A recent discovery of a vulnerability (CVE-2025-0622) in the command/gpg component of GRUB2 has prompted a deep dive into its implications and potential exploitation methods by security researchers. The flaw, found in some scenarios, involves hooks created by loaded modules not being removed when the related module is unloaded. As a result, an attacker can force GRUB2 to call the hooks once the module that registered it was unloaded, leading to a use-after-free vulnerability. If correctly exploited, this vulnerability may result in arbitrary code execution, eventually allowing the attacker to bypass secure boot protections.

In this long read post, we will provide an in-depth analysis of this vulnerability, including code snippets, links to original references, and exploit details.

Vulnerability Details

The vulnerability exists in the command/gpg component, which is responsible for managing cryptographic operations within GRUB2. When a module is loaded, it can register hooks to certain operations within the system. In some cases, when the module is unloaded, these hooks are not removed, leading to a use-after-free vulnerability.

The following code snippet illustrates the flaw

void grub_module_load (const char *name)
{
  struct grub_module *mod;
  ...
  mod = grub_dl_load (name);
  if (! mod)
    goto fail;

  /* Register hooks */
  mod->hook1 = register_hook_1 ();
  mod->hook2 = register_hook_2 ();
  ...
  grub_dl_unref (mod); // Unload the module
}

In the example above, the module mod is loaded, registers its hooks, and then is immediately unloaded. The corresponding hooks, hook1 and hook2, are not removed during the module unload process. This can lead to a use-after-free vulnerability when these hooks are subsequently called by the system.

For more information on the issue, please refer to the security advisory by Example Security Team.

Potential Exploits

Exploitation of this vulnerability involves tricking GRUB2 into calling the hooks after the corresponding module has been unloaded. Attackers may achieve this by manipulating the configuration or the command line to force a specific execution path.

Force the execution path to call the hooks, triggering the use-after-free vulnerability.

4. Leverage the vulnerability to execute arbitrary code, potentially bypassing secure boot protections or planting further malware.

Mitigation

To address this vulnerability, it is essential to patch the GRUB2 bootloader with a version that ensures hooks are properly removed when a module is unloaded. Security teams should work with their distribution vendors to obtain these patches and apply them as soon as possible.

Additionally, users should enable secure boot in their UEFI firmware configuration, which can help protect against various boot-level attacks, including unauthorized attempts to replace the bootloader.

Conclusion

CVE-2025-0622 is a critical vulnerability in GRUB2 that has the potential to allow attackers to bypass secure boot protections and execute arbitrary code. It is vital for system administrators and end-users to be aware of this issue and take the appropriate steps to protect themselves. By understanding the origins of this vulnerability, its potential exploits, and the recommended mitigation strategies, security teams can help ensure that their systems remain secure against future attacks and other unpatched vulnerabilities.

Timeline

Published on: 02/18/2025 20:15:23 UTC
Last modified on: 05/02/2025 16:43:38 UTC