It is possible to cause Sendmail to stop responding (effectively creating a Denial of Service attack) using the ETRN command. When the ETRN command is read by Sendmail, it calls fork(). Parent process generates no output - only child-generated
output is sent, so the parent won't be notified on send()/write() failure. This enables attacks to drop the connection (after sending a lot of ETRNs), causing the parent process to get stuck, doing repeatedly fork() and sleep(5), no more ETRNs requests are in need of processing.
This vulnerability allows an attacker to spawn any amount of 'unusable' Sendmail child processes, causing Sendmail to hang for a long period of time (and it can be done using low network bandwidth and resources). Direct result - all server memory consumed (causing Linux 2.0 kernels to crash with messages like 'no memory for Sendmail', 'no memory for klogd' etc.)
