An industry-wide issue was found in the way many modern microprocessor designs have implemented speculative execution of instructions (a commonly used performance optimization). There are three primary variants of the issue which differ in the way the speculative execution can be exploited.

Variant CVE-2017-5753 triggers the speculative execution by performing a bounds-check bypass. It relies on the presence of a precisely-defined instruction sequence in the privileged code as well as the fact that memory accesses may cause allocation into the microprocessor's data cache even for speculatively executed instructions that never actually commit (retire). As a result, an unprivileged attacker could use this flaw to cross the syscall boundary and read privileged memory by conducting targeted cache side-channel attacks.

Mozilla Firefox before 28.0, Firefox ESR 24.x before 24.4, Thunderbird before 24.4, and SeaMonkey before 2.25 might allow local users to gain privileges by modifying the extracted Mar contents during an update.

Mozilla Firefox before 27.0 and SeaMonkey before 2.24 allow remote attackers to bypass the Same Origin Policy and obtain sensitive information by using an IFRAME element in conjunction with certain timing measurements involving the document.caretPositionFromPoint and document.elementFromPoint functions.

Mozilla Firefox before 26.0, Firefox ESR 24.x before 24.2, Thunderbird before 24.2, and SeaMonkey before 2.23 do not recognize a user's removal of trust from an EV X.509 certificate, which makes it easier for man-in-the-middle attackers to spoof SSL servers in opportunistic circumstances via a valid certificate that is unacceptable to the user.

The Portable Tool Library (aka PTLib) before 2.10.10, as used in Ekiga before 4.0.1, does not properly detect recursion during entity expansion, which allows remote attackers to cause a denial of service (memory and CPU consumption) via a crafted PXML document containing a large number of nested entity references, aka a "billion laughs attack."