What is a Web Application Firewall (WAF)?
A Web application firewall (WAF) protects websites and web applications by monitoring, filtering, and blocking malicious HTTP/S traffic, defending against threats like SQL injection, cross-site scripting (XSS), and other OWASP Top 10 vulnerabilities. WAFs act as a specialized defense layer, inspecting traffic based on security rules to identify and block harmful requests before they reach the application, ensuring the integrity and security of the web service.
Positioned as a reverse proxy, a WAF inspects incoming and outgoing web traffic, enabling detection and mitigation of suspicious requests before they reach the web application(s). WAFs operate using a combination of predefined rules and dynamic analysis techniques to identify potentially harmful behavior or patterns.
WAFs can be deployed as hardware appliances, software solutions, or cloud-based services, offering flexible integration into a variety of infrastructures. The primary goal of a WAF is to ensure that only safe, properly structured requests make it to the application, protecting sensitive user data and business processes from exploitation.
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Web applications are a frequent target for attackers because they often contain vulnerabilities that can be exploited to compromise sensitive data, disrupt services, or deface content. A breach at the application layer typically allows threat actors direct access to databases and internal resources, resulting in significant financial, operational, and reputational damage.
Traditional firewalls and network security devices are not equipped to inspect and understand application-layer traffic, leaving many modern threats undetected if a dedicated WAF is not in place. Organizations are compelled to deploy WAFs as they often face regulatory requirements regarding data protection and privacy, such as PCI DSS or GDPR.
Compliance aside, WAFs provide a proactive defense against known threats and zero-day vulnerabilities by applying rigorous traffic inspection, anomaly detection, and adaptive response mechanisms.
Related content: Read our guide to WAF cyber security.
OWASP Top 10/ Application-Layer Attack Protection
The OWASP Top 10 outlines the most critical security risks to web applications, including SQL injection, cross-site scripting, and insecure deserialization. Modern WAFs offer native protection against these prevalent threats by inspecting incoming HTTP/S requests and blocking attacks before they can exploit application vulnerabilities.
Beyond basic blocking, WAFs provide real-time visibility and logging of attempted attacks, assisting security teams in understanding threat patterns and reinforcing weak spots. This detailed insight allows organizations to prioritize remediation efforts according to real-world risks and adapt their security policies in response to emerging threats.
Deep HTTP/S Traffic Inspection
Deep HTTP/S inspection enables a WAF to analyze application traffic at a granular level, inspecting full request and response payloads rather than just headers. This capability is crucial for identifying complex, obfuscated, or multi-stage attacks that may bypass simpler filtering mechanisms. By thoroughly examining HTTP/S data, a WAF can enforce strict security policies.
The inspection process often includes decoding encoded payloads, scrutinizing embedded scripts, and validating request structure against application expectations. This level of scrutiny ensures that malicious content or data exfiltration attempts are caught early, while legitimate user interactions remain unaffected.
Signature-based + Behavior / Anomaly Detection
Signature-based detection leverages known attack patterns and rule sets to identify and block threats, effective for defending against common exploits that have been previously cataloged. However, attackers frequently vary their techniques to circumvent these rules. To address this, WAFs also employ behavioral and anomaly detection, continuously learning normal traffic patterns and flagging deviations that may indicate novel attacks or suspicious behavior.
This combination of methods significantly strengthens a WAF's defensive capability, providing coverage against both established threats and new, emerging tactics. By correlating multiple detection approaches, WAFs minimize false negatives, adapt to evolving threat landscapes, and offer actionable intelligence for swift incident response.
Virtual Patching
Virtual patching is the process of mitigating vulnerabilities at the WAF layer, often before the underlying application code can be updated. When a new vulnerability is discovered, a virtual patch can be created quickly to block specific exploit methods, buying valuable time for development teams to perform permanent code fixes.
By implementing virtual patches, organizations reduce their attack surface and likelihood of exploitation, even during periods between vulnerability disclosure and actual remediation. This also helps organizations maintain compliance with regulatory standards, as virtual patches can be documented as compensating controls for known risks in audit scenarios.
