Meltdown is a novel attack that exploits a vulnerability in the way the processor enforces memory isolation.
At a high level, the root cause of Meltdown‘s simplicity and strength are side effects caused by out-of-order execution, which is an important performance feature of modern processors designed to overcome latencies of busy execution units (for example, a memory fetch unit waiting for data arrival from memory). Rather than stalling the execution, modern processors run operations out-of-order, that is, they look ahead and schedule subsequent operations on available execution units of the core. —CACM, “Meltdown: Reading Kernel Memory from User Space”
Meltdown is a hardware vulnerability affecting Intel x86 microprocessors, IBM POWER processors, and some ARM-based microprocessors. It allows a rogue process to read all memory, even when it is not authorized to do so.
Meltdown affects a wide range of systems. At the time of disclosure (2018), this included all devices running any but the most recent and patched versions of iOS, Linux, macOS, or Windows. Accordingly, many servers and cloud services were impacted, as well as a potential majority of smart devices and embedded devices using ARM-based processors (mobile devices, smart TVs, printers and others), including a wide range of networking equipment. A purely software workaround to Meltdown has been assessed as slowing computers between 5 and 30 percent in certain specialized workloads, although companies responsible for software correction of the exploit are reporting minimal impact from general benchmark testing.
Meltdown was issued a Common Vulnerabilities and Exposures ID of CVE-2017-5754, also known as Rogue Data Cache Load (RDCL), in January 2018. It was disclosed in conjunction with another exploit, Spectre, with which it shares some characteristics. The Meltdown and Spectre vulnerabilities are considered “catastrophic” by security analysts. The vulnerabilities are so severe that security researchers initially believed the reports to be false. —Wikipedia, “Meltdown (security vulnerability)“