As cyberattacks on web applications grow more advanced, businesses face increasing risks. Shockingly, 26% of data breaches are caused by application-level attacks, exploiting known vulnerabilities.

A Web Application Firewall or WAF is specifically built to address this gap. It filters incoming web traffic, blocking attacks before they reach your application. 

In this article, we’ll break down how WAFs work, the threats they defend against, and how they fit into a broader, layered defense strategy for web application protection.

What is a Web Application Firewall (WAF)?

If you manage or rely on web applications, you already know how exposed they can be to cyber threats. 

A Web Application Firewall (WAF) helps you protect these applications by analyzing traffic between your web app and the internet. It intercepts every request your application receives, checking for harmful intent or suspicious activity. If a threat is detected, the WAF blocks it before it can cause damage.

At its core, a WAF is built to address the kinds of threats that exploit weaknesses in how your application operates. 

For example, attacks like SQL injection and Cross-Site Scripting (XSS) often rely on users submitting unvalidated inputs that are then used to manipulate your application or steal sensitive data. A WAF is equipped to detect these patterns to block malicious requests before they reach your application.

Beyond its role in preventing attacks, a Web Application Firewall also helps with regulatory compliance. For example, if your business processes payments, standards like PCI DSS require you to take steps to prevent breaches and safeguard customer data. A WAF helps you meet these compliance requirements by preventing threats that target your application’s vulnerabilities.

But having said that, understand that WAF is not a silver bullet. You need to view a WAF as part of a larger security strategy. A WAF works best alongside other solutions, such as Intrusion Detection Systems (IDS) or network firewalls, to provide more comprehensive protection.

How do WAFs differ from traditional firewalls?

WAFs and traditional firewalls are both critical in defending your systems, but they are two tools that work differently because they solve different problems.

A WAF is specifically designed to protect web applications, analyzing the traffic between your application and the internet to detect malicious behavior that could exploit your app’s design flaws.

As we mentioned above, attacks like SQL injection or cross-site scripting try to insert harmful code into user inputs and steal data or manipulate how your application functions. A WAF detects these threats by analyzing the content of HTTP and HTTPS requests, filtering out anything that doesn’t look safe.

Traditional firewalls, on the other hand, are focused on securing the overall network. They operate at lower levels of the OSI model, network (Layer 3), and transport (Layer 4) and make decisions based on things like IP addresses, ports, and protocols. 

Their job is to block unauthorized access to your network, control data flow, and prevent attackers from breaching the perimeter. However, they don’t inspect the details of web traffic, which means they’re not equipped to handle threats specifically targeting applications.

How does a WAF work?

A WAF protects your web application by filtering and analyzing incoming and outgoing HTTPS traffic. Its job is to detect harmful requests and block them before they reach your application.

Let’s take a closer look at the specific functions of a Web Application Firewall.

how does a web application firewall (waf) work?

Monitoring and filtering traffic

When a user interacts with your application, the request doesn’t go directly to your server. A WAF intercepts these requests first. The interception allows it to review the request in detail, breaking it down into components such as headers, query parameters, and data payloads.

Malicious traffic often carries signs like unexpected inputs, hidden scripts, or commands that deviate from normal patterns. The WAF identifies these threats and filters them out. So, for example, a WAF might block a request containing embedded SQL commands designed to manipulate your database.

Providing security policies and rule-based protection

WAFs rely on a combination of preset and custom rules to detect and respond to threats:

  • Preset rules: Most WAFs come with a library of predefined rules that identify common attack types. These rules are regularly updated to account for new threats.
  • Custom rules: You can also create rules specific to your application. For example, if certain parts of your app handle sensitive inputs, you can write a custom rule to monitor those fields for unusual patterns or block specific types of data.

Rules might identify threats by spotting specific keywords, characters, or behaviors in requests. A WAF can restrict the number of requests from a single IP address over a short period, reducing the risk of Distributed Denial-of-Service (DDoS) attacks targeting your application.

Performing anomaly detection and behavioral analysis

Modern WAFs use advanced techniques like anomaly detection and behavioral analysis to catch threats that don’t fit predefined patterns.

First, they learn what normal traffic looks like for your application and then identify deviations that may signal an attack. For example, a sudden surge in traffic from a single session or unusual activity from an IP address could trigger the WAF to take action.

It’s a particularly useful approach to detect zero-day vulnerabilities like new and unknown threats that don’t yet have clear rules or signatures.

Blocking and alerting

When the WAF identifies a threat, it blocks the request before it can reach your application. This automated response ensures that malicious traffic is neutralized in real time.

At the same time, the WAF generates an alert for your security team, giving you details about the blocked request and the nature of the threat. These alerts can help your team to investigate further, fine-tune rules, and strengthen defenses.

Key types of WAFs 

WAFs come in three primary forms: network-based, host-based, and cloud-based. The choice depends on your organization’s specific factors, like application architecture, performance requirements, and resources.

In the table below, you can find the key types of WAFs, their descriptions, pros, and cons.

types of web application firewalls

Common security threats mitigated by WAFs

There are several security threats that you can effectively mitigate with a Web Application Firewall. Let’s look at the most common ones.

SQL injection

SQL injection attacks are designed to manipulate your database by injecting malicious commands into a query. 

For example, an attacker might send a query like apple' OR 1=1;--, which forces the database to return all records instead of the intended result. This can expose sensitive data or compromise the entire database.

A WAF monitors incoming requests for suspicious patterns associated with SQL injection. It looks for certain keywords or characters, such as SELECT, UNION, or ', that don’t align with normal application behavior. 

When the WAF detects a risky request, it blocks it before it interacts with the database, preventing potential damage.

