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Intel Reptar Flaw Patch For CPU Vulnerability Released

Wajahat Raja

November 29, 2023 - TuxCare expert team

Intel recently released multiple fixes for a high-severity vulnerability dubbed Reptar. The CVE-2023-23583 has a CVSS score of 8.8 and, when exploited, has the potential for privilege escalation, information disclosure, and a denial of service (DoS) condition. The Intel Reptar flaw patch has been released despite there being no evidence of the vulnerability being exploited. 

In this blog, we’ll discuss how threat actors could potentially exploit this high severity Intel flaw and Intel’s take on the matter. 

Potential Exploits Without The Intel Reptar Flaw Patch


Threat actors could potentially exploit the Reptar vulnerability in CPUs if they have local code execution on the operating system or on a guest virtual machine (VM). Without CPU vulnerability mitigation, this flaw could be exploited for privilege escalation or disclosing confidential information. 

Cybercriminals, in a multi-tenant virtualized environment, can also exploit the vulnerability of a guest VM. Such an exploit would cause the host server to crash, resulting in a DoS condition for all users on the server. Organizations, as part of their CPU firmware fixes, should check for BIOS/UEFI updates with their system manufacturers. 

Unignored Instruction Prefixes And Reptar


The name “Reptar” for CVE-2023-23583 originates from the “rep” instruction prefix that should be ignored but isn’t. Tavis Ormandy, a security researcher at Google, has unveiled that this high severity Intel flaw can be traced back to the way instruction prefixes on CPUs with fast short repeat move (FSRM) capabilities are processed. 

An instruction set for such CPUs can be accessed through human-readable machine code presented in assembly language. Codes written in this programming language are used to operate directly with CPU instructions that support prefixes, which change the way they function.  

The code “rep movsb” contains the prefix “rep,” which, in an ideal scenario, would mean repeat for the instruction “movsb.” In the code “rex.rbx rep movsb,” the prefix being is used to allocate additional bits. However, the prefix in this case is not required for this instruction and should be ignored, but it isn’t. 

Researchers at Google have found that such prefixes in CPU where FSRM is evident are interpreted unusually, which then results in the vulnerability being referred to as Reptar. 

Reptar Intel Security Advisory


In a guidance issued on November 14th, 2023, Intel said, “Intel does not expect this issue to be encountered by any non-malicious real-world software. Redundant REX prefixes are not expected to be present in code nor generated by compilers. Malicious exploitation of this issue requires execution of arbitrary code.”

Commenting further on what could have been one of the most severe CPU security vulnerabilities, Intel emphasized that the potential for privilege escalation was identified in their internal security validation protocols.  

As far as patching high-severity flaws is concerned, Intel has released an updated microcode for all the affected processors. According to the Intel chip security update, the affected CPUs include:

  • Xeon D and 3rd and 4th generation Xeon Scalable server CPUs.
  • Mobile or desktop versions of 10th, 11th, 12th, and 13th generation Intel Core processors. 


Intel Reptar flaw patch contains microcode for all affected processors. Although there is currently no evidence of malicious activities resulting from the vulnerability, if exploited, it can lead to privilege escalation, a DoS condition, or information disclosure. It’s worth mentioning that another Intel CPU vulnerability dubbed “Downfall” came to light a couple of months ago. 

At the time, risks of information and password theft were high. Such vulnerabilities and their potential for being exploited are alarming. Therefore, organizations must adopt proactive cyber security measures to improve their security posture and safeguard systems and data. 

The source for this piece includes articles in The Hacker News and CSO.


Intel Reptar Flaw Patch For CPU Vulnerability Released
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Intel Reptar Flaw Patch For CPU Vulnerability Released
Discover the Intel Reptar flaw patch, the vulnerability, and the possible damages of potential exploits. Stay informed, be protected!
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