As described by [Intel](https://www.openwall.com/lists/oss-security/2023/11/14/4):
Under certain microarchitectural conditions, Intel has identified cases where execution of an instruction (REP MOVSB) encoded with a redundant REX prefix may result in unpredictable system behavior resulting in a system crash/hang, or, in some limited scenarios, may allow escalation of privilege from CPL3 to CPL0.
and by [Red Hat](https://access.redhat.com/security/cve/CVE-2023-23583):
A security vulnerability was found in some Intel processors. Execution of REP MOVSB instructions with a redundant REX prefix may result in execution continuing at an incorrect EIP address after a micro-architectural event occurs, potentially allowing privilege escalation, information disclosure and/or a denial of service via local access.
as well as in [CVE-2023-23583](https://www.cve.org/CVERecord?id=CVE-2023-23583).
More detail is available via these [links to Intel's website](https://www.openwall.com/lists/oss-security/2023/11/14/7) and in the public disclosure by [Tavis Ormandy](https://lock.cmpxchg8b.com/reptar.html) from [Google](https://cloud.google.com/blog/products/identity-security/google-researchers-discover-reptar-a-new-cpu-vulnerability).
