Linux Forensics
Initial Information Gathering
Basic Information
First of all, it's recommended to have some USB with good known binaries and libraries on it (you can just get a ubuntu and copy the folders /bin, /sbin, /lib, and /lib64), then mount the USN, and modify the env variables to use those binaries:
Once you have configured the system to use good and known binaries you can start extracting some basic information:
Suspicious information
While obtaining the basic information you should check for weird things like:
root processes usually run with low PIDS, so if you find a root process with a big PID you may suspect
Check registered logins of users without a shell inside
/etc/passwdCheck for password hashes inside
/etc/shadowfor users without a shell
Memory Dump
LiME supports 3 formats:
Raw (every segment concatenated together)
Padded (same as raw, but with zeroes in right bits)
Lime (recommended format with metadata
LiME can also be use to send the dump via network instead of storing it on the system using something like: path=tcp:4444
Disk Imaging
Shutting down
First of all you will need to shutdown the system. This isn't always an option as some times system will be a production server that the company cannot afford to shutdown.
There are 2 ways of shutting down the system, a normal shutdown and a "plug the plug" shutdown. The first one will allow the processes to terminate as usual and the filesystem to be synchronized, but I will also allow the possible malware to destroy evidences. The "pull the plug" approach may carry some information loss (as we have already took an image of the memory not much info is going to be lost) and the malware won't have any opportunity to do anything about it. Therefore, if you suspect that there may be a malware, just execute the sync command on the system and pull the plug.
Taking an image of the disk
It's important to note that before connecting to your computer anything related to the case, you need to be sure that it's going to be mounted as read only to avoid modifying the any information.
Disk Image pre-analysis
Imaging that you receive a disk image with no more data.
Search for known Malware
Modified System Files
Some Linux systems have a feature to verify the integrity of many installed components, providing an effective way to identify unusual or out of place files. For instance, rpm -Va on Linux is designed to verify all packages that were installed using RedHat Package Manager.
Malware/Rootkit Detectors
Read the following page to learn about tools that can be useful to find malware:
Search installed programs
Package Manager
On Debian-based systems, the /var/ lib/dpkg/status file contains details about installed packages and the /var/log/dpkg.log file records information when a package is installed.
On RedHat and related Linux distributions the rpm -qa --root=/ mntpath/var/lib/rpm command will list the contents of an RPM database on a subject systems.
Other
Not all installed programs will be listed by the above commands because some applications are not available as packages for certain systems and must be installed from source. Therefore, a review of locations such as /usr/local and /opt may reveal other applications that have been compiled and installed from source code.
Another good idea is to check the common folders inside $PATH for binaries not related to installed packages:
Inspect AutoStart locations
Scheduled Tasks
Services
It is extremely common for malware to entrench itself as a new, unauthorized service. Linux has a number of scripts that are used to start services as the computer boots. The initialization startup script /etc/inittab calls other scripts such as rc.sysinit and various startup scripts under the /etc/rc.d/ directory, or /etc/rc.boot/ in some older versions. On other versions of Linux, such as Debian, startup scripts are stored in the /etc/init.d/ directory. In addition, some common services are enabled in /etc/inetd.conf or /etc/xinetd/ depending on the version of Linux. Digital investigators should inspect each of these startup scripts for anomalous entries.
/etc/inittab
/etc/rc.d/
/etc/rc.boot/
/etc/init.d/
/etc/inetd.conf
/etc/xinetd/
/etc/systemd/system
/etc/systemd/system/multi-user.target.wants/
Kernel Modules
On Linux systems, kernel modules are commonly used as rootkit components to malware packages. Kernel modules are loaded when the system boots up based on the configuration information in the /lib/modules/'uname -r' and /etc/modprobe.d directories, and the /etc/modprobe or /etc/modprobe.conf file. These areas should be inspected for items that are related to malware.
Other AutoStart Locations
There are several configuration files that Linux uses to automatically launch an executable when a user logs into the system that may contain traces of malware.
/etc/profile.d/* , /etc/profile , /etc/bash.bashrc are executed when any user account logs in.
∼/.bashrc , ∼/.bash_profile , ~/.profile , ∼/.config/autostart are executed when the specific user logs in.
/etc/rc.local It is traditionally executed after all the normal system services are started, at the end of the process of switching to a multiuser runlevel.
Examine Logs
Look in all available log files on the compromised system for traces of malicious execution and associated activities such as creation of a new service.
Pure Logs
Logon events recorded in the system and security logs, including logons via the network, can reveal that malware or an intruder gained access to a compromised system via a given account at a specific time. Other events around the time of a malware infection can be captured in system logs, including the creation of a new service or new accounts around the time of an incident. Interesting system logons:
/var/log/syslog (debian) **or /var/log/messages** (Redhat)
Shows general messages and info regarding the system. Basically a data log of all activity throughout the global system.
/var/log/auth.log (debian) **or /var/log/secure** (Redhat)
Keep authentication logs for both successful or failed logins, and authentication processes. Storage depends on system type.
cat /var/log/auth.log | grep -iE "session opened for|accepted password|new session|not in sudoers"
/var/log/boot.log: start-up messages and boot info.
/var/log/maillog or var/log/mail.log: is for mail server logs, handy for postfix, smtpd, or email-related services info running on your server.
/var/log/kern.log: keeps in Kernel logs and warning info. Kernel activity logs (e.g., dmesg, kern.log, klog) can show that a particular service crashed repeatedly, potentially indicating that an unstable trojanized version was installed.
