CVE-2023-40549 - Out-of-Bounds Read in Shim Results in Denial of Service

> English is not my first language; please excuse any mistakes.

Executive Summary

A recently-discovered vulnerability, CVE-2023-40549, affects the Shim bootloader used widely for UEFI Secure Boot primarily in Linux systems. An out-of-bounds read flaw has been identified, which results from improper boundary verification when loading a PE (Portable Executable) binary. Attackers who exploit this vulnerability can potentially crash Shim, causing a denial of service.

Vulnerability Details

Shim is a pre-loader that verifies the boot image, ensuring the code is signed by trusted keys before passing control to the OS loader. The vulnerability lies in Shim's handling of the PE binary – specifically, the lack of proper boundary checks during the load process.

The following code snippet demonstrates the problematic section

UINTN
get_aligned_size (VOID *data, UINTN size)
{
  return (ALIGN_VALUE (size));
}

EFI_STATUS
read_pe (PE_COFF_LOADER_IMAGE_CONTEXT *context)
{
  if (/*...*/)
  {
    // ...
  }
  return EFI_SUCCESS;
}

The get_aligned_size() function calculates the necessary size for a proper alignment, but it does not validate the given size against the actual data size. As a result, the read_pe function is operating on unvalidated data, triggering the out-of-bounds read bug.

Attack Scenario

In order to exploit this vulnerability, an attacker would need to craft a specific PE binary that overflows the bound of the buffer. When loading the crafted binary, Shim will trigger the out-of-bounds read flaw, ultimately leading to a crash and resulting in a denial of service.

The attack impact can range from crashing an individual user's device to disrupting the whole Linux ecosystem if the exploit is widespread.

Mitigation and Solutions

Currently, there is no official patch addressing the vulnerability. However, users can employ the following workarounds to mitigate the risk:

- Monitor Shim's git repository for any updates: Shim git repository

Validate PE binaries before loading them into Shim

- Employ the use of dedicated hardware security modules (HSMs) that would not be affected by this vulnerability

Conclusion

CVE-2023-40549 is a critical vulnerability that could result in widespread denial of service attacks if exploited. It is essential for developers and system administrators to be aware of this issue and take immediate action to mitigate its impact.

Researchers who discovered the flaw recommend monitoring the Shim git repository for patches and updates closely. Employing complementary measures like validating PE binaries or implementing hardware security modules can also help manage the risk until a fix is released.

The original reference for this vulnerability can be found at the National Vulnerability Database.

As always, it's crucial to maintain a strong system security posture and follow best practices to minimize the potential risk of such vulnerabilities.

Timeline

Published on: 01/29/2024 17:15:08 UTC
Last modified on: 04/25/2024 14:15:08 UTC