Bot and Automated Attack Mitigation
Malicious bots conduct a range of automated attacks, including credential stuffing, scraping, vulnerability scanning, and denial-of-service attacks. WAFs are equipped with mechanisms to detect and mitigate these threats, often through device fingerprinting, CAPTCHA challenges, analysis of request patterns, and rate limiting. This helps in distinguishing legitimate users from automated, potentially harmful traffic.
Effective bot mitigation protects not only the application's availability and performance but also protects business data and user privacy. As bots become more sophisticated, leveraging headless browsers and mimicking human behavior, advanced WAFs continuously update their algorithms and blocking techniques.
DDoS and Rate Limiting
Distributed Denial of Service (DDoS) attacks aim to overwhelm web applications with massive traffic, rendering them inaccessible to legitimate users. WAFs defend against DDoS by employing a combination of high-capacity filtering, source validation, geo-blocking, and real-time traffic analysis to dissipate attack traffic. They do this without impacting genuine requests, ensuring business continuity during volumetric or application-layer attacks.
Rate limiting further enforces security by restricting the number of requests an entity can make within a certain timeframe. This hampers brute force login attempts, API abuse, or resource-intensive spam activity.
SSL/TLS Support
Modern web applications rely heavily on SSL/TLS encryption to protect data as it travels between clients and servers. For effective inspection and defense, a WAF must support decryption and re-encryption of SSL/TLS traffic. This allows it to analyze secure payloads for threats that may be hidden within encrypted sessions, which would otherwise evade detection by traditional security tools.
SSL/TLS support in a WAF also enforces strong encryption policies, ensures the application uses up-to-date ciphers, and can help prevent protocol downgrade attacks. Properly managed, this feature ensures that sensitive information is not only transmitted securely but also scrutinized for anomalies or exploits without compromising user privacy or data integrity.
API Protection
Automated API discovery plus schema- and business-logic enforcement are now core WAF capabilities: modern WAF/WAAP solutions continuously discover and catalog API endpoints, validate requests against API schemas (a positive-security model), and apply behavioral learning to detect token manipulation, parameter tampering, business-logic abuse and API-targeted bots or account-takeover attempts. These controls let a WAF block API-specific abuse (including scraping, credential stuffing and malformed/invalid payloads) in real time while reducing false positives through learned business-logic context.
Client-side Protection
Protection that extends into the browser monitors and hardens the client-side attack surface by mapping and continuously monitoring third-party scripts and browser-side supply-chain components, detecting risky or malicious changes and preventing data exfiltration from the user’s browser. When combined with server-side WAF controls and bot management, client-side protections provide end-to-end coverage for sensitive user data (payment and PII) that attackers try to harvest via injected/skimmer scripts or compromised third-party resources.
Radware Cloud WAF is a cloud-native web application firewall that protects applications and APIs from a broad spectrum of web threats, including OWASP Top 10 vulnerabilities, bot attacks, and data leakage. Delivered as part of Radware’s Cloud Application Protection Service, it combines machine learning, advanced threat intelligence, and automation to provide continuous, adaptive protection with minimal manual effort.
Key features include:
- Automated rule generation: Analyzes applications and automatically creates precise security policies to detect and block threats without overblocking.
- Threat intelligence–driven defense: Leverages global attack data to identify and mitigate emerging vulnerabilities and exploit patterns in real time.
- Bot and API protection: Uses device fingerprinting and AI-powered API discovery to prevent abuse from malicious bots and unauthorized API usage.
- Data leak prevention: Blocks transmission of sensitive data such as credentials, credit card numbers, and personal identifiers.
- Compliance and certifications: NSS Labs recommended, ICSA Labs certified, and PCI-DSS compliant for robust enterprise-grade security.
- Integrated Layer-7 protection: Includes web DDoS mitigation and client-side protection for a full-stack security approach.
Barracuda Web Application Firewall offers protection for web applications, APIs, and mobile app backends by combining ease of use with strong security capabilities. It defends against a range of threats including OWASP Top 10 attacks, zero-days, DDoS, and bot-based exploits.
