DDoS attacks: types, defenses and the current threat situation

In the first half of 2024, the BSI recorded a massive increase in high-volume DDoS attacks in Germany. What once required complex infrastructure now costs just a few euros per hour on the dark web. DDoS has evolved from a hacker’s tool into a mass-market product - and a weapon in geopolitical conflicts.

What is a DDoS attack?

Distributed Denial of Service (DDoS) is an attack in which a target system (server, network, service) is brought to a standstill by an overload of requests from many distributed sources (hence "distributed"), preventing legitimate users from accessing it.

Difference between DoS and DDoS:

DoS (Denial of Service): Attack from a single system - easy to block
DDoS: Attack from thousands or millions of compromised systems (botnet) - very difficult to block because the traffic comes from IP addresses that appear legitimate

Goal: To destroy availability. No data access, no infiltration - just downtime.

Motivations:

Politically motivated (hacktivism, state actors)
Extortion ("Ransom DDoS" - pay up or we’ll cripple you)
Competitive sabotage
Distraction tactic for other attacks (e.g., during a DDoS, logs are flooded, other attacks are obscured)
Script kiddie motivation (demonstrating power)

DDoS Types

Layer 3/4 - Volumetric Attacks

Goal: Exhaust network bandwidth (Gbps to Tbps).

Amplification Attacks: The attacker sends small requests to public services - and forges the sender IP as the victim’s IP. The services respond with much larger packets directly to the victim:

Attack Type	Amplification Factor
DNS Amplification	up to 50-100×
NTP Amplification	up to 556×
SSDP Amplification	up to 30×
Memcached Amplification	up to 51,000×

UDP Floods: Massive volumes of UDP packets sent to random ports. The target responds with an ICMP "Destination Unreachable" message - exhausting CPU and bandwidth.

ICMP Flood (Ping Flood): Overwhelming the target with massive ping requests.

Record-breaking attack: In November 2021, Microsoft recorded a DDoS attack with 3.47 Tbps of bandwidth targeting an Azure customer - the largest attack ever recorded.

Layer 4 - Protocol Attacks

Objective: Exhaust server resources by exploiting protocol vulnerabilities.

SYN Flood: The attacker sends massive amounts of TCP SYN packets (connection requests) but never responds to the SYN-ACK. The target maintains many half-open connections in the "Wait" queue:

Normal:   Client → SYN → Server → SYN-ACK → Client → ACK (connection established)
SYN Flood: Attacker → SYN → Server → SYN-ACK → [no response, slot occupied]
           Thousands of times simultaneously → Server connection queue full

ACK Flood, RST Flood, URG Flood: Various variants using TCP flags.

Layer 7 - Application Layer Attacks

Goal: Exhaust web server or application resources through seemingly legitimate HTTP requests - often effective even with low attack bandwidth.

HTTP GET/POST Flood: Thousands of legitimate-looking HTTP requests overload backend servers or databases.

Slowloris: Keeps many HTTP connections open simultaneously by intentionally sending requests extremely slowly (byte by byte). The server connection pool becomes exhausted.

RUDY (R-U-Dead-Yet): POST requests with extremely small content chunks over a long period of time - keeps Apache/Nginx worker threads blocked.

HTTP/2 Rapid Reset: New attack from 2023 - exploits the HTTP/2 streaming mechanism to reset requests en masse and send new ones. Record attack with 398 million requests per second.

Ransom DDoS (RDDoS)

Criminals threaten a company with a DDoS attack and demand a ransom (typically: 1-5 BTC). This is often followed by a small demonstration attack to establish credibility.

BSI Recommendation: Do not pay - there is no guarantee the attack will not occur after payment. Instead: Activate DDoS protection and report the attack.

The Threat Landscape in 2024

BSI Situation Report 2024: In the first half of 2024, the number of high-volume DDoS attacks rose sharply. Particularly affected: public administrations, financial service providers, and critical infrastructure.

Pro-Russian hacktivists (Killnet, NoName057, Anonymous Sudan) launched targeted attacks against German and European targets - politically motivated in the context of the war in Ukraine:

Bundestag, federal agencies
Airports (Frankfurt, Berlin)
Banks (Commerzbank, Deutsche Bank)
Media and news portals

IoT botnets: Poorly secured IoT devices (routers, cameras, smart home devices) are being recruited en masse into botnets. The Mirai botnet and its variants remain active.

