What are WAF Bypass Attacks?

Bypass attacks occur then attackers find ways to work around a WAF's defenses. Learn how they work, their consequences and how to prevent them

A Web Application Firewall (WAF) is like a security guard for your website or web application. It analyzes incoming traffic, filtering out attacks like SQL injection or cross-site scripting that try to steal data or damage your online assets. They are essential for protecting your company’s online presence.

While WAFs offer robust protection, they’re not invincible. Cunning attackers can leverage various techniques to bypass their defenses, leaving your web applications vulnerable.

Here’s a deeper look at the challenges:

  • Obfuscation and Encoding: Attackers can disguise malicious code using techniques like encoding characters, splitting payloads across multiple requests, or leveraging unusual formatting. This sleight of hand can trick the WAF into overlooking the true nature of the attack.
  • Exploiting WAF Logic: Adept attackers might spend time studying a WAF’s rule sets and logic. They can then craft requests that exploit loopholes or manipulate the decision-making process, sneaking malicious code past the filter.
  • Zero-Day Exploits: Occasionally, hackers find weaknesses within an application that can be exploited by attackers before the vendor can release a fix. These are also known as 0-day vulnerabilities.
  • Targeting Application Vulnerabilities: If the underlying web application has security flaws, attackers might be able to bypass the firewall entirely and aim directly at those weaknesses.

Sadly, there is no easy way to know the exact number of bypass attacks that have occurred in the past. This is due to factors such as under-reporting, lack of a central database for reports and varied definitions of what constitutes a successful attack. It is likely that the true number is much higher than any that might get officially reported.

What Are the Consequences of a WAF Bypass Attack?

Since bypass attacks essentially let attackers slip through your defenses, the potential damage caused by them can be severe. They can lead to:

  • Data Breaches: One of the most serious outcomes of a WAF bypass is the unauthorized access and exfiltration of sensitive data. Attackers can exploit vulnerabilities in the web application to steal data such as personal information, credit card numbers, passwords, and proprietary business information. This can lead to significant financial losses, damage to reputation and even legal liabilities due to data privacy legislation like the GDPR (General Data Protection Regulation) and HIPAA (Health Insurance Portability and Accountability Act), among many others.
  • Unauthorized Access and System Compromise: By bypassing the WAF, attackers can gain unauthorized access to restricted areas of the web application or underlying systems. This can enable them to modify content, inject malicious scripts, establish backdoors for future access, or escalate privileges to gain further control over the system.
  • Denial of Service (DoS) or Distributed Denial of Service (DDoS) Attacks: With the WAF bypassed, attackers can more easily launch DoS or DDoS attacks, overwhelming the web application with traffic to render it unavailable to legitimate users. This can disrupt business operations, lead to loss of revenue, and damage customer trust.
  • Defacement: An attacker might deface the web application by altering its visual appearance or content, typically for political messages, vandalism, or to undermine trust in the targeted organization. Defacement can harm an organization’s image and require time and resources to restore the original content.
  • Malicious Code Injection: Attackers can exploit a WAF bypass to inject malicious code, such as cross-site scripting (XSS) payloads or malware, into the web application. This can lead to further attacks against users of the application, such as stealing cookies, session hijacking, or delivering malware to users’ devices.

Which Kinds of WAFs Are More Vulnerable to Bypass Attacks?

As we previously discussed, there are two main types of WAFs: signature-based and score-based. Generally, signature-based WAFs are considered more susceptible to bypass attacks. This is because attackers can specifically target the known security vulnerabilities that the WAF signatures are designed to detect. Also, novel (zero-day) attacks are more effective, since they won’t be detected until they are added to the signature database.

How to Prevent WAF Bypass Attacks?

There are many measures you can take to avoid falling victim to a WAF Bypass attack. The first one is to keep your WAF software and its rule sets up-to-date, specially if you use a signature-based system. This helps protect against the latest bypass techniques and Zero-Day attacks.

You should also establish a test and review policy, conducting regular penetration testing to see if attackers could find ways to bypass your WAF. This lets you fix vulnerabilities before they are exploited. Also, remember that a WAF is just one layer of protection on your defense system. It is essential to implement good coding practices, secure input validation, and other security measures to protect your applications.

Last, select a WAF provider with a strong reputation for security and a commitment to constantly improving their product. If possible, prefer score-based WAFs, since they are less vulnerable to zero-day attacks.

WAF bypass attacks are a serious concern. However, a well-chosen and carefully maintained WAF remains a powerful shield for your web applications, especially when combined with other modern security practices.

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