Reptar is a CPU vulnerability discovered in late 2023, affecting a number of recent families of Intel x86 CPUs.
According to The Register, the following CPU families are vulnerable: Alder Lake, Raptor Lake and Sapphire Rapids.
[1] The Reptar vulnerability relates to processing of x86 instruction prefixes in ways that lead to unexpected behavior.
[2][3] The vulnerability can be exploited in a number of ways, potentially leading to information leakage, denial of service, or privilege escalation.
[5] Intel have released new microcode in an out-of-band patch to mitigate the vulnerability, which it calls "redundant prefix".