I was just wondering if a virus could melt a CPU by altering te voltage in the bios. :)
(and no, I'm not planning on making that kind of a virus...)
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It's definitely possible if the chip failsafe can be overriden. See for example the research paper "Potential thermal security risks". From page 4:
Permanent Damage – Disabling the Failsafe. The Intel failsafe cannot be disabled as far as we know, but older motherboards for AMD AthlonXP processors did not provide an adequate failsafe.  Although that problem was quickly rectified, even newer motherboards may allow the failsafe to be bypassed by disabling the thermal-emergency shutdown procedure in the BIOS. Obviously, we did not try this, but the same scenario that engaged the failsafe with the Pentium 4 would overheat and quickly destroy the processor. This is not reported to criticize AMD systems, but simply to show the importance of a failsafe mechanism that is hardwired. AMD has built a thermal trip mechanism in its latest generation of processors: the Athlon64. Similar to the P4, once the failsafe is engaged the CPU will shutdown its internal clock and inform the motherboard to reset through a THERMTRIP pin .
Accelerated aging caused by overheating (also discussed in that paper) is another, less spectacular but perhaps more likely attack.
A third possibility is an attack overclocking a CPU beyond recommended tolerances. For example, although an artificial overclocking lock may be present, it may not be secure, as in the case of the Intel 925X chip.
Note that similar attacks are possible on other pieces of hardware (see the question Can some software physically damage hardware? for details).
It is possible that a virus can make an application take up 100% of the CPU cycles - causing heat if it is done for a prolonged period of time, but unless it also manages to reflash / damage the BIOS, the thermal sensors of any modern motherboard will shut it down before it reaches any critical temperature.
Sure, it's possible, but probably unlikely. Many of the higher-end motherboards that are targeted at gamers include dynamic overclocking utilities that can be used to modify clock speeds, heat thresholds, and most importantly voltages. Most motherboards only allow these values to be changed while in the BIOS setup, but some allow for changes while running Windows.
Fortunately, many recent CPUs have also become a bit more defensive, at least on the heat front. They tend to include circuitry in them that shuts down the CPU if it detects overheating situations.
In order to exploit this, a malware maker would need to care enough about a particular motherboard (or perhaps brand and generation) to go to the trouble of attacking it.
It is extremely unlikely that any software could melt a modern CPU (as in I bet it has never happened except if done intentionally by trained experts physically disabling the hardware :-) ). Modern processors since Intel P4 and Opteron have a hardware thermal shutdown trip point that cannot be altered by software including BIOS.
I believe there are methods in which a virus can modify the cpu voltage. I mean there certainly are HDD-wipe viruses out there.
Anything manipulable can be manipulated.
I'd think a virus creator somehow has to read each motherboards unique "ID" or somewhat. Enabling the virus to access the mobo.
I mean there are software out there where you can manipulate your cpu/gpu voltage on-the-fly while the PC system is running in an OS environment. Taking this into account, yes, I believe it is doable somehow.
That being said, nowadays such virus would be moot, since almost any modern cpu has an internal sensor which enables it to shut down once it reaches critical temperatures.
But it really isn't a relevant concern yet.
Back in the mid nineties I remember hearing of a, for lack of better word, rootkit that would gain control of the hardware, spike the voltage then open and close all of the logical contacts at once.
Never saw it in action so I don't know if it was vaporware or not.