What Is A WAF (Web Application Firewall)?

A web application firewall (WAF) acts as a robust digital safeguard for your web applications, providing a critical line of defense against a wide array of application layer attacks. These attacks encompass a variety of threats, including but not limited to cross-site scripting (XSS), SQL injection and cookie poisoning. Such vulnerabilities often serve as the primary entry points for breaches, providing attackers with access to your organization’s most sensitive data.

What is a WAF?

Watch this Radware Minute episode with Radware’s Uri Dorot to learn what a web application firewall is, why it is important to have one, how it works, and what you should look for when choosing one.

By implementing an effective WAF, you equip yourself with the necessary tools to counteract these threats, thereby protecting your systems from attempts to extract sensitive information through exploitation. The WAF operates by meticulously managing the incoming HTTP traffic to your web application.

Engineered to combat a diverse range of threats such as cross-site request forgery, XSS, file inclusion and SQL injection, a WAF not only maintains the integrity and security of your web applications but also ensures the confidentiality and safety of your data against unauthorized access and cyber-attacks. It’s important to stay updated with the latest vulnerabilities that are common in web applications, such as broken access control and broken authentication, because understanding these vulnerabilities can help in configuring the WAF more effectively.

Essentially, a WAF serves as your frontline defense against ever evolving cyberthreats and plays a pivotal role in preserving the security and integrity of your web applications and data. WAFs are required to protect against all types of attacks on the OWASP Top 10 list, and a good WAF must also protect against unknown and zero-day attacks that go beyond the OWASP Top 10.

How Does A WAF Work?

When a WAF is deployed in front of a web application, a protective shield is placed between the web application and the internet that monitors all the traffic between the application and the end user(s). A WAF protects the web apps by filtering, monitoring and blocking any malicious HTTP/S traffic traveling to the web application, and also prevents any unauthorized data from leaving the application by adhering to a set of policies that help determine what traffic is malicious and what traffic is safe. Just as a proxy server acts as an intermediary to protect the identity of a client, in a traditional deployment, a WAF operates in similar fashion but in the reverse—called a reverse proxy—acting as an intermediary that protects the web app server from a potentially malicious client.


Figure 1: Basic WAF Architecture.

WAF Security Models

Web application firewalls (WAFs) typically employ three security models:

Positive Security Model (Whitelisting): This approach, also known as the “allow list” method, leverages machine learning and behavior modeling algorithms to determine the type of traffic that the WAF permits. In essence, it denies all requests by default and only allows those that are known to be trusted. This method is less resource-intensive than the negative security model and provides a list of IP addresses that are known to be safe. However, one downside of the positive security model is that it could unintentionally block benign traffic.

Negative Security Model (Blacklisting): Contrary to the positive security model, the “block list” method is based on up-to-date signatures against known vulnerabilities. It defines the type of traffic that the WAF denies, while the rest is accepted. This approach is good if you have omniscient knowledge of every single vulnerability that could ever exist for a single product. However, it requires constant updates to blacklist the next threats.

Hybrid Approach: This approach combines the strengths of both the positive and negative security models. It uses a combination of allow and block lists to determine what gets through. Some WAFs take a hybrid approach by using allow lists, but add in a secondary layer of blocklist checking for the most common types of attacks. This approach provides balanced and comprehensive security coverage, ensuring that your web applications are protected against both known and potential threats.

Each of these approaches has its own strengths and weaknesses, and the choice between them depends on the specific needs and circumstances of your web application environment. It’s also important to note that no single approach can provide complete security, and they are often used in conjunction with other security measures to provide a comprehensive defense strategy.

The Difference Between WAF and Firewall

A traditional firewall and a web application firewall (WAF) serve different, but complementary, roles in the realm of cybersecurity.

A network firewall operates primarily at the network and transport layers (layers 3 and 4 of the OSI model). Its primary function is to separate a secure zone from a less secure zone and control communications between the two. It acts as a barrier that prevents unauthorized access to the network as a whole. Network firewalls handle lower layers and are typically associated with protecting the network infrastructure. They monitor and control incoming and outgoing network traffic based on predetermined security rules.

On the other hand, a WAF provides protection at the application layer (layer 7 of the OSI model). A WAF protects web applications by monitoring and guarding Hypertext Transfer Protocol (HTTP) traffic. It sits between external users and web applications to analyze all HTTP communication. It then detects and blocks malicious requests before they reach users or web applications. As a result, WAFs secure business-critical web applications and web servers from zero-day threats and other application-layer attacks.

In essence, while a network firewall provides a first line of defense against a broad range of threats to the network and data centers, a WAF offers specialized, application-level protection to detect and block a variety of threats specific to web applications. Both play crucial roles in a comprehensive cybersecurity strategy.

