The Mozilla Foundation yesterday issued a security update for Firefox and Firefox Extended Support Release, which were found to contain an actively exploited, critical vulnerability in the IonMonkey JIT compiler.

“Incorrect alias information in IonMonkey JIT compiler for setting array elements could lead to a type confusion,” reads an official advisory posted by Mozilla, citing the two elements as StoreElementHole and FallibleStoreElmenet. “We are aware of targeted attacks in the wild abusing this flaw.”

Designated CVE-2019-17026, the zero-day bug was reported by researchers at Qihoo 360 ATA. The problem has been fixed with the release of Firefox 72.0.1 and Firefox ESR 68.4.1

Mozilla provided no further context on how the flaw has been exploited.