trust-dns and trust-dns-server are vulnerable to remotely triggered denial-of-service attacks, consuming both network and CPU resources. DNS messages with the QR=1 bit set are responded to with a FormErr response. This allows creating a traffic loop, in which these FormErr responses are sent nonstop between vulnerable servers. There are two scenarios how this can be exploited: 1) Create a loop between two instances of trust-dns, consuming network resources, or 2) consuming the CPU of a single instance. With two instances A and B an attacker sends a DNS query with a spoofed source IP address to A. A replies with a FormErr to B. Now both servers with ping-pong the message back and forth until by chance the packet is dropped in the network. Multiple spoofed packets can be sent by the attacker, increasing resource consumption. A single server can get locked up replying to itself. Same setup as above, but now A sends the reply to itself. The packet is sent out as fast as the CPU and network stack manage. This locks up a CPU core. Multiple packets from the attacker consume multiple CPU cores. The issue is patched in versions 0.22.1 and above.