Cross-Site Scripting

XSS attacks inject malicious scripts into your web application and target users who visit your site. These scripts can hijack sessions, steal data, or trick users into downloading malware. 

For example, a malicious script might display harmless content on your page but execute harmful actions in the user’s browser.

WAFs help prevent XSS by scanning incoming requests for indicators like <script> tags or JavaScript code. They also filter out suspicious input, blocking scripts before they can execute and protecting users from unintended consequences.

Cross-Site Request Forgery

An attacker might trick an authenticated user into performing an unintended action, like transferring money or changing account settings, by embedding a malicious request in a link or form.

To counter Cross-Site Request Forgery (CSRF), WAFs check for security tokens that validate legitimate requests. If a request doesn’t include a valid token, the WAF flags it as suspicious and prevents it from executing. This ensures that unauthorized actions are stopped before they can take effect.

Application layer Distributed Denial-of-Service

Application layer DDoS attacks overwhelm your application by sending a flood of requests designed to exhaust resources and disrupt normal operations. Unlike network-layer DDoS, these attacks are harder to detect because they mimic legitimate traffic.

A WAF can identify patterns indicative of application-layer DDoS, like an unusually high number of requests from a single source or region. 

Other threats

In addition to these, WAFs protect against other risks as well, such as:

  • Path traversal which exploits directory vulnerabilities to access files outside the intended structure.
  • File inclusion to execute unauthorized or harmful scripts.

Benefits of using a WAF for web application security

Attackers are constantly looking for ways to exploit vulnerabilities in web applications. A WAF can help you address these risks by not only blocking threats but also creating a more secure, compliant, and trusted environment for your users. 

Here’s how implementing a WAF can benefit your organization.

Enhanced security posture

Your web application is constantly processing user inputs like form submissions, API calls, or file uploads. Such a functionality opens the door to attacks like SQL injection, where an attacker manipulates queries to access sensitive data. 

A WAF inspects each incoming request, analyzes its structure for malicious patterns, and blocks anything that could harm your application or its users.

Without a WAF, detecting and stopping these attacks becomes a manual, reactive process. With one, you can address threats in real time, reducing the risk of data breaches, downtime, or application exploitation.

Improved compliance

If your business handles sensitive data like payment information or personal details, regulations probably require you to implement strong security measures. 

For example, PCI DSS 4.0 explicitly mandates the use of a WAF for protecting web applications. A WAF not only helps you meet these requirements but also demonstrates to regulators, auditors, and customers that you’re taking security seriously.

Cost-effective security layer

Recovering from an attack can be costly. Patching vulnerabilities, restoring systems, handling downtime, and dealing with potential legal fallout all add up. 

A WAF reduces these risks by addressing threats before they cause harm. So, it is an investment in which, instead of reacting to incidents after they happen, you’re preventing them entirely. This saves not only money but also time, which your team can redirect to building and improving your applications.

Greater confidence for users and stakeholders

Users want to trust that their data is safe when they interact with your application. A WAF is a tangible measure that shows you prioritize security. It protects users from threats like stolen credentials, session hijacking, or malware delivery, all things that can affect trust quickly.

Limitations and challenges of WAFs

A Web Application Firewall is a powerful tool for protecting your web application, but it’s not without its challenges. Like any security measure, a WAF requires careful integration into your broader strategy. 

Let’s explore these limitations and how you can address them.

Performance impact

One challenge with WAFs is that they inspect every request to your application. While this is essential for security, it can slow things down.  In fact, most traditional WAFs often struggle to scale because they require individual deployments on each node or instance, which increases complexity as your infrastructure grows.

On high-traffic websites, this added layer of processing may cause noticeable delays, frustrating users who expect fast load times. If your application handles heavy traffic or real-time transactions, this latency can be particularly problematic. 

The CrowdSec WAF solves this by using lightweight components that benefit from the memory of past behaviors within the central LAPI and a shared vision of an attacker’s behavior across multiple endpoints, allowing for more efficient protection.

False positives and false negatives

WAFs sometimes block legitimate requests (false positives), which can disrupt your business or users. On the flip side, some threats slip through undetected (false negatives), leaving your application vulnerable. 

Both situations are risky, either your users face unnecessary hurdles, or your defenses fail when it matters most. To address this, CrowdSec WAF relies on a combination of methods, including reporter trust score and diversity, machine profiling, cross-checking data sources to guarantee information consistency, and more. Check out how we collect, curate, and safeguard the accuracy of our data here. 

Maintenance and tuning

Cyber threats evolve constantly, and your WAF rules need to keep up. Left untuned, a WAF can become less effective, either missing new threats or creating unnecessary noise with outdated rules. 

Regular updates and monitoring are critical but can be resource-intensive for your team.

One of CrowdSec WAF’s biggest advantages over other Web Application Firewalls is its collaborative effort towards virtual patching. CrowdSec Network submits well-tested virtual patching rules that CrowdSec teams can curate in record time.

Not a complete solution

You need to recognize what a WAF can and can’t do. While it is great at protecting your web application from application-layer attacks, it doesn’t address every risk. 

For example, network-layer threats or endpoint vulnerabilities require other tools. A WAF works best as part of a layered security strategy, where different tools cover different types of risks.

Make WAF part of your preemptive security strategy

A WAF is a necessary layer of protection in a hostile digital environment. It is effective at defending against application-level attacks, but relying solely on it is a mistake. 

As threats grow more sophisticated, a single defense mechanism isn’t enough. Your security strategy must be holistic, combining the WAF with other tools such as intrusion detection systems, network firewalls, and continuous monitoring. 

CrowdSec is here to offer you a well-rounded approach that covers the vast majority of your security needs.

References and further reading