The Pros and Cons of Blocking Requests with Empty User-Agent Headers Using ModSecurity


ModSecurity is an open-source web application firewall (WAF) that is highly configurable and customizable. It offers an array of rules and filters to help secure your web application. One common rule used is blocking requests that come with an empty User-Agent header. While this might seem like a good idea at first glance, there are both advantages and disadvantages to implementing such a rule. Let’s delve into the details.


1. Reduced Server Load

One of the benefits of blocking requests with an empty User-Agent is the potential reduction in server load. Web scrapers and automated bots often make requests to websites without setting a User-Agent. By blocking such requests, you could reduce the number of non-legitimate hits your server has to handle.

2. Improved Security

Bots with no User-Agent are often up to no good. They could be scraping your site, attempting SQL injection attacks, or even launching DDoS attacks. Blocking these requests at the firewall level can add an extra layer of security to your web application.

3. Easier Log Analysis

When you block requests with empty User-Agent headers, it can simplify the process of analyzing your server logs. This filtering can make it easier to identify problematic or malicious requests that are worth your attention.

4. Minimal Impact on Real Users

Most web browsers and legitimate bots (like Google’s crawler) include a User-Agent header in their HTTP requests. Therefore, this rule typically has minimal impact on real users and legitimate web traffic.


1. False Positives

While it’s true that many bad bots send requests with empty User-Agent headers, there are also some legitimate use-cases where a User-Agent might be missing. Some API clients, for example, might not set this header. Therefore, you could inadvertently block legitimate traffic.

2. Easy to Circumvent

Blocking requests based solely on an empty User-Agent header is relatively easy for a malicious actor to circumvent. All they need to do is set a fake User-Agent in their requests, and they will bypass this rule.

3. Additional Server Load

Though one of the benefits is reduced server load, the act of processing ModSecurity rules itself consumes server resources. If you’re running a particularly high-traffic website, the overhead of evaluating this rule for every incoming request could be non-negligible.

4. Maintenance Overhead

Adding any new rule to ModSecurity, or any WAF, adds some degree of maintenance overhead. Rules need to be tested, monitored, and updated as necessary. Especially for a rule that could have false positives, this may mean periodic tweaking and auditing.


Blocking requests with empty User-Agent headers using ModSecurity can be a double-edged sword. While it does offer the potential for improved security and reduced server load, it is not a foolproof strategy. If you decide to implement this rule, be aware of its limitations and be prepared to monitor its impact closely.

As with any security measure, this should be just one part of a multi-layered defense strategy. Always keep your web application’s specific needs and context in mind when making such decisions.

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