Hack #5. Protect Your Logs from Tampering

Use file attributes to prevent intruders from removing traces of their break-ins.

In the course of an intrusion, an attacker will more than likely leave telltale signs of his actions in various system logs. This is a valuable audit trail that should be well protected. Without reliable logs, it can be very difficult to figure out how the attacker got in, or where the attack came from. This information is crucial in analyzing the incident and then responding to it by contacting the appropriate parties involved [Hack #125]. However, if the break-in attempt is successful and the intruder gains root privileges, what’s to stop him from removing the traces of his misbehavior?

This is where file attributes come in to save the day (or at least make it a little better). Both Linux and the BSDs have the ability to assign extra attributes to files and directories. This is different from the standard Unix permissions scheme in that the attributes set on a file apply universally to all users of the system, and they affect file accesses at a much deeper level than file permissions or ACLs [Hack #4]. In Linux, you can see and modify the attributes that are set for a given file by using the lsattr and chattr commands, respectively. Under the BSDs, you can use ls -lo to view the attributes and use chflags to modify them.

One useful attribute for protecting log files is append-only. When this attribute is set, the file cannot be deleted, and writes are only allowed to append to the end of the file.

To set the append-only flag under Linux, run this command:

# chattr +a 
            
               filename
            
         

Under the BSDs, use this:

# chflags sappnd 
            
               filename
            
         

See how the +a attribute works by creating a file and setting its append-only attribute:

# touch /var/log/logfile
# echo "append-only not set" > /var/log/logfile
# chattr +a /var/log/logfile
# echo "append-only set" > /var/log/logfile
bash: /var/log/logfile: Operation not permitted

The second write attempt failed, since it would overwrite the file. However, appending to the end of the file is still permitted:

# echo "appending to file" >> /var/log/logfile
# cat /var/log/logfile
append-only not set
appending to file

Obviously, an intruder who has gained root privileges could realize that file attributes are being used and just remove the append-only flag from the logs by running chattr -a. To prevent this, you’ll need to disable the ability to remove the append-only attribute. To accomplish this under Linux, use its capabilities mechanism. Under the BSDs, use the securelevel facility.

The Linux capabilities model divides up the privileges given to the all-powerful root account and allows you to selectively disable them. To prevent a user from removing the append-only attribute from a file, you need to remove the CAP_LINUX_IMMUTABLE capability. When present in the running system, this capability allows the append-only attribute to be modified. To modify the set of capabilities available to the system, use a simple utility called lcap (http://snort-wireless.org/other/lcap-0.0.6.tar.bz2.

To unpack and compile the tool, run this command:

# tar xvfj lcap-0.0.6.tar.bz2 && cd lcap-0.0.6 && make
         

Then, to disallow modification of the append-only flag, run:

# ./lcap CAP_LINUX_IMMUTABLE
# ./lcap CAP_SYS_RAWIO
         

The first command removes the ability to change the append-only flag, and the second command removes the ability to do raw I/O. This is needed so that the protected files cannot be modified by accessing the block device on which they reside. It also prevents access to /dev/mem and /dev/kmem, which would provide a loophole for an intruder to reinstate the CAP_LINUX_IMMUTABLE capability.

To remove these capabilities at boot, add the previous two commands to your system startup scripts (e.g., /etc/rc.local). You should ensure that capabilities are removed late in the boot order, to prevent problems with other startup scripts. Once lcap has removed kernel capabilities, you can only reinstate them by rebooting the system.

The BSDs accomplish the same thing through the use of securelevels. The securelevel is a kernel variable that you can set to disallow certain functionality. Raising the securelevel to 1 is functionally the same as removing the two previously discussed Linux capabilities. Once the securelevel has been set to a value greater than 0, it cannot be lowered. By default, OpenBSD will raise the securelevel to 1 when in multiuser mode. In FreeBSD, the securelevel is –1 by default.

To change this behavior, add the following line to /etc/sysctl.conf:

kern.securelevel=1

Before doing this, you should be aware that adding append-only flags to your log files will most likely cause log rotation scripts to fail. However, doing this will greatly enhance the security of your audit trail, which will prove invaluable in the event of an incident.