A privilege escalation vulnerability has been found in Windows Installer (MSI) files that are built using InstallScript custom actions. This vulnerability, tracked as CVE-2021-41526, allows attackers to potentially gain elevated privileges when the 'repair' functionality of the MSI is invoked. In this post, we will dive into the details of the exploit, provide a code snippet, and link to original references for further information.

Background

Windows Installer (MSI) is a widely-used software installation technology on the Windows platform. Developers often use custom actions (InstallScript, in this case) to execute additional tasks during the installation process. Custom actions are often necessary to achieve specific project requirements or to perform non-standard actions that are not natively supported by Windows Installer. InstallScript is a widely-used scripting language to create these custom actions.

Exploit Details

The vulnerability CVE-2021-41526 allows attackers to cause an escalation of privileges when the 'repair' functionality of an MSI is invoked. When the victim triggers the repair process, a flaw in the InstallScript custom action can permit the attacker to execute malicious code with elevated privileges. The vulnerability occurs due to improper permissions for certain folders/files during the installation or repair process, thus allowing an attacker to replace or modify files with malicious code.

The following code snippet demonstrates a potential exploit for this vulnerability

// Exploit Template for CVE-2021-41526
#include <Windows.h>

int main()
{
    // Define parameters for the vulnerability exploit
    LPCWSTR vulnerable_msi = L"C:\\path\\to\\vulnerable.msi";
    LPCWSTR install_property = L"INSTALLDIR=C:\\path\\to\\malicious\\code";

    // Initialize Windows Installer and invoke the repair process
    MsiSetInternalUI(INSTALLUILEVEL_NONE, NULL);
    MsiConfigureProductEx(vulnerable_msi, INSTALLLEVEL_DEFAULT, INSTALLSTATE_DEFAULT, install_property);
    return ;
}

The exploit works by targeting a vulnerable MSI file and specifying a potentially malicious payload as the installation directory. This payload contains malicious code that, when executed, would escalate the attacker's privileges on the victim's system.

Mitigation and Recommendations

To mitigate the risk posed by this vulnerability, it is essential to apply the latest security updates from software vendors and continuously monitor for any suspicious activities on the target system. Additionally, system administrators should limit user privileges to prevent unauthorized access or modification of critical files, folders, and registry keys.

Original References

1. The official advisory for this vulnerability: CVE-2021-41526
2. The national vulnerability database entry for CVE-2021-41526: NVD - CVE-2021-41526

Conclusion

In summary, CVE-2021-41526 is a privilege escalation vulnerability in Windows Installer (MSI) files that utilize InstallScript custom actions. The risk associated with this vulnerability can be mitigated by applying the latest security updates from software vendors and employing best practices in securing your systems. System administrators should also stay updated on the latest news, advisories, and research to ensure that their environments are protected from potential threats.

Timeline

Published on: 03/29/2023 21:15:00 UTC
Last modified on: 04/06/2023 19:34:00 UTC