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Old 04-15-2016, 09:12 AM   #1
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More Alerts Alert (TA14-017A) UDP-Based Amplification Attacks (Revised 12 Apr 2016)

Systems Affected

Certain application-layer protocols that rely on User Datagram Protocol (UDP) have been identified as potential attack vectors:
  • DNS
  • NTP
  • SNMPv2
  • NetBIOS
  • SSDP
  • CharGEN
  • QOTD
  • BitTorrent
  • Kad
  • Quake Network Protocol
  • Steam Protocol
  • RIPv1
  • Multicast DNS (mDNS)
  • Portmap/RPC

A Distributed Reflective Denial of Service (DRDoS) attack is a form of Distributed Denial of Service (DDoS) that relies on the use of publicly accessible UDP servers, as well as bandwidth amplification factors, to overwhelm a victim system with UDP traffic.


UDP, by design, is a connection-less protocol that does not validate source IP addresses. Unless the application-layer protocol uses countermeasures such as session initiation in VOIP (voice over IP), it is very easy to forge the IP packet datagram to include an arbitrary source IP address [1]. When many UDP packets have their source IP address forged to the victim IP address, the destination server (or amplifier) responds to the victim (instead of the attacker), creating a reflected Denial of Service (DoS) Attack.

Recently, certain UDP protocols have been found to have particular responses to certain commands that are much larger than the initial request. Previously, attackers were limited linearly by the number of packets directly sent to the target to conduct a DoS attack; now a single packet can generate tens or hundreds of times the bandwidth in its response. This is called an amplification attack, and when combined with a reflective DoS attack on a large scale using multiple amplifiers and targeting a single victim, DDoS attacks can be conducted with relative ease.

To measure the potential effect of an amplification attack, a metric called the bandwidth amplification factor (BAF) is used. BAF can be calculated as the number of UDP payload bytes that an amplifier sends to answer a request, compared to the number of UDP payload bytes of the request [2 (link is external)] [3 (link is external)].

The list of known protocols—and their associated bandwidth amplification factors—are listed below. US-CERT offers thanks to Christian Rossow for providing this information. For more information on bandwidth amplification factors, please see Christian's blog (link is external) and associated research paper (link is external).
The complete Notification may be found at

Hope this helps some.


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