mitigations.md

Mitigation Strategies Analysis for faisalman/ua-parser-js

Mitigation Strategy: 1. Input Validation and Sanitization (Pre-Parsing)

Description:

1. Identify Entry Points: Locate all code sections where user-agent strings are passed to ua-parser-js.
2. Implement Length Check: Before calling ua-parser-js functions, enforce a maximum length on the user-agent string.

   const MAX_UA_LENGTH = 512; // Adjust as needed
   if (userAgentString && userAgentString.length > MAX_UA_LENGTH) {
       // Handle oversized string (reject, truncate, or use fallback)
       console.warn(`Oversized User-Agent: ${userAgentString}`);
       userAgentString = 'Unknown'; // Example fallback
   }

3. (Optional) Character Whitelisting (High Effort, Limited Applicability): If your application only expects user-agents from a very restricted set of sources, create a regular expression to allow only those expected characters. This is rarely practical for general web applications.

   // Example (VERY restrictive - adjust to your needs)
   const uaWhitelistRegex = /^[a-zA-Z0-9\s\/\.\(\)\-]+$/;
   if (userAgentString && !uaWhitelistRegex.test(userAgentString)) {
       // Handle invalid characters
   }

4. Directly Modify Input: The key here is that the validation happens before any ua-parser-js methods are invoked.

Threats Mitigated:

• ReDoS (Regular Expression Denial of Service): Severity: High. Length limits significantly reduce the attack surface. Whitelisting (if feasible) provides very strong protection.

Impact:

• ReDoS: Substantial risk reduction. Length limits are a crucial first step.

Currently Implemented:

• Specify where this pre-parsing validation is implemented (e.g., "Middleware function validateUA in middleware/ua-validation.js").

Missing Implementation:

• List any locations where user-agent strings are passed to ua-parser-js without this validation.

Mitigation Strategy: 2. Timeout Mechanism (During Parsing)

Description:

1. Locate Parsing Calls: Find all instances where ua-parser-js methods are used to parse the user-agent (e.g., parser.setUA(userAgent).getResult()).
2. Wrap with Timeout: Wrap the ua-parser-js call within a Promise that includes a timeout. This is the core of this mitigation.

   async function parseUserAgentWithTimeout(userAgentString, timeoutMs) {
       return new Promise((resolve, reject) => {
           const timer = setTimeout(() => {
               reject(new Error('User-agent parsing timed out'));
           }, timeoutMs);
   
           try {
               const parser = new UAParser(); // Create parser *inside* the promise
               const result = parser.setUA(userAgentString).getResult();
               clearTimeout(timer);
               resolve(result);
           } catch (error) {
               clearTimeout(timer);
               reject(error);
           }
       });
   }

3. Short Timeout: Use a timeout between 50ms and 200ms.
4. Handle Timeouts: In the .catch() block:
   • Log: Record the user-agent and the timeout.
   • Fallback: Use a safe default value or reject the request. Do not retry without modification.

   parseUserAgentWithTimeout(userAgent, 100)
       .then(result => { /* Process result */ })
       .catch(error => {
           console.error("UA parsing error:", error, "UA:", userAgent);
           if (error.message === 'User-agent parsing timed out') {
               // Use a fallback result
           }
       });

5. Direct Interaction: This mitigation directly controls how ua-parser-js is executed.

Threats Mitigated:

• ReDoS (Regular Expression Denial of Service): Severity: High. The timeout prevents CPU exhaustion.

Impact:

• ReDoS: Very high risk reduction. This is the most critical direct mitigation.

Currently Implemented:

• Specify where the timeout is implemented (e.g., "Utility function parseUA in utils/ua.js").

Missing Implementation:

• List any places where ua-parser-js is used without a timeout.

Mitigation Strategy: 3. Alternative Parsing Strategies (Within ua-parser-js Context)

Description:

1. Fork and Modify (Last Resort): If you've identified a specific regular expression within ua-parser-js that's causing problems, and updates aren't addressing it, you could (as a last resort) fork the library and modify that regular expression. This is highly discouraged unless you have a deep understanding of the library and regular expressions. It also creates a maintenance burden.
2. Contribute Back (If Forking): If you do fork and fix a vulnerability, submit a pull request to the original ua-parser-js repository to benefit the community.
3. Direct Modification: This involves directly changing the internal workings of ua-parser-js (via a fork).

Threats Mitigated:

• Specific ReDoS Vulnerabilities: Severity: High (if the problematic regex is successfully modified).

Impact:

• ReDoS: Potentially complete mitigation for the specific vulnerability addressed, but introduces maintenance overhead.

Currently Implemented:

• Likely "Not Implemented" unless you've already forked the library.

Missing Implementation:

• "Forking and modification not considered unless other mitigations fail and a specific, unpatched vulnerability is identified."