CVE-2023-40550 - Out-of-Bounds Read in Shim Exposes Sensitive Data at Boot

*Published: June 2024*

A recently disclosed vulnerability, CVE-2023-40550, affects the Shim bootloader—a critical piece of software that plays an essential role in Secure Boot on Linux-based systems. This flaw enables unauthorized reading of sensitive data right at the system's earliest stage. In this article, we’ll break down what went wrong, how this issue exposes data, show a code snipplet, and explain the impact in straightforward terms.

What is Shim and SBAT?

Shim is a small bootloader that bridges standard Linux bootloaders (like GRUB) with systems implementing Secure Boot. It authenticates OS loaders, making sure only signed code is allowed to start.

SBAT stands for Shim Boot Advanced Targeting. SBAT is a protocol for tracking boot component versions, which helps administrators quickly revoke vulnerable or untrusted bootloaders and kernels.

The Vulnerability: Out-of-Bounds Read

CVE-2023-40550 arises from a logic error in Shim’s SBAT handling. Shim fails to validate certain buffer boundaries when parsing SBAT information, leading to out-of-bounds reads—the software reads data from memory it isn’t supposed to.

Why Is This Bad?

Since Shim operates very early during boot—before normal OS protection mechanisms are enabled—it has high privileges. An attacker exploiting this issue could read any memory content adjacent to the SBAT buffer, potentially exposing secrets like:

Kernel parameters

Attackers could leverage this exposure for further attacks, or simply access sensitive data that should never leave the secure boot environment.

Original References

- Red Hat Security Advisory (RHSA-2024-XXXX)
- Shim GitHub Repository
- Ubuntu Security Notice (USN-XXXX-1)

Vulnerable Code Snipplet

Below is a simplified version, based on public reports and GitHub discussions. The problem lies in how Shim parses the SBAT section from a PE binary.

// Vulnerable code: fails to check the upper bound of 'limit'
while (*current != '\') {
    // Read SBAT line from the buffer
    size_t len = strcspn(current, "\n");

    // Out-of-bounds read if 'current + len' goes past buffer end
    process_sbat_line(current, len);

    current += len;
    if (*current == '\n')
        current++;
}

What’s wrong?
The code does not validate if current + len is still inside the allocated sbat buffer. An attacker could craft a malformed SBAT section to trick Shim into reading unintended memory regions.

Craft a malicious bootloader

- Insert a specially formed SBAT string that lacks a null terminator or newlines, pushing Shim beyond the SBAT buffer when parsing.

Shim Reads Out-of-Bounds Data

- During validation, Shim reads secrets or sensitive memory, which could be logged or accessed by later-stage malware.

Harvest Sensitive Data

- Obtain credentials, cryptographic keys, or boot parameters, increasing the risk of privilege escalation or Secure Boot bypass.

How to Stay Safe

- Update Shim immediately: Vendors have released patched versions that include proper bounds checking.

Conclusion

CVE-2023-40550 is a critical reminder of how even tiny mistakes in early-stage system software can have big consequences. Stay up-to-date with security patches, especially for anything related to Secure Boot and the bootloader chain. A single overlooked check can put your system's deepest secrets at risk—don’t wait to patch!

Timeline

Published on: 01/29/2024 17:15:08 UTC
Last modified on: 04/29/2024 14:15:07 UTC