DDoS Protection Measures

Layer 1: Network Capacity and Anycast

Bandwidth: More bandwidth than the attacker has - for volumetric attacks. Hardly feasible for individual companies, but cloud providers have Tbps of capacity.

Anycast Routing: Traffic is distributed globally across many PoPs (Points of Presence). This "dilutes" the attack traffic within the global infrastructure.

BGP Blackholing / RTBH: In an emergency, the attacked IP address is “zero-routed” for external traffic - your own customers can no longer access it, but the attack stops. Last resort.

Layer 2: DDoS Protection Services

Cloud-based scrubbing service: Traffic is first redirected to the DDoS protection provider’s scrubbing center. There, malicious traffic is filtered out, and legitimate traffic is forwarded to the origin:

Internet → [Cloudflare/Akamai/AWS Shield] → [Scrubbing] → Origin Server

Well-known DDoS protection services:

Cloudflare Magic Transit: Network-layer protection, handles over 200 Tbps
Akamai Prolexic: Enterprise-grade, suitable for critical infrastructure
AWS Shield Advanced: Integrated into AWS infrastructure
Radware DDoS Protection
Deutsche Telekom / Telekom Security: Local provider for BSI-sensitive companies

On-Premises DDoS Appliances:

Arbor Networks (NETSCOUT)
Radware DefensePro
F5 DDoS Hybrid Defender

Layer 3: CDN and Web Application Firewall

For Layer 7 attacks, CDN providers (Cloudflare, Fastly, Akamai) offer essential protection features:

Rate Limiting: X requests per IP per time window
Bot Management: Distinguishing between human users and bots
Challenge Pages: CAPTCHA or JavaScript challenge for suspicious requests
IP Reputation: Blocking known botnet IPs

WAF Rules Against Layer 7 Attacks:

Slowloris: Timeouts for slow connections
HTTP Flood: Request rate limiting by IP/fingerprint

Layer 4: Incident Response

DDoS Response Plan: Who does what when the attack begins?

Detection → Classification → Escalation → Mitigation → Communication
  │
  ├── Alert in monitoring (network bandwidth, error rate)
  │
  ├── Determine cause: DDoS or genuine traffic spike?
  │
  ├── Activate DDoS protection (contact upstream provider if necessary)
  │
  ├── Notify ISP for BGP blackholing if necessary
  │
  └── Communication: internal (management), external (customers, BSI/CERT-Bund if necessary)

Report to BSI/CERT-Bund: Operators of critical infrastructure (KRITIS) and important facilities under NIS2 are required to report serious DDoS attacks.

Layer 5: Proactive Hardening

Anycast DNS: Distribute DNS servers via Anycast - prevents DNS amplification on your own infrastructure.

BCP38 / Ingress Filtering: Network routers filter packets with spoofed source IPs (spoofed packets) - every ISP should implement this, but unfortunately it is not yet standard.

SYN Cookies: Server-side countermeasure against SYN floods without having to maintain half-open connections.

Redundancy: Multiple data centers, geographic distribution, automatic failover.

DDoS and NIS2

For operators of essential and important facilities under NIS2:

Art. 21: Security measures must address availability
Art. 23: Mandatory reporting of significant disruptions (including DDoS if essential services are affected)
Within 24 hours: Early warning to CSIRT/BSI
Within 72 hours: Initial assessment of the incident

Costs and Economic Losses

Direct costs of a DDoS attack: starting at 7 EUR/hour on the dark web
Costs of damage to victims: on average 50,000-500,000 EUR per hour of downtime (Depending on the industry)
E-commerce: 100,000 EUR+ per hour of downtime for medium-sized online stores
Financial sector: Millions of EUR per hour for critical trading platforms

Conclusion

DDoS attacks are easy to buy, hard to stop, and can bring critical business processes to a standstill. The most effective protection combines cloud-based DDoS mitigation (scrubbing services), a well-configured CDN/WAF layer, sufficient network capacity - and a proven incident response plan to ensure a quick and coordinated reaction in the event of an attack.