Fragmented UDP Flood Attack

A Fragmented UDP Flood Attack is a form of Distributed Denial-of-Service (DDoS) attack that exploits the User Datagram Protocol (UDP) and IP fragmentation mechanisms to overwhelm target systems, network infrastructure, or security appliances.


How It Works

In this attack, adversaries send a high volume of oversized UDP packets that exceed the standard Maximum Transmission Unit (MTU). These packets are deliberately fragmented into smaller IP fragments. The fragmentation is crafted so that the target system must expend computational resources to reassemble the fragments, even if the complete original packet is never received or is intentionally malformed.

The following diagram shows visualization of the attack:


Since UDP is connectionless and stateless, there’s no handshake or session validation to confirm the legitimacy of the data stream. This characteristic allows attackers to spoof IP addresses and continuously transmit massive numbers of fragmented packets to a targeted IP or subnet.


Impact

  • Resource Exhaustion: Target systems are forced to track and reassemble incomplete or malformed fragments, consuming CPU cycles and memory buffers.

  • Bandwidth Saturation: Flooding the network with fragmented traffic can saturate available bandwidth, affecting legitimate users.

  • Application Downtime: Services relying on the affected system may experience delays, interruptions, or complete failure.


Real-World Incidents

  • Mirai Botnet Attacks (2016): The Mirai botnet launched massive DDoS attacks using various techniques, including UDP floods. Notably, it targeted the French hosting provider OVH with attacks peaking at 1.1 terabits per second, overwhelming their infrastructure and causing significant disruptions.

  • Attacks on Russian Sites (2023): Hacktivist groups employed fragmented UDP flood attacks against Russian websites, sending large volumes of fragmented packets to overwhelm and disrupt services.


Detection and Mitigation

  • Fragment Reassembly Monitoring: Monitor for excessive fragment reassembly attempts from the same source or targeting the same destination.

  • Rate Limiting and Filtering: Implement rate-limiting for fragmented packet flows and filter non-standard UDP traffic at the network edge.

  • Anti-DDoS Services: Employ cloud-based DDoS mitigation providers capable of absorbing and filtering out malformed or fragmented traffic before it reaches your network.


Common Targets

  • Internet-facing servers (e.g., DNS, VoIP, gaming servers)

  • Network infrastructure (e.g., routers, firewalls)

  • Security appliances vulnerable to reassembly overhead


Sergei Penchuk, CISSP

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