Hackers exploited Windows 0-day for 6 months after Microsoft knew of it

The vulnerability Lazarus exploited, tracked as CVE-2024-21338, offered considerably more stealth than BYOVD because it exploited appid.sys, a driver enabling the Windows AppLocker service, which comes preinstalled in the Microsoft OS. Avast said such vulnerabilities represent the “holy grail,” as compared to BYOVD.

In August, Avast researchers sent Microsoft a description of the zero-day, along with proof-of-concept code that demonstrated what it did when exploited. Microsoft didn’t patch the vulnerability until last month. Even then, the disclosure of the active exploitation of CVE-2024-21338 and details of the Lazarus rootkit came not from Microsoft in February but from Avast 15 days later. A day later, Microsoft updated its patch bulletin to note the exploitation.

It’s unclear what caused the delay or the initial lack of disclosure. Microsoft didn’t immediately have answers to questions sent by email.

Whatever the reason, the six-month wait gave Lazarus a much more efficient and stealthy way to install FudModule. Once in place, the rootkit allowed Lazarus to bypass key Windows defenses such as Endpoint Detection and Response, Protected Process Light—which is designed to prevent endpoint protection processes from being tampered with—and the prevention of reading memory and code injection by unprotected processes. Avast’s Vojtěšek explained:

From the attacker’s perspective, crossing from admin to kernel opens a whole new realm of possibilities. With kernel-level access, an attacker might disrupt security software, conceal indicators of infection (including files, network activity, processes, etc.), disable kernel-mode telemetry, turn off mitigations, and more. Additionally, as the security of PPL (Protected Process Light) relies on the admin-to-kernel boundary, our hypothetical attacker also gains the ability to tamper with protected processes or add protection to an arbitrary process. This can be especially powerful if lsass is protected with RunAsPPL as bypassing PPL could enable the attacker to dump otherwise unreachable credentials.

The researcher went on to write:

If an attacker, despite all of these hurdles, manages to exploit a zero-day vulnerability in a built-in driver, they will be rewarded with a level of stealth that cannot be matched by standard BYOVD exploitation. By exploiting such a vulnerability, the attacker is in a sense living off the land with no need to bring, drop, or load any custom drivers, making it possible for a kernel attack to be truly fileless. This not only evades most detection mechanisms but also enables the attack on systems where driver allowlisting is in place (which might seem a bit ironic, given that CVE-2024-21338 concerns an AppLocker driver).

While we can only speculate on Lazarus’ motivation for choosing this third approach for crossing the admin-to-kernel boundary, we believe that stealth was their primary motivation. Given their level of notoriety, they would have to swap vulnerabilities any time someone burned their currently used BYOVD technique. Perhaps they also reasoned that, by going beyond BYOVD, they could minimize the need for swapping by staying undetected for longer.

Vulnerability or not, patch ASAP

Independent researcher Kevin Beaumont called the company’s handling of the vulnerability “another clanger from Microsoft.”