General features include:
- Easy deployment with minimal setup and no specialized training required
- Available as on-premises appliance, virtual machine, or cloud-native service
- Integrated with CI/CD tools via REST API for DevOps-friendly automation
- Hardened SSL/TLS stack and application delivery features (load balancing, caching, routing)
- Supports integration with AD, LDAP, RADIUS, and SAML for secure access control
Features for website protection include:
- Blocks OWASP Top 10 threats, zero-day exploits, and application-layer DDoS attacks
- Advanced bot protection using machine learning to distinguish between good and bad bots
- API protection with schema-aware security for REST, JSON, XML, and WSDL interfaces
- Automated API rule generation from OpenAPI definitions to reduce configuration time
- Two-factor authentication support for enhanced backend access security
Imperva Web Application Firewall offers a low-maintenance solution for protecting web applications and APIs across cloud, on-premises, or hybrid environments. It stands out for enabling most customers to confidently operate in blocking mode due to its near-zero false positive rate, made possible by managed rules.
General features include:
- Near-zero false positives allow confident use of blocking mode
- Managed rules continuously updated by Imperva’s Threat Research Labs
- Machine learning-driven incident correlation reduces alert fatigue
- Automated deployment and management using Terraform and IaC practices
- Full enterprise-grade SSL management with automated renewal
Features for website protection include:
- Blocks OWASP Top 10 threats like SQL injection and XSS with high accuracy
- Real-time threat detection and proactive virtual patching via managed rules
- Protects APIs and web applications from automated attacks and data breaches
- Provides centralized visibility into attack sources, methods, and severity
- Ensures seamless inspection of encrypted traffic through SSL termination
Cloudflare Web Application Firewall provides protection against a range of application-layer attacks, including zero-day threats. Built on Cloudflare’s globally distributed network, the WAF processes billions of requests daily and applies threat intelligence, machine learning, and real-time analysis to block malicious activity at the edge.
General features include:
- Backed by a global network that handles over 100 million requests per second
- Uses machine learning to detect and block zero-day threats in real time
- Simple setup with no need for professional services or manual tuning
- Integrates with Cloudflare’s full security stack for broader protection
- Combines managed rulesets (including OWASP) with support for custom rules
Features for website protection include:
- Blocks OWASP Top 10 attacks such as SQL injection and XSS by default
- Detects and prevents credential stuffing and account takeover attempts
- Scans uploaded files to detect and block malware before it reaches backend systems
- Applies advanced rate limiting and security rules to mitigate abuse and targeted threats
- Continuously updated rules protect against new vulnerabilities with minimal delay
Fortinet FortiWeb is a web application firewall to secure web applications and APIs against both known vulnerabilities and advanced, zero-day attacks. It uses a dual-layer machine learning system to detect threats in real time while minimizing false positives, reducing the operational burden typically associated with WAF management.
General features include:
- Protects against known and unknown threats, including AI-generated zero-day exploits
- Dual-layer machine learning reduces false positives and manual tuning
- Available as hardware appliance, virtual machine, SaaS, or cloud instance
- Integrated with Fortinet Security Fabric (e.g., FortiGate, FortiSandbox)
- Advanced threat analytics simplify incident response and playbook execution
Features for website protection include:
- Defends against OWASP Top 10 threats, DDoS, bots, and advanced APTs
- Identifies and blocks malicious bots while allowing legitimate automation (e.g., search crawlers)
- Automatically discovers and secures APIs, generating schema-based positive security models
- Scans uploaded content for malware using AI-powered inline inspection
- Prevents credential theft and malicious command-and-control (C2) communication
Conclusion
Web application firewalls have become essential for securing websites against a growing array of sophisticated threats targeting the application layer. Their ability to block injection attacks, mitigate bot abuse, enforce rate limits, and inspect encrypted traffic ensures that both legacy and modern web applications remain secure and available. Effective WAFs also reduce the time to respond to zero-day vulnerabilities through features like virtual patching and adaptive threat detection.