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:
In other cases you need to download [**LiME**](https://github.com/504ensicsLabs/LiME) from github can compile it with correct kernel headers. In order to **obtain the exact kernel headers** of the victim machine, you can just **copy the directory**`/lib/modules/<kernel version>` to your machine, and then **compile** LiME using them:
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.
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.
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.
```bash
#RedHat
rpm -Va
#Debian
dpkg --verify
debsums | grep -v "OK$" #apt-get install debsums
```
### Malware/Rootkit Detectors
Read the following page to learn about tools that can be useful to find malware:
{% page-ref page="../malware-analysis.md" %}
## Search installed programs
### Package Manager
On Debian-based systems, the _**/var/ lib/dpkg/status**_ file contains details about installed packages and the _**/var/log/dpkg.log**_ file records information when a package is installed.
On RedHat and related Linux distributions the **`rpm -qa --root=/ mntpath/var/lib/rpm`** command will list the contents of an RPM database on a subject systems.
**Not all installed programs will be listed by the above commands** because some applications are not available as packages for certain systems and must be installed from source. Therefore, a review of locations such as _**/usr/local**_ and _**/opt**_ may reveal other applications that have been compiled and installed from source code.
```bash
ls /opt /usr/local
```
Another good idea is to **check** the **common folders** inside **$PATH** for **binaries not related** to **installed packages:**
```bash
#Both lines are going to print the executables in /sbin non related to installed packages
ls -l /usr/lib/cron/tabs/ /Library/LaunchAgents/ /Library/LaunchDaemons/ ~/Library/LaunchAgents/
```
### 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.
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**_ , _**∼/.config/autostart**_ are executed when the specific user logs in.
## 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.
* **/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
{% hint style="info" %}
Linux system logs and audit subsystems may be disabled or deleted in an intrusion or malware incident. In fact, because logs on Linux systems generally contain some of the most useful information about malicious activities, intruders routinely delete them. Therefore, when examining available log files, it is important to look for gaps or out of order entries that might be an indication of deletion or tampering.
{% endhint %}
### Command History
Many Linux systems are configured to maintain a command history for each user account:
* **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
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.
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 `mactime` feature from **Sleuth Kit** directly.
* Check for **unexpected scripts** inside **$PATH** \(maybe some sh or php scripts?\)
* Files in `/dev` use 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 –print`
* Review 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 \(last ones usually\).
The **MBR** occupies the **sector 0 of the disk** \(the first sector\) and it's used to indicate the partitions of the disc. This sector is essential to indicate the PC what and from where a partition should be mounted.
It allows up to **four partitions** \(at most **just 1** can be active/**bootable**\). However, if you need more partitions you can use **extended partitions**.
**Ext2** is the most common filesystem for **not journaling** partitions \(**partitions that don't change much**\) like the boot partition. **Ext3/4** are **journaling** and are used usually for the **rest partitions**.
All block groups in the filesystem have the same size and are stored sequentially. This allows the kernel to easily derive the location of a block group in a disk from its integer index.
Every block group contains the following pieces of information:
* A copy of the filesystem’s superblock
* A copy of the block group descriptors
* A data block bitmap which is used to identify the free blocks inside the group
* An inode bitmap, which is used to identify the free inodes inside the group
* inode table: it consists of a series of consecutive blocks, each of which contains a predefined Figure 1 Ext2 inode number of inodes. All inodes have the same size: 128 bytes. A 1,024 byte block contains 8 inodes, while a 4,096-byte block contains 32 inodes. Note that in Ext2, there is no need to store on disk a mapping between an inode number and the corresponding block number because the latter value can be derived from the block group number and the relative position inside the inode table. For example, suppose that each block group contains 4,096 inodes and that we want to know the address on disk of inode 13,021. In this case, the inode belongs to the third block group and its disk address is stored in the 733rd entry of the corresponding inode table. As you can see, the inode number is just a key used by the Ext2 routines to retrieve the proper inode descriptor on disk quickly
* data blocks, containing files. Any block which does not contain any meaningful information, it is said to be free.
The Superblock contains a description of the basic size and shape of this file system. Usually only the Superblock in Block Group 0 is read when the file system is mounted but each Block Group contains a duplicate copy in case of file system corruption.
