Silencing the Guard







Risk Rating:


Probably the most formidable control is the alarm control. The ability to draw attention when something goes wrong and bring down the cavalry to handle an attack are powerful weapons in any battle. When protecting the Linux deployment, the alarm control is still the most formidable weapon—except when it's abused.

Assuming the alarm is properly deployed and monitored, the only means of getting past it without incident is to cut it off before it can alert anyone, circumvent it by finding a path to assets it does not protect, or trigger it all the time and for no reason until it's either disabled or the valid alarm is obscured by the invalid ones.

Cutting off the alarm before it can alert anyone may be too difficult, though. The path to the guard is often much shorter than to the alarm itself. Intercepting the guard is sometimes a more feasible option than attempting to cut off the alarm. Slower alerts, such as log files, however, can be deleted, and this step is important in penetrating an asset gateway. However, deleting log files only works once the attacker has access and is not the best choice for network-based alerts.

Circumventing alarm controls is often possible for network-based sensors but not for system access where log files record changes to files, permissions, and actions. Since movement in a system is limited to the Linux system environment, it is not possible to move about a system unnoticed and untracked. However, most network sensors work with black lists, so all the attacker needs to do is make the attack appear as proper traffic or unrecognizable as known traffic at all so the black list cannot make a match to a known attack type.

The final technique is a potent but noisy one. It depends on noise to drown out the valid information about an ongoing attack. A typical human reaction is to turn off the alerts when they all seem to be invalid. A detection system may just be overwhelmed and drop the traffic it cannot handle, leaving it unverified.

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