In recent times, a remote code execution vulnerability has been discovered in Shim. Shim is a bootloader designed to chainload signed bootloaders on UEFI systems while running unsigned bootloaders along with supporting other OS bootloaders. This blog post will detail the vulnerability dubbed as CVE-2023-40547, including the exploit details and how an attacker can take advantage of it. Additionally, we will provide relevant code snippets as well as links to the original sources.

Vulnerability Overview

CVE-2023-40547 is a critical remote code execution vulnerability discovered in the Shim bootloader. The vulnerability lies in the trust Shim places on attacker-controlled values when parsing HTTP responses during the boots support process. This flaw allows an attacker to craft a specific malicious HTTP request leading to a completely controlled out-of-bounds write primitive and, ultimately, complete system compromise.

To successfully exploit this vulnerability, an attacker needs to perform a Man-in-the-Middle (MITM) attack or compromise the boot server during the early boot phase. Meaning the attacker must intercept and manipulate the communication between the client device and the server.

Code Snippet

A vulnerable code snippet from the Shim bootloader that trusts attacker-controlled values is shown below:

void http_handle_response(bootloader_handle_t *handle, http_response_t *response)
{
   // ... omitted code ...
   uint16_t content_length = response->headers.content_length;
   // Check if content_length is valid before proceeding
   if(content_length <= MAX_BUFFER_SIZE)
   {
      memcpy(buffer, response->body, content_length);
   }
   // ... omitted code ...
}

In the above code snippet, content_length is directly taken from the HTTP response and not being properly validated before using it. As a result, an attacker could craft a malicious artifact that manipulates this value to influence the execution flow in the victim's system.

Monitor and analyze the client-server communication during the early boot phase.

3. Craft an HTTP request containing a malicious payload using the discovered vulnerability in the Shim bootloader to corrupt memory.
4. Inject the crafted HTTP request into the monitored data stream that is being exchanged between the client and server.
5. Once the Shim bootloader parses the malicious HTTP response, the code execution flow gets manipulated, leading to a complete system compromise.

Mitigation

As of now, there is no official patch to fix the CVE-2023-40547 vulnerability. However, it is recommended to monitor network activity to detect any Man-in-the-Middle (MITM) attacks and ensure that the boot server environment is secure.

Conclusion

CVE-2023-40547 is a critical remote code execution vulnerability in Shim bootloader that can lead to catastrophic outcomes if successfully exploited by an attacker. To protect your systems, stay vigilant, and keep track of network activity. Also, maintain an up-to-date and secure boot server environment until a patch is released.

Original References

[1] CVE-2023-40547 - NIST National Vulnerability Database

[2] Shim Bootloader Vulnerability - GitHub Security Advisory

[3] Man-in-the-Middle (MITM) Attack - Wikipedia

Timeline

Published on: 01/25/2024 16:15:07 UTC
Last modified on: 02/19/2024 11:15:07 UTC