Blog | G5 Cyber Security

Empty User Agent: Robot or Not?

G5 Cyber Security

TL;DR

No, you can’t reliably assume an empty user agent means it’s a robot. While many bots don’t set a user agent, legitimate users and tools might also have missing user agents due to configuration issues or privacy settings. Treating all requests with empty user agents as robots will likely lead to false positives.

Understanding User Agents

The User-Agent string is sent by your browser (or other application) to identify itself to the web server. It helps servers tailor content appropriately. A typical user agent looks like this:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36

However, it’s not always present.

Why Empty User Agents Happen

  1. Bots: Many (but not all) bots intentionally leave the user agent blank or set a generic value to avoid easy detection.
  2. Privacy Tools: Some privacy-focused browsers and extensions deliberately remove or modify the user agent string.
  3. Headless Browsers: When running automated tests with headless browsers (like Puppeteer or Selenium), the user agent might not be configured by default.
  4. Configuration Errors: Misconfigured proxies, firewalls, or custom HTTP clients can strip out the user agent header.
  5. Old Clients: Very old web clients may not support sending a User-Agent header.

Why You Can’t Just Block Empty Agents

Blocking all requests without a user agent will block legitimate traffic, potentially breaking functionality for real users and important services.

What to Do Instead: A Multi-Layered Approach

  1. Don’t rely on the User Agent alone. It’s easily spoofed or missing.
  2. Check other headers: Look at Accept, Accept-Language, and Connection headers for unusual patterns.
  3. Rate Limiting: Implement rate limiting to slow down suspicious activity regardless of the user agent.
  4. CAPTCHAs: Use CAPTCHAs for high-risk actions or when you suspect bot activity.
  5. Behavioral Analysis: Monitor request patterns (e.g., speed, pages visited) to identify bots based on their behaviour. This is the most effective method but requires more complex setup.
  6. Bot Detection Services: Consider using a commercial bot detection service that uses advanced techniques to identify and block malicious bots.

Example (Python – Checking for Empty User Agent)

This example shows how to check if the user agent is empty in Python using the requests library:

import requests

url = "https://example.com"
headers = {"User-Agent": ""}
response = requests.get(url, headers=headers)

if not response.request.headers.get('User-Agent'):
    print("Empty User Agent detected!")
else:
    print("User Agent present: ", response.request.headers['User-Agent'])

Important Note: This code only *detects* an empty user agent; it doesn’t make any decisions about blocking or allowing the request.

Conclusion

An empty user agent is a signal, not definitive proof of a robot. Use it as part of a broader strategy for identifying and mitigating malicious activity, but avoid making blanket assumptions based on this single header.

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