Free unofficial fixes are now available for a new zero-day flaw in Windows Themes that allows hackers to remotely harvest a target's NTLM credentials.
NTLM has been extensively exploited in NTLM relay attacks, in which threat actors force susceptible network devices to authenticate against servers under their control, and in pass-the-hash attacks, in which attackers exploit system vulnerabilities or deploy malicious software to steal NTLM hashes (hash passwords) from target systems.
Once they acquire the hash, the attackers can impersonate the affected user, gaining access to sensitive data and expanding laterally throughout the now-compromised network. Microsoft indicated a year ago that it will drop the NTLM authentication technology in Windows 11.
ACROS security experts uncovered the new Windows Themes zero-day (which has yet to be assigned a CVE ID) while working on a micropatch for a flaw tracked as CVE-2024-38030 that might reveal a user's credentials (reported by Akamai's Tomer Peled), which was itself a workaround for another Windows Themes spoofing vulnerability (CVE-2024-21320) fixed by Microsoft in January.
According to Peled, "when a theme file specified a network file path for some of the theme properties (specifically BrandImage and Wallpaper), Windows would automatically send authenticated network requests to remote hosts, including user's NTLM credentials when such a theme file would be viewed in Windows Explorer.”
"This meant that merely seeing a malicious theme file listed in a folder or placed on the desktop would be enough for leaking user's credentials without any additional user action," ACROS Security CEO Mitja Kolsek stated.
Even though Microsoft fixed CVE-2024-38030 in July, ACROS Security discovered another vulnerability that attackers may use to steal a target's NTLM credentials on all fully updated Windows versions, from Windows 7 to Windows 11 24H2.
"So instead of just fixing CVE-2024-38030, we created a more general patch for Windows themes files that would cover all execution paths leading to Windows sending a network request to a remote host specified in a theme file upon merely viewing the file," Kolsek added.
The firm is now offering free and unofficial security updates for this zero-day flaw via its 0patch micropatching service for all affected Windows versions until official patches from Microsoft are available, which have already been applied to all online Windows systems running the company's 0patch agent.
To install the micropatch on your Windows device, first create a 0patch account and then install the 0patch Agent. If no specific patching policy prevents it, the micropatch will be applied immediately without the need for a system restart once the agent is activated.
However, it is crucial to remember that in this case, 0patch only delivers micropatches for Windows Workstation, as Windows Themes does not work on Windows Server until the Desktop Experience feature is deployed.