Checklists — System is Compromised or Hacked — Part 1

As in my previous Blog where i explained how i came to know if my system is hacked or compromized ( link here). Here in this blog i will explain what basic things we can check on our system when we have doubt if our system is compromized .

This Blogs have 3 parts

• List of Checks which can determine if system is compromised or hacked — Part 1
• List of checks which can give a direction how system is compromised or hacked — Part 2
• What preventive steps (specially infra related) can be taken care to avoid hacking or to make system more secure — Part 3

Here , i am assuming system is Linux system with Centos installed .

List of Checks which can determine if system is compromised or hacked

• Generally when hacker break into a linux system it is high chance that it will alter you main packages like openssh,kernel etc.. , So first if of please check if these packages are altered or there are some changes in the files or binaries provided by these packages . Following are commands to check on Centos

sudo rpm -qa | grep openssh | xargs -I '{}' sudo rpm -V '{}'

• If therr are files shown by above command in which you did not change anything then it means there is high chance your system is compromised
• Run rootkit Hunter to check if you system is compromised

Download rkhunter.tar.gz

copy it in /root and goto /root

tar zxvf rkhunter-1.4.2.tar.gz

cd rkhunter-1.4.2/

sh installer.sh --layout default --install

changes in /etc/rkhunter.conf ENABLE_TESTS="all" DISABLE_TESTS="none" HASH_CMD=SHA1 HASH_FLD_IDX=4 PKGMGR=RPM 7

/usr/local/bin/rkhunter --propupd

/usr/local/bin/rkhunter --update

/usr/local/bin/rkhunter -c -sk 10.

note output or check and copy /var/log/rkhunter.log

you can also check the link for using rkhunter

• Check /var/log/secure to check if there are many authentication failure requests and someone trying brute force to enter in to system
• following will be the comand :

[root@localhost ~]# less /var/log/secure | grep 'authentication failures'

• and output will be something like :

Apr 25 12:48:46 localhost sshd[2391]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.29.14 user=root

Apr 25 12:49:33 localhost sshd[2575]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.29.14 user=root

• In above output you can see the rhost from where login attempt are made . If you see lots of entries like this then also check if at some point of time some login attempt will be successful from any of attempting rhosts . In secure logs accepted logs will looks something like as follows :
• Check in Processes if some unusual process is running and consuming high CPU using top and ps commands .

Command to list all process running in system : ps aux | less

• Also check using top command if some unusual process trying to utiize high cpu
• Check if there is some unusual entry in crontab of all users made on system
• Check if in id_rsa.pub , if some attacker has somehow made its entry in .ssh folder in every users’s home directory .

This was the Part 1 of the Blog , In later Parts i will explain some further checklist to ensure that you system will remain less hackable .

Thankyou .

Originally published at http://hello-worlds.in on April 25, 2021.