Types of Web Application Firewalls and Deployment Options

A WAF can be implemented in various ways, each with its own unique advantages and disadvantages. There are three primary types of WAF:

1. Network-based WAF: This is generally a hardware-based solution that is installed locally on the network infrastructure. It offers low latency and high performance, which is crucial for real-time applications. However, it is also expensive and requires physical maintenance. Network-based WAFs are typically used by large organizations that have the resources to manage and maintain the physical equipment.

2. Software-based WAF: This type of WAF is managed by a service provider that offers the WAF on a “security-as-a-service” basis. Software-based WAFs provide additional customization options and are typically less expensive than network-based WAFs. However, their filtering and monitoring processes may be slower, as they are run on top of a virtual machine. These WAFs can be deployed for different servers, offering a lot of flexibility.

3. Cloud-based WAF: Cloud-based WAFs offer an affordable and easy-to-implement option. They usually offer a turnkey installation that is as simple as a change in DNS to redirect traffic. Cloud-based WAFs also have a minimal upfront cost, as users pay monthly or annually for security as a service. They can offer a solution that is consistently updated to protect against the newest threats without any additional work or cost on the user’s end. The drawback of a cloud-based WAF is that users hand over the responsibility to a third party. This means that some WAF features can be a “black box” for them.

Ideally, a WAF should provide the option to be deployed either in-line, where the solution can serve as a “middleman,” or as an API-based, out-of-path (OOP) service. An API-based, OOP deployment can offer several unique advantages that enable it to be optimized for multi-cloud environments. It enables application requests to go directly from the client to the application server without interruption. Benefits include reduced latency, no traffic redirection, increased uptime, and comprehensive protection across heterogeneous environments.

What Are the Key Capabilities of A WAF?

A web application firewall (WAF) is a critical component in ensuring the security of web applications. It should ideally possess a range of capabilities to provide comprehensive protection against both known and unknown threats. Here are some of the key capabilities a WAF should have:

Dual Security Models
A WAF should ideally combine both positive and negative security models. This combination allows the WAF to mitigate known web application attacks, such as access violations, attacks disguised behind Content Delivery Networks (CDNs), API manipulations and assaults, HTTP/S floods, brute force assaults, and others. Additionally, this combination also provides protection against unknown attacks and vulnerabilities, such as zero-day assaults.

Deep and Complete Coverage of the OWASP Top 10: A WAF should provide extensive security coverage that includes all the vulnerabilities listed in the OWASP Top 10. This ensures that the most critical security risks to web applications are well protected.

Protection Beyond the OWASP Top 10: In addition to covering the OWASP Top 10, a WAF should also provide protection against unknown and zero-day attacks. This means that the WAF should be capable of securing web applications from threats that are not yet known or have not been previously encountered.

Real-Time Policy Optimization
A WAF should leverage behavioral-based, machine-learning algorithms to create and optimize security policies in real-time. This capability ensures comprehensive protection while producing minimal to no false positives. It also provides automatic detection and protection of new applications as they are added to a network.

Core Features
The core features of a WAF should include the ability to filter network traffic based on geo-blocking, IP groups, blocklists, allowlists, whitelisting and blacklisting.

API Discovery and Protection
A WAF should provide API discovery and protection that offers visibility, enforcement and mitigation of all forms of API abuse and manipulation, whether for on-premises or cloud-hosted environments.

Built-In DDoS Protection
To counter application-layer DDoS attacks, a WAF should have built-in DDoS protection.

Integration with Bot Management Solutions
A WAF should have the ability to integrate with bot management solutions to detect and mitigate sophisticated, human-like bots.

Client-side Protection: A WAF should also include client-side protection to secure end users from attacks embedded in the application supply chain. This extends the protection against supply-chain attacks via third-party APIs to offer protection against attacks that do not go through the server and are thus not detected by traditional WAFs.

Data Leakage Prevention Mechanisms
To protect sensitive user data, such as personally identifiable information (PII), a WAF should have data leakage prevention mechanisms.

These capabilities ensure that a WAF can provide robust and comprehensive protection for web applications against a wide range of threats.

Why Do You Need a WAF?

In today's digital landscape, the need for a web application firewall (WAF) has become more critical than ever. Here are several reasons why you need a WAF:

Compliance Requirement: In many verticals today, having a WAF is not just a security best practice—it's a compliance requirement. Regulatory standards such as PCI DSS mandate the use of a WAF to protect against web application attacks.