/var/log/dmesg: a repository for device driver messages. Use dmesg to see messages in this file.
/var/log/faillog: records info on failed logins. Hence, handy for examining potential security breaches like login credential hacks and brute-force attacks.
/var/log/cron: keeps a record of Crond-related messages (cron jobs). Like when the cron daemon started a job.
/var/log/daemon.log: keeps track of running background services but doesn’t represent them graphically.
/var/log/btmp: keeps a note of all failed login attempts.
/var/log/httpd/: a directory containing error_log and access_log files of the Apache httpd daemon. Every error that httpd comes across is kept in the error_log file. Think of memory problems and other system-related errors. access_log logs all requests which come in via HTTP.
/var/log/mysqld.log or /var/log/mysql.log : MySQL log file that records every debug, failure and success message, including starting, stopping and restarting of MySQL daemon mysqld. The system decides on the directory. RedHat, CentOS, Fedora, and other RedHat-based systems use /var/log/mariadb/mariadb.log. However, Debian/Ubuntu use /var/log/mysql/error.log directory.
/var/log/xferlog: keeps FTP file transfer sessions. Includes info like file names and user-initiated FTP transfers.
/var/log/* : You should always check for unexpected logs in this directory
Command History
Many Linux systems are configured to maintain a command history for each user account:
~/.bash_history
~/.history
~/.sh_history
~/.*_history
Logins
Using the command last -Faiwx it's possible to get the list of users that have logged in.
It's recommended to check if those logins make sense:
Any unknown user?
Any user that shouldn't have a shell has logged in?
This is important as attackers some times may copy /bin/bash inside /bin/false so users like lightdm may be able to login.
Note that you can also take a look to this information reading the logs.
Application Traces
SSH: Connections to systems made using SSH to and from a compromised system result in entries being made in files for each user account (∼/.ssh/authorized_keys and ∼/.ssh/known_keys). These entries can reveal the hostname or IP address of the remote hosts.
Gnome Desktop: User accounts may have a ∼/.recently-used.xbel file that contains information about files that were recently accessed using applications running in the Gnome desktop.
VIM: User accounts may have a ∼/.viminfo file that contains details about the use of VIM, including search string history and paths to files that were opened using vim.
Open Office: Recent files.
MySQL: User accounts may have a ∼/.mysql_history file that contains queries executed using MySQL.
Less: User accounts may have a ∼/.lesshst file that contains details about the use of less, including search string history and shell commands executed via less
USB Logs
It is interesting to know all the USBs that have been used and it will be more useful if you have an authorized list of USB to find "violation events" (the use of USBs that aren't inside that list).
Installation
Examples
Review User Accounts and Logon Activities
Examine the /etc/passwd, /etc/shadow and security logs for unusual names or accounts created and/or used in close proximity to known unauthorized events. Also check possible sudo brute-force attacks. Moreover, check files like /etc/sudoers and /etc/groups for unexpected privileges given to users. Finally look for accounts with no passwords or easily guessed passwords.
Examine File System
File system data structures can provide substantial amounts of information related to a malware incident, including the timing of events and the actual content of malware. Malware is increasingly being designed to thwart file system analysis. Some malware alter date-time stamps on malicious files to make it more difficult to find them with time line analysis. Other malicious code is designed to only store certain information in memory to minimize the amount of data stored in the file system. To deal with such anti-forensic techniques, it is necessary to pay careful attention to time line analysis of file system date-time stamps and to files stored in common locations where malware might be found.
Using autopsy you can see the timeline of events that may be useful to discover suspicions activity. You can also use the
mactimefeature from Sleuth Kit directly.Check for unexpected scripts inside $PATH (maybe some sh or php scripts?)
Files in
/devuse to be special files, you may find non-special files here related to malware.Look for unusual or hidden files and directories, such as “.. ” (dot dot space) or “..^G ” (dot dot control-G)
setuid copies of /bin/bash on the system
find / -user root -perm -04000 –printReview date-time stamps of deleted inodes for large numbers of files being deleted around the same time, which might indicate malicious activity such as installation of a rootkit or trojanized service.
Because inodes are allocated on a next available basis, malicious files placed on the system at around the same time may be assigned consecutive inodes. Therefore, after one component of malware is located, it can be productive to inspect neighbouring inodes.
Also check directories like /bin or /sbin as the modified and/or changed time of new or modified files me be interesting.
It's interesting to see the files and folders of a directory sorted by creation date instead alphabetically to see which files/folders are more recent (last ones usually).
You can check the most recent files of a folder using ls -laR --sort=time /bin
You can check the inodes of the files inside a folder using ls -lai /bin |sort -n
Compare files of different filesystem versions
Find added files
Find Modified content
Find deleted files
Other filters
-diff-filter=[(A|C|D|M|R|T|U|X|B)…[*]]
Select only files that are Added (A), Copied (C), Deleted (D), Modified (M), Renamed (R), have their type (i.e. regular file, symlink, submodule, …) changed (T), are Unmerged (U), are Unknown (X), or have had their pairing Broken (B). Any combination of the filter characters (including none) can be used. When * (All-or-none) is added to the combination, all paths are selected if there is any file that matches other criteria in the comparison; if there is no file that matches other criteria, nothing is selected.
Also, these upper-case letters can be downcased to exclude. E.g. --diff-filter=ad excludes added and deleted paths.
Note that not all diffs can feature all types. For instance, diffs from the index to the working tree can never have Added entries (because the set of paths included in the diff is limited by what is in the index). Similarly, copied and renamed entries cannot appear if detection for those types is disabled.
References
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