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, 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, but not all characteristics. The Meltdown and Spectre vulnerabilities are considered “catastrophic” by security analysts.The vulnerabilities are so severe that, initially, security researchers believed the reports to be false.

Several procedures to help protect home computers and related devices from the Meltdown and Spectre security vulnerabilities have been published. Meltdown patches may produce performance loss. Spectre patches have been reported to significantly reduce performance, especially on older computers; on the newer eighth-generation Core platforms, benchmark performance drops of 2–14 percent have been measured. On 18 January 2018, unwanted reboots, even for newer Intel chips, due to Meltdown and Spectre patches, were reported. Nonetheless, according to Dell: “No ‘real-world’ exploits of these vulnerabilities [i.e., Meltdown and Spectre] have been reported to date [26 January 2018], though researchers have produced proof-of-concepts.”Further, recommended preventions include: “promptly adopting software updates, avoiding unrecognized hyperlinks and websites, not downloading files or applications from unknown sources … following secure password protocols … [using] security software to help protect against malware (advanced threat prevention software or anti-virus).”

On 25 January 2018, the current status and possible future considerations in solving the Meltdown and Spectre vulnerabilities were presented.

On 15 March 2018, Intel reported that it will redesign its CPU processors to help protect against the Meltdown and related Spectre vulnerabilities (especially, Meltdown and Spectre-V2, but not Spectre-V1), and expects to release the newly redesigned processors later in 2018. On 8 October 2018, Intel is reported to have added hardware and firmware mitigations regarding Spectre and Meltdown vulnerabilities to its latest processors.