CVE-2023-40547

What it actually does

The flaw causes Shim to trust attacker-supplied values while parsing an HTTP response during network boot, resulting in an out-of-bounds write primitive that grants complete control of the early-boot execution environment.

Why it matters to the business

Successful exploitation during network boot can lead to full system compromise of any machine relying on the affected boot infrastructure, creating high-risk exposure for large-scale deployments that use PXE or HTTP boot servers.

How an attack unfolds

An attacker must first obtain a man-in-the-middle position or compromise the boot server, then craft a malicious HTTP response containing specially chosen values that trigger the out-of-bounds write when Shim parses the response, resulting in arbitrary code execution before the operating system loads.

How to fix it

Upgrade rhboot/shim to commit 5914984a1ffeab841f482c791426d7ca9935a5e6 or later; rebuild and redeploy any custom Shim binaries; restrict network boot traffic to trusted servers only.

How to confirm the fix worked

Confirm the fixed commit hash is present in the Shim source or binary; test network boot against a patched build to verify the vulnerable parsing path is no longer reachable.

Typical attack path

1. Position as MITM or compromise the HTTP/PXE boot server
2. Intercept or serve a malicious HTTP response during early boot
3. Trigger out-of-bounds write via crafted response values
4. Execute attacker-controlled code in the pre-OS environment

How to detect it

Review boot-server access and HTTP response logs for unexpected or malformed responses around boot events; examine any systems that fail to boot or exhibit unusual early-boot behavior after network exposure.

What your security team should do

• Apply the referenced commit to all Shim instances used for network boot
• Harden or replace any boot servers that serve HTTP responses to Shim
• Disable network boot where not strictly required
• Monitor boot-server logs for anomalous HTTP responses