Volume and Diversity of Attacks: With the increasing volume and diversity of web application attacks, it's virtually impossible to protect applications without a WAF. A WAF protects web applications from a wide range of attacks such as cross-site forgery, server-side request forgery, file inclusion and SQL injection, among others. It also safeguards applications and websites against the most critical security vulnerabilities (visit OWASP Top 10 to see the full list) including, but not limited to:

  • Injection Attacks: This category includes both cross-site scripting (XSS) and SQL Injection attacks. In these types of code injection attacks, adversaries insert malicious scripts or SQL statements into a legitimate website or web application’s database query software. This can potentially allow the attacker to steal sensitive information, impersonate the user, or modify or delete information in the database.
  • Security Misconfiguration: This category includes various types of attacks that exploit misconfigurations in web applications, including some forms of application-layer DDoS attacks.
  • Vulnerable and Outdated Components: This category addresses risks associated with using components with known vulnerabilities, which can be exploited by attackers.

Agile Development Methodologies: While these methodologies allow for rapid development and deployment, they can also introduce new vulnerabilities into web applications if not properly managed. A WAF can provide the necessary security controls to mitigate these risks.

Shift to the Cloud: As more organizations move their operations to the cloud, they must also contend with the unique security challenges that this environment presents A WAF can provide consistent security policies across on-premises and cloud environments.

Increased Use of Web-Based Software or SaaS Applications: These applications can be targeted by attackers due to their widespread use and internet-facing nature. A WAF can protect these applications by filtering, monitoring, and analyzing HTTP and HTTPS traffic.

Remote Workforces: The shift to remote work has expanded the attack surface for many organizations, as employees access corporate resources from various locations and devices. A WAF can provide the necessary security controls to protect these remote access points.

In addition to the above, it’s important to consider zero-day attacks, which occur when a hacker discovers and exploits a previously unknown vulnerability in a software application before the software developer has had a chance to create and distribute a patch. There are two perspectives to consider when discussing zero-day attacks:

1. Exploitation of Zero-Day Vulnerabilities: This is when an attacker exploits a vulnerability that is not yet known to the public or the vendor.

2. Unrecognized Attacks by Current Defense Systems: From the perspective of protection solutions such as WAF, DDOS and bot mitigation, a zero-day attack can also refer to an attack that the current defense system doesn’t recognize as an attack because there is no signature for it yet.

By incorporating a WAF into their security infrastructure, organizations can significantly enhance their ability to defend against these and other application-level attacks. This can help protect sensitive data, maintain application availability, and ensure compliance with various regulatory standards.

Web Application Attack Trends

There has been a 171% year-on-year increase in malicious web application transactions from 2022 to 2023, a steep rise compared to the 128% rise from 2021 to 2022.

Top web application security violations per type since 2021

Figure 2: Top web application security violations per type since 2021. Source: Radware 2024 Global Threat Analysis Report

WAF Market Dynamics and Trends

The Web Application Firewall (WAF) market is evolving rapidly, driven by several key factors and trends:

1. Democratization of AI Tools: The latest advancements in AI and the democratization of generative AI tools have made malicious scripts and injections easily available for any wannabe hacker. This has led to a surge in the number of attack attempts and more sophisticated attacks.

2. Integration with Other Systems: WAF providers are continuously improving their offerings by integrating their products with other systems such as Security Information and Event Management (SIEM) systems, Application Security Testing (AST), and Web Access Management (WAM). This integration allows for a more comprehensive security approach, enhancing the ability to detect and respond to threats.

3. Use of AI in Detection, Monitoring, Analytics, and Mitigation Engines: Vendors are developing WAF solutions based on a positive security model that utilizes machine learning algorithms to analyze HTTP requests. This advanced technology helps in accurately identifying and mitigating potential threats, thereby enhancing the overall security posture.

4. Increase in IoT Devices: The proliferation of internet of things (IoT) devices has led to an increased focus on data privacy norms. Organizations are likely to invest in WAF solutions that offer IoT-specific features such as device fingerprinting and protocol validation to ensure compliance and enhance security.

5. Demand for Enhanced Threat Intelligence: Enterprises are seeking WAF solutions that provide enhanced threat intelligence, extended protection, and a variety of out-of-the-box integrations. These features enable organizations to better understand the threat landscape and respond effectively to potential attacks.

6. Focus on New Detection Methods: There is a growing focus on developing new detection methods to prevent web attacks and minimize false positives. This not only improves the accuracy of threat detection but also reduces the chances of legitimate traffic being blocked.

7. Consolidation of Solutions and Transition from WAF to WAAP: The market is witnessing a trend towards the consolidation of solutions, moving towards a single pane of glass approach. This involves transitioning from traditional WAF solutions to web application and API protection (WAAP), which provides a more comprehensive and modernized approach to application security.

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