National Cyber Awareness System:
Certain UDP protocols have been identified as potential attack vectors:
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, it is very easy to forge the IP packet datagram to include an arbitrary source IP address . When many UDP packets have their source IP address forged to a single address, the server responds to that victim, 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, 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  .
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 bandwith amplificatication factors, please see Christian's blog and associated research paper.
In March 2015, Software Engineering Institute CERT issued Vulnerabilty Note (VU#550620) describing the use of mDNS in DRDoS attacks. Attackers can leverage mDNS by sending more information than can be handled by the device, thereby causing a DoS. 
In July 2015, Akamai Technologies' Prolexic Security Engineering and Research Team (PLXsert) issued a threat advisory describing a surge in DRDoS attacks using the Routing Information Protocol version one (RIPv1). Malicious actors are leveraging the behavior of RIPv1 for DDoS reflection through specially crafted request queries .
In August 2015, Level 3 Threat Research Labs reported a new form of DRDoS attack that uses portmap. Attackers leverage the behavior of the portmap service through spoofed requests and flood a victim’s network with UDP traffic. 
Attackers can utilize the bandwidth and relative trust of large servers that provide the above UDP protocols to flood victims with unwanted traffic, a DDoS attack.
Detection of DRDoS attacks is not easy because of their use of large, trusted servers that provide UDP services. Network operators of these exploitable services may apply traditional DoS mitigation techniques. In addition, watch out for abnormally large responses to a particular IP address, which may indicate that an attacker is using the service to conduct a DRDoS attack.
Source IP Verification
Because the UDP requests being sent by the attacker-controlled clients must have a source IP address spoofed to appear as the victim’s IP, the first step to reducing the effectiveness of UDP amplification is for Internet service providers (ISPs) to reject any UDP traffic with spoofed addresses. The Network Working Group of the Internet Engineering Task Force (IETF) released Best Current Practice 38 in May 2000 and Best Current Practice 84 in March 2004. These documents describe how an ISP can filter network traffic on their network to reject packets with source addresses not reachable via the actual packet’s path  . Recommended changes would cause a routing device to evaluate whether it is possible to reach the source IP address of the packet via the interface that transmitted the packet. If it is not possible, then the packet most likely has a spoofed source IP address. This configuration change would substantially reduce the potential for many popular types of DDoS attacks. As such, we highly recommend that all network operators perform network ingress filtering if possible. Note that such filtering will not explicitly protect a UDP service provider from being exploited in a DRDoS because all network providers must use ingress filtering to eliminate the threat completely.
Limiting responses to UDP requests is another potential mitigation to this issue. This may require testing to discover the optimal limit that does not interfere with legitimate traffic. The IETF released Request for Comment 2475 and Request for Comment 3260 that describe some methods to shape and control traffic  . Most network devices today provide these functions in their software.