CVE-2023-40548

What it actually does

Heap-based buffer overflow in 32-bit Shim caused by unchecked addition of a user-controlled value parsed from a PE binary, used in subsequent memory allocation, resulting in memory corruption during the boot phase.

Why it matters to the business

Potential boot-time crashes or data integrity failures on affected 32-bit Linux systems using Shim for Secure Boot, increasing operational downtime risk for organizations relying on Debian or upstream Shim builds, though low exploit maturity reduces immediate urgency.

How an attack unfolds

An attacker supplies a specially crafted PE binary containing a large value that triggers an integer overflow during size calculation in Shim's 32-bit parsing logic; when Shim processes the binary at boot, the resulting heap overflow corrupts memory and may allow control-flow hijacking or denial of service.

How to fix it

Upgrade rhboot/shim to commit 5914984a1ffeab841f482c791426d7ca9935a5e6 or later; on Debian 12/13 install shim package version 5.16.11-1 or newer; rebuild and redeploy any custom Shim builds incorporating the fix; test boot integrity after update.

How to confirm the fix worked

Verify installed Shim binary or package version against fixed versions (5.16.11-1 or commit 5914984a1ffeab841f482c791426d7ca9935a5e6); inspect boot logs for absence of corruption symptoms on 32-bit systems; re-run any existing Secure Boot validation tests.

Typical attack path

1. Craft malicious PE binary with oversized header field
2. Place binary in a location loaded by Shim during boot (e.g., EFI partition or bootloader chain)
3. Trigger system boot or reboot on 32-bit target
4. Exploit memory corruption to crash or potentially execute attacker-controlled code

How to detect it

Review early boot logs and firmware event logs for unexpected crashes or memory errors during Shim initialization; look for failed Secure Boot measurements or integrity check failures on 32-bit systems; no widespread in-the-wild exploitation artifacts have been observed.

What your security team should do

• Prioritize patching all 32-bit Debian systems and custom Shim deployments within standard change windows
• Audit bootloader configurations to ensure only signed, updated Shim binaries are used
• Monitor EPSS and KEV status for any future increase in exploit activity