Slowloris & R.U.D.Y.: Detecting and Blocking Low-and-Slow Application Layer DDoS Attacks

Introduction: The Stealth Threat Redefined

While traditional Distributed Denial-of-Service (DDoS) attacks overwhelm servers with massive traffic, Low-and-Slow Application Layer DDoS attacks have emerged as a more insidious threat. These attacks paralyze targets with minimal resource consumption, with Slowloris and R.U.D.Y. (R-U-Dead-Yet?) being two primary weapons. According to Cloudflare's 2022 report, over 35% of application-layer attacks now employ these techniques, with an average detection delay of 72 minutes. This article dissects their mechanisms and provides actionable detection and mitigation strategies.

Anatomy of Low-and-Slow Attacks

Slowloris: The Art of Connection Exhaustion

Created by security researcher RSnake in 2009, Slowloris was designed to exploit concurrent connection handling flaws in traditional web servers like Apache. Its elegance lies in requiring just one ordinary computer to paralyze targets. It gained notoriety after being weaponized by the hacktivist group Anonymous to crash government websites.

• Attack Workflow:

1. Establish numerous HTTP connections with incomplete headers (e.g., missing \r\n\r\n).
2. Periodically send junk bytes (e.g., X-a: b\r\n) to keep connections alive.
3. A single thread can occupy 200-400 concurrent connections.

• Impact Formula:

Service Collapse Threshold = Server Max Connections / (Attack Threads × Connections per Thread)  

Example: Nginx’s default 10,000-connection limit can be breached with just 25 attack threads.

R.U.D.Y.: Poisoning POST Requests

The name taunts victims with "R-U-Dead-Yet?" – a nod to its destructive power. First observed in 2014, R.U.D.Y. weaponizes servers’ indefinite retention of unfinished POST requests, achieving persistent resource occupation with bandwidth consumption as low as 1 Kbps. Its debut caused extended outages for major e-commerce platforms.

Attack Workflow:

1. ​Target Reconnaissance
   R.U.D.Y. first scans the target website to catalog all web forms vulnerable to malicious submissions, including contact forms, search bars, and comment sections.
2. ​Crafted POST Requests
   Attackers send valid POST requests with enormous Content-Length headers (e.g., 10GB), tricking servers into reserving resources for anticipated data.
3. ​Low-and-Slow Data Drip
   Instead of completing the request, R.U.D.Y. transmits payloads one byte at a time with ​10-second delays between each byte. This maintains connections at speeds as low as 0.1 Bps.
4. ​Connection Persistence
   Whenever servers attempt to terminate inactive connections (typically after 60-300 seconds), R.U.D.Y. re-initiates requests to refill the connection table.

Impact Amplification:

Server threads/processes hang indefinitely awaiting data completion. With precise timing control, ​a single attacker can occupy 10,000+ connections without triggering volumetric alarms. The attack culminates in complete exhaustion of server worker pools, often crashing applications within 15-30 minutes.

How to Detect Slow DDoS Attacks

When your website suddenly slows down or even crashes, traditional firewalls might not respond—because these attacks disguise themselves as normal visitors! This is the cunning nature of slow DDoS attacks (like Slowloris and R.U.D.Y.).

3-Step Self-Check: Is Your Website Under Attack?

1. Observe User Action Times

• Normal: Submitting a form takes 3 seconds
• Under attack: The same action takes over 3 minutes

2. Check Server Connection Counts

• Log into your server control panel (like cPanel)
• Check if "active connections" suddenly spike
• Normal small to medium websites: Typically <500 connections
• Under attack: Could exceed 5000+ connections

3. Compare Traffic Patterns

• Use free tools (like Google Analytics)
• Compare attack periods with normal periods:
  • Page load times (a 300%+ increase is a red flag)
  • User dwell times (abnormally long times may signal an attack)

3 Easy Ways to Stop Slow Attacks

1. Enable Connection Limits

Just like a restaurant limits dining time per table, set limits on your server:

# Simple configuration example (suitable for non-technical users)
limit_conn per_ip 50;  # Maximum 50 connections per IP
client_header_timeout 10s; # Disconnect if no data is sent in 10 seconds

2. Set Up Automatic Alerts

Use free tools like UptimeRobot to:

• Monitor site speed every minute
• Get instant SMS/email alerts if response exceeds 3 seconds

3. Emergency Response Plan

If already under attack:

• Enable CAPTCHA challenges (5-minute Cloudflare setup)
• Block suspicious IP ranges (check logs for high-frequency IPs)
• Request traffic scrubbing from your hosting provider

The Best Choice: EdgeOne DDoS Protection

Tencent EdgeOne provides ​multi-layered defense against both L3/L4 volumetric attacks and sophisticated L7 application-layer threats, with preconfigured protection policies enabled by default. Its real-time traffic monitoring and AI-powered scrubbing engine achieve ​99.99% attack traffic filtering accuracy within 50 milliseconds.

Specialized Defense: CC Attack Mitigation System

As a variant of application-layer DDoS, Challenge Collapse (CC) attacks (HTTP/HTTPS Floods) aim to exhaust server resources through malicious request storms. EdgeOne deploys a three-tier defense mechanism:

1.  ​Adaptive Frequency Control

• Dynamically identifies abnormal IPs through baseline learning and imposes request rate limits per IP.
• Defense Scenarios: High-frequency concurrent requests (e.g., 500+ API calls/second).
• Typical Use Case: Blocking bot-driven seat-grabbing attacks during e-commerce flash sales.

2. ​Slow Attack Protection

• Blocks connections with transfer rates <1 KB/s using session-rate modeling.
• Precision Detection: Identifies R.U.D.Y.'s signature 1-byte/10-second drip attacks.
• Threshold: Auto-blocks sessions with transfer rates 70% below baseline.​

3. Intelligent Client Filtering (Enabled by Default)

• ​Real-Time JS Challenge: Validates client legitimacy through browser execution.
• ​Header Analysis: Detects anomalies across 25+ headers (e.g., abnormal Accept-Language distributions).
• ​IP Reputation Database: Flags traffic from proxies/VPNs and known malicious IPs.
• Action: Triggers CAPTCHA for high-risk sessions (false positive rate <0.1%).
   

Conclusion

Low-and-slow DDoS attacks like Slowloris and R.U.D.Y. are stealthy threats that bypass traditional defenses by mimicking legitimate traffic. Unlike brute-force volumetric attacks, they silently exhaust server resources, often going undetected for hours. Key takeaways:

1. Detection Requires Behavioral Insight

• Monitor connection durations, incomplete requests, and abnormally low data rates.
• Free tools like Google Analytics and UptimeRobot can reveal early warning signs.

2. Mitigation Demands Layered Defense

• Server Hardening: Enforce connection limits (e.g., limit_conn in Nginx).
• Reverse Proxies: Filter attacks before they reach your origin server.
• AI-Powered Solutions: Tools like Tencent EdgeOne use behavior analysis and real-time traffic scrubbing to neutralize threats.

FAQs

Q1: Upgrading to expensive servers will stop attacks?

A1: Attackers can cripple a $10,000 server for just $1.

Q2: Only big websites get targeted?

A2: No. In Q4 2024, industries like Marketing & Advertising (3rd most attacked) and ransom attacks on smaller services (12% of victims) proved attackers target vulnerabilities, not just size. (Source: Cloudflare report for 2024 Q4).

Q3: Are WordPress sites more vulnerable?
A3: Not inherently, but 43% of attacked CMS sites run WordPress (Source: Sucuri). Plugins like Wordfence add basic protection.

Q4: Do slow attacks steal data?
A4: No. Their goal is to crash your site, not steal information. But they often distract from real data breaches!

Q5: How long do slow attacks typically last?

A5: Short: 1-6 hours (common for small sites). Extended: Days/weeks (targeting enterprises)

Q6: Will a VPN protect my website?
A6: No. VPNs protect your browsing, not your server. Use a WAF (Web Application Firewall) instead.