In 2025, a serious vulnerability was discovered in GRUB2, the widely-used boot loader for Linux systems. CVE-2025-0624 exposes systems to remote code execution during network boot, even on environments with Secure Boot enabled. This flaw lets attackers on the same network gain control over affected machines at the earliest point of their boot process.

In this post, I’ll explain how the flaw works, go over the vulnerable code, and describe a basic exploitation strategy.

What’s the problem?

When a computer’s firmware is set to do a network boot (PXE), it looks for a configuration file—usually from a TFTP server. With GRUB2, users (or in some scenarios, attackers) can control certain environment variables passed into the bootloader.

The bug: GRUB2’s network boot code copies data from an environment variable into an internal buffer, using grub_strcpy(). But before doing this, it allocates the buffer *without* checking how big the environment variable is. So if the variable is very large, GRUB2 will overflow the buffer, potentially overwriting critical memory.

Because this happens so early in the boot process, and over the network, it allows attackers on the same LAN to run arbitrary code—even bypassing Secure Boot.

Let’s look at the simplified vulnerable code

char *env_value = grub_env_get("net_default_server");
char *buffer;
int len = 256; // Fixed buffer size

buffer = grub_malloc(len);
grub_strcpy(buffer, env_value); // No length check!

The buffer is always 256 bytes, but env_value can be any length.

- grub_strcpy() copies everything, without stopping for length. A long environment variable here will overflow the buffer.

In the official upstream grub source code, this logic is buried within the network/PXE boot handling modules.

How attackers exploit this

Attackers on the same network as a target set up a rogue PXE/TFTP/DHCP server. They can serve DHCP options or configuration files that poison the net_default_server environment variable with a payload much larger than 256 bytes and containing shellcode or ROP gadgets.

Victim machine starts network boot.

2. PXE negotiation: Attacker’s rogue server responds and provides malicious configuration, including a huge value for the vulnerable environment variable.

GRUB2 copies this value with grub_strcpy into a 256-byte buffer, overflowing it.

4. With carefully crafted data, the attacker overwrites return addresses or function pointers used by GRUB2, redirecting execution to their code.

Attacker’s payload executes in GRUB2’s context, before any OS protection loads.

If Secure Boot is enabled, it may only verify the bootloader’s signature—not the values of environment variables loaded in this way. So an attacker could launch unsigned boot kits, rootkits, or even load a completely different OS image.

Here’s a simplified PoC environment variable used to trigger the overflow

# This would be delivered via DHCP/tftp server in a real attack
malicious_value = "A" * 300 + shellcode_bytes

And in a properly configured rogue PXE environment (using dnsmasq or similar)

# Example dnsmasq config for a rogue PXE server
dhcp-option=17,192.168.1.200 # Points TFTP root
pxe-service=x86PC,"PXE GRUB",pxelinux
# ...additional directives to serve malicious config...

When grub fetches its config/environment, it imports the oversized variable and triggers the overflow.

Impact and Defense

- Remote code execution: Attackers can run arbitrary code before the OS boots—the holy grail of exploitation.

Mitigation

- Patch to latest GRUB2 immediately after updates are released.
- Disable or restrict netboot/PXE on systems unless absolutely necessary.
- Use hardware network isolation to block rogue DHCP/TFTP servers.

References

- CVE-2025-0624 (National Vulnerability Database)
- GRUB2 Official Site
- Upstream GRUB2 Git Source
- Network Boot and PXE security risks (US-CERT)

Conclusion

*CVE-2025-0624* is not just a rare theoretical bug. It’s a dangerous real-world flaw affecting systems that trust network boot—common in schools, offices, and server rooms. Thanks to early-stage memory handling errors, attackers can gain system-level code execution without touching disk, and without tripping Secure Boot’s limitations.

If your organization uses GRUB2 with PXE boot, treat this as a critical incident—patch now, and double-check your network’s boot security!

*Stay safe, and always dig deeper into bootloader security.*

Timeline

Published on: 02/19/2025 19:15:15 UTC
Last modified on: 03/31/2025 04:15:15 UTC