A WAF creates a shield between a web app and the Internet; this shield can help mitigate many common attacks.
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A WAF or Web Application Firewall helps protect web applications by filtering and monitoring HTTP traffic between a web application and the Internet. It typically protects web applications from attacks such as cross-site forgery, cross-site-scripting (XSS), file inclusion, and SQL injection, among others. A WAF is a protocol layer 7 defense (in the OSI model), and is not designed to defend against all types of attacks. This method of attack mitigation is usually part of a suite of tools which together create a holistic defense against a range of attack vectors.
By deploying a WAF in front of a web application, a shield is placed between the web application and the Internet. While a proxy server protects a client machine’s identity by using an intermediary, a WAF is a type of reverse-proxy, protecting the server from exposure by having clients pass through the WAF before reaching the server.
A WAF operates through a set of rules often called policies. These policies aim to protect against vulnerabilities in the application by filtering out malicious traffic. The value of a WAF comes in part from the speed and ease with which policy modification can be implemented, allowing for faster response to varying attack vectors; during a DDoS attack, rate limiting can be quickly implemented by modifying WAF policies.
A WAF that operates based on a blocklist (negative security model) protects against known attacks. Think of a blocklist WAF as a club bouncer instructed to deny admittance to guests who don’t meet the dress code. Conversely, a WAF based on an allowlist (positive security model) only admits traffic that has been pre-approved. This is like the bouncer at an exclusive party, he or she only admits people who are on the list. Both blocklists and allowlists have their advantages and drawbacks, which is why many WAFs offer a hybrid security model, which implements both.
A WAF can be implemented one of three different ways, each with it’s own benefits and shortcomings:
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