HULK (HTTP Unbearable Load King) is a class of application-layer (Layer 7) HTTP flood attack that deliberately generates large volumes of unique, cache-busting requests to overwhelm web servers and application stacks. Originally released as a research/stress-testing tool in 2012, HULK’s technique—randomizing query strings, headers and URIs to defeat caching and signature detection—has been adopted and adapted by attackers to build scalable, hard-to-detect HTTP floods. Because these attacks mimic legitimate HTTP behavior at scale, defending against HULK-style floods requires behavior-based detection, adaptive rate control and tight integration between application security and network defenses.
HULK-style attacks generate a high volume of HTTP requests that are intentionally randomized so that each request appears unique to caches and simple signature-based filters. Attack clients vary the request path, query parameters, HTTP headers (User-Agent, Referer), and sometimes use randomized cookie values to ensure requests bypass cache and trigger full application processing.
The attack targets CPU, memory and connection/thread resources on web servers and application stacks—rather than network bandwidth—by forcing expensive dynamic content generation, database queries or authentication checks. Because requests resemble normal user traffic in structure, IP-based blocking and static signatures are often ineffective against distributed, proxy-backed campaigns.
Modern variants add layers of sophistication: proxy chaining, TLS/HTTPS support, coordinated botnet orchestration, and adaptive request pacing. These enhancements allow attackers to scale HULK-like floods while complicating blocklist-based defenses.
Typical indicators of a HULK-style event include sustained spikes in HTTP GET/POST rates, unusually high URL entropy (many unique URIs per second), elevated server CPU and memory usage, and a rise in application errors (500s) due to exhausted resources. Because the requests often look syntactically valid, anomaly detection must analyze request patterns, header variance and session characteristics rather than rely on payload signatures.
Open-source implementations and proof-of-concept tools—original HULK scripts and later modules—have provided attackers with templates for randomized HTTP floods. Combined with botnets and proxy services, these modules can generate distributed HTTP floods that appear origin-address diverse and temporally variable.
Some modern attack chains couple HULK-style floods with credential-stuffing attempts or API abuse, using the same randomized request patterns to mask malicious activity. Others use small, repeated bursts with gap intervals (low-and-slow) to evade threshold-based detectors while degrading service over time.
HULK-style HTTP floods have been implicated in numerous application-layer outages and incident response cases across industries, from gaming and media to finance and public-sector websites. Industry telemetry shows an ongoing prevalence of HTTP flood campaigns that leverage randomized request patterns—effectively making HULK a template rather than a single, discrete tool. Radware and other vendors continue to observe adaptive HTTP flood signatures emerging from commercial botnets and rented booter services.
Given the continued growth of APIs and dynamic web applications, application-layer floods remain a high-impact threat: they are relatively inexpensive to launch and can directly translate into lost revenue, degraded user experience and costly mitigation efforts.
HULK-style attacks defeat legacy defenses because they exploit the very assumptions those defenses make: that high-volume traffic is inherently malicious and that identical requests imply an attack. HULK flips this model by generating high volume composed of non-repeating, valid requests. As a result, IP blacklisting, static rate caps and signature matching frequently underperform; defenses must instead profile behavioral baselines and apply context-aware mitigation.
Another failure mode is siloed tooling: WAFs that are not tightly integrated with network DDoS systems, or DDoS appliances that lack application-layer awareness, create gaps attackers can exploit. Successful mitigation requires orchestration between the WAF, application security controls and edge/ cloud scrubbing services.
Application-layer DDoS requires a layered defense that can detect anomalous behavior quickly, apply adaptive controls to preserve user experience, and scale mitigation without wholesale blocking. Key mitigation tactics include:
Implement real-time behavioral analytics that monitor request entropy, session patterns and header anomalies. Apply dynamic rate controls that are traffic-aware—throttling or challenging only the anomalous portion of flows rather than the entire client population. Use graduated mitigations (soft throttles, JS challenges, CAPTCHA) before escalating to blocking to minimize user friction.
How Radware helps: DefensePro deploys adaptive behavioral algorithms at line-rate, detecting anomalies in request patterns and session behavior without relying on static signatures. Radware’s Cloud DDoS Protection Service scales mitigation elastically and coordinates diversion for overflow scenarios.
Integrate WAF logic with behavioral detection to enforce positive security models and block unexpected query patterns. Ensure the WAF supports auto-learning modes and can surface newly observed legitimate structures to reduce false positives.
How Radware helps: Cloud WAF Service applies adaptive learning to distinguish legitimate URL structures from randomized attack traffic and can apply application-specific policy enforcement to stop HULK-style requests.
Employ session-aware throttles, per-endpoint quotas, and progressive challenge flows that escalate only for suspicious sessions. Preserve legitimate client experience by validating JavaScript execution or performing device fingerprinting before applying hard blocks.
How Radware helps: Web DDoS Protection generates real-time, behavior-based L7 signatures and challenge flows that mitigate randomized HTTP floods while minimizing collateral damage to legitimate users.
Treat APIs as first-class assets: enforce schema validation, strong authentication, and strict rate limits. Monitor API patterns for unusually high uniqueness or malformed payloads that indicate programmatic misuse.
How Radware helps: Cloud Application Protection Service includes API-aware modules for schema validation, adaptive throttling and integrated bot defenses to stop API-targeted HULK-style activity.
Maintain runbooks tuned for L7 events, pre-authorize scrubbing diversion where relevant, and integrate WAF and DDoS telemetry into SOC workflows. Use threat intelligence to identify emerging HTTP flood patterns and known abusive sources.
How Radware helps: Threat Intelligence Subscriptions provide live indicators of attacks and malicious infrastructure, while Cloud Network Analytics gives cross-customer telemetry and forensic views to accelerate classification.
Run simulated L7 flood exercises to validate detection thresholds, practice diversion workflows, and ensure escalation paths between NOC, SOC and legal/communications teams are clear. Maintain pre-authorized contact points with ISPs and cloud scrubbing partners to reduce administrative lag during live incidents.
Instrument WAF and application logs for rapid correlation, and preserve packet captures when possible to support signature development and post-incident analysis.
HULK’s original approach—randomized, cache-busting HTTP requests—remains a foundational technique for modern application-layer floods. As attackers adopt proxy networks, TLS support and AI-assisted request generation, defenders must rely on adaptive detection, coordinated WAF/DDoS controls, and managed mitigation services to maintain availability. Key takeaways: profile application behavior, instrument APIs, use graduated mitigation to protect UX, and integrate threat intelligence to stay ahead of evolving HTTP flood campaigns.
To learn more about how Radware can safeguard your organization from HULK-style HTTP floods and other advanced application-layer DDoS attacks, contact us now.