CVE-2023-40546 - Shim Vulnerability Leads to Crash in ESL Variable Creation Error Handling

A critical flaw has been identified in the popular Shim bootloader used in the Linux ecosystem. This vulnerability, registered under the identifier CVE-2023-40546, exposes systems running Shim to a potential crash when attempting to create a new ESL (EFI Signature List) variable. The issue lies in the error handling mechanism of Shim, which uses an incorrect number of parameters in its logging function, leading to a crash under specific circumstances.

In this post, we'll take a deep dive into this vulnerability, discuss its implications, and highlight possible mitigation strategies and solutions.

The Flaw

Shim acts as a bridge between Linux distributions and UEFI (Unified Extensible Firmware Interface) firmware. It allows Linux distributions to run securely alongside other operating systems, such as Windows. Shim verifies the boot loader's signature before allowing it to run, ensuring the system's integrity during boot.

The vulnerability CVE-2023-40546 was discovered in Shim's error handling process when creating a new ESL variable. If for some reason, Shim is unable to create the new variable, it attempts to print an error message to inform the user. The problem lies in the logging function utilized in this scenario, as the number of parameters passed to it does not match the format string it expects. Under certain conditions, this mismatch can cause Shim to crash, leaving the system inoperable.

Here's the code snippet showcasing the issue

/* buggy logging function */
void log_error(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);
}

/* error handling in Shim */
if (efi_status != EFI_SUCCESS) {
    log_error("Failed to create new variable: %r\n", efi_status);
}

In the above example, the log_error function is called with two parameters, but the format string within the function only expects a single parameter. This discrepancy between the expected and received parameters causes the crash.

Exploit Details

As of now, there are no known exploits actively targeting this vulnerability in the wild. However, considering the widespread use of Shim in the Linux community, it is crucial to understand and address this issue. A potential attacker could exploit this vulnerability to cause a denial of service (DoS) on the target machine, crippling the boot process and rendering the system inoperable.

Original References and Acknowledgments

This vulnerability was responsibly disclosed to the Shim maintainers, who have since addressed the issue and released a patch. More details on the vulnerability can be found by referring to the following sources:

Shim project's home page: [ExampleLink3](#)

We would like to extend our thanks to the security researcher who discovered this vulnerability and the Shim maintainers for their timely response and remediation.

To protect your systems from the risks associated with CVE-2023-40546, you should

1. Update Shim to the latest version available for your Linux distribution. The maintainers have released a new version that includes a patch for this vulnerability.
2. Keep your Linux distribution and UEFI firmware up-to-date, and ensure that all security patches are applied regularly.

Following these guidelines should help you mitigate the impact of this vulnerability and keep your systems protected.

Conclusion

CVE-2023-40546 is a significant vulnerability in the Shim bootloader. Although there are currently no known exploits targeting this flaw, it is essential to recognize its potential impact, especially considering the widespread use of Shim in the Linux ecosystem. By understanding the issue and following best practices for updating your systems, you can protect your infrastructure from possible exploitation.

Timeline

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