When the Morris worm went around, one of the ways it got in was through sendmail, using the "DEBUG" command. Right after it happened, some wise-ass sent around an email telling everybody to edit their sendmail binary (with Emacs of course), search for "DEBUG", and replace the "D" will a NULL, thus disabling the "DEBUG" command.

What that actually did was change the "DEBUG" command into the "" command.

At the time I was running a mailing list from the University of Maryland, and often had to check Sun email addresses by telnetting to sun.com port 25, pressing return a couple of times to flush the telnet negotiation characters, then going "EXPN some-email-address".

So the day after the Morris worm, I go "telnet sun.com 25", hit return a couple of times, then "EXPN foobar", and it dumps out a huge torrent of debugging information, because I had accidentally switched it into debug mode by entering a blank line!

I reported it to [email protected], and they fixed it. But it's kinda silly that they would have applied such a ham fisted patch to their sendmail demon like that, based on an email that some dude on the internet sent around!

https://www.rapid7.com/db/modules/exploit/unix/smtp/morris_s...

http://www.cs.unc.edu/~jeffay/courses/nidsS05/attacks/seely-...

https://spaf.cerias.purdue.edu/tech-reps/823.pdf