled device model information can trigger catastrophic backtracking in the device detection regex pattern, causing exponential CPU consumption and potential system exhaustion.
The operational impact of this vulnerability extends beyond simple performance degradation to encompass full denial-of-service conditions that can compromise entire web applications or services. When an attacker crafts malicious Sec-CH-UA-Model headers with carefully constructed patterns designed to exploit the regex engine's backtracking behavior, the application processing these requests experiences dramatic increases in CPU utilization that can render the service unavailable to legitimate users. This vulnerability directly aligns with CWE-400, which categorizes excessive resource consumption as a primary concern for denial-of-service attacks, and maps to ATT&CK technique T1499.004 for network denial of service through resource exhaustion.
Mitigation strategies require immediate upgrading to UAParser.js version 2.0.10 or later, where the vulnerability has been patched by implementing consistent input length limits across all user-agent data processing pathways. Security teams should also consider additional protective measures including rate limiting on client hint headers, input sanitization of Sec-CH-UA-Model values before processing, and monitoring for unusual CPU utilization patterns that may indicate exploitation attempts. The fix addresses the core issue by applying the same UA_MAX_LENGTH restrictions to client hints data that were previously applied only to traditional user-agent strings, ensuring consistent validation regardless of the data source or processing method employed by the application stack.