VU#417980: Implementations of UDP-based application protocols are vulnerable to network loops

VU#417980: Implementations of UDP-based application protocols are vulnerable to network loops


A novel traffic-loop vulnerability has been identified against certain implementations of UDP-based applications protocols. An unauthenticated attacker can use maliciously-crafted packets against a UDP-based vulnerable implementation of application protocols (e.g., DNS, NTP, TFTP) that can lead to Denial-of-Service (DOS) and/or abuse of resources.


The User Datagram Protocol (UDP) is a simple, connectionless protocol that is still commonly used in many internet-based applications. UDP has a limited packet-verification capability and is susceptible to IP spoofing. Security researchers have identified that certain implementations of the UDP protocol in applications can be triggered to create a network-loop of seemingly never-ending packets. Software implementations of UDP-based application protocols DNS, NTP, TFTP, Echo (RFC862), Chargen (RFC864), and QOTD (RFC865) were specifically found to be vulnerable to such network loops.

As an example, if two application servers have a vulnerable implementation of said protocol, an attacker can initiate a communication with the first server, spoofing the network address of the second server (victim). In many cases, the first server will respond with an error message to the victim, which will also trigger a similar behavior of another error message back to the first server. This behavior has been demonstrated to be resource exhausting and can cause services to become either unresponsive or unstable.


Successful exploitation of this vulnerability could result in the following scenarios:
1. Overload of a vulnerable service, causing it to become unstable or unusable.
2. DOS attack of the network backbone, causing network outage to other services.
3. Amplification attacks that involve network loops causing amplified DOS or DDOS attacks.


CERT/CC recommends that you apply the latest patch provided by the affected vendor that addresses this vulnerability in the vendor-specific implementations. Review the vendor-specific information below. If the product is end-of-life/unsupported, vendors will be unlikely to release a patch; thus, we recommend replacing the device.

Protect or replace UDP applications

When possible, protect UDP-based applications using network firewall rules and/or other access-control lists to prevent unauthorized access. If the same service can be implemented using a TCP or with any request-validation capability (e.g., Message-Authenticator) available in the UDP-based application protocol, implement such protection to prevent unknown or spoofed requests. It is recommended that you disable unnecessary and unused UDP services that may be enabled as part of your operating system to prevent exposure of these services for abuse.

Deploy anti-spoofing

Network providers should deploy available anti-spoofing techniques (BCP38) such as Unicast Reverse Path Forwarding (uRPF) to prevent IP spoofing in protecting their internet-facing resources against spoofing and abuse.

Enforce network rate-limiting

Service providers should employ network rate-limiting capabilities, such Quality-of-Service (QoS) to protect their network from abuse from network loops and amplifications and to ensure their critical resources/services are protected.


Thanks to the reporters Yepeng Pan and Christian Rossow from the CISPA Helmholtz Center for Information Security, Germany. This document was written by Elke Drennan and Vijay Sarvepalli.

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