How can the Magic SysRq key be dangerous for linux users?

Question

The magic SysRq key is known for linux users to perform some actions when the system freeze , but it considered as dangerous command for users who have a physical access to the system:

Some people view this key as giving access to dangerous system-level commands to anyone who has physical access to the keyboard or serial console.[12] It has been argued that this perceived security risk is illusory, as anyone with physical access to the computer would already have the capability to compromise its security.[13] The advent of the procfs interface has rekindled debate over this subject.

How can the Magic SysRq key be dangerous for linux users ?

Answer 1

It can be used to display CPU registers (which could contain bits of confidential information), forcibly unmount filesystems or reboot the computer, among other things (denial of service vulnerability).

I wouldn't say it is dangerous though. If you have physical access to the server there so much you can do without even using the "magic" key. A lot of distributions for example allow rebooting the server with Ctrl+Alt+Del without being logged in, but even if that's disabled, just unplug the power cable.

Once an attacker has physical access, all bets are off.

Answer 2

This is arguably dangerous, and doesn't require physical access. Writing to the file /proc/sysrq-trigger has the same effect as typing the physical key combination.

This:

echo "c" > /proc/sysrq-trigger

causes a crash on RHEL 7.5.