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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 ?

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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.

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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.

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    That's not dangerous at all, as you need to be root to use that file.
    – Joseph Sible-Reinstate Monica
    Commented Sep 17, 2018 at 14:24
  • Hence the "arguably" - it is an increase in attack surface which I was unaware of - as I'm sure others will have been.
    – Graham Nicholls
    Commented Sep 17, 2018 at 15:03

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