Researcher Drops YellowKey, GreenPlasma Windows Zero-Days

A disgruntled security researcher this week publicly disclosed two zero-day vulnerabilities in Windows that enable BitLocker bypass and privilege escalation. BitLocker, Windows’ built-in full-volume encryption feature, relies on TPM (Trusted Platform Module) to deliver hardware-based security, protecting users’ data from unauthorized access if the device is stolen or lost. On Tuesday, a cybersecurity researcher known as Chaotic Eclipse and Nightmare Eclipse published proof-of-concept (PoC) code that allows an attacker with physical access to a machine running Windows 11 to bypass BitLocker and gain unrestricted access to the storage volume. The exploit has been dubbed YellowKey. This is not the first time Chaotic Eclipse has disclosed unpatched vulnerabilities in Microsoft products, and the researcher previously suggested they are displeased with the tech giant’s handling of vulnerability reports.  According to the researcher, the underlying issue for YellowKey is a well-hidden vulnerability without an explicit root cause, and could be a backdoor intentionally planted into BitLocker. The researcher’s exploit chain begins with copying a PoC folder to a USB drive and plugging it into a Windows machine that has BitLocker on, albeit copying the files to the EFI partition could do the trick without the removable drive. Next, one would need to reboot the device to Windows Recovery Environment (WinRE) by holding the Shift key while clicking ‘Restart’, then immediately release Shift and press and hold the Ctrl key until a command prompt window spawns, providing access to the protected volume. “Now, why would I say this is a backdoor? The component that is responsible for this bug is not present anywhere (even on the internet) except inside WinRE image, and what makes it raise suspicions is the fact that the exact same component is also present with the exact same name in a normal Windows installation, but without the functionalities that trigger the BitLocker bypass issue,” Chaotic Eclipse notes. Several security researchers, including Kevin Beaumont, KevTheHermit, and Will Dormann, have tested the exploit and confirmed it works even against recent Windows 11 builds. Chaotic Eclipse warned that YellowKey also works on devices protected with a TPM PIN (the user-defined, pre-boot authentication code required to unlock the machine), but refrained from publishing the PoC for this bypass. According to security researchers who tested the PoC exploit, such as JaGoTu, the success of a TPM PIN attack appears to depend on the WinRe implementation.   YellowKey reminds of a Windows vulnerability discovered a decade ago, which enabled BitLocker bypass by holding SHIFT+F10 pressed during feature updates in Windows 10. It spawned a shell providing admin privileges while BitLocker was disabled. The second zero-day Windows exploit dropped by Chaotic Eclipse is named GreenPlasma and allows attackers to elevate their privileges to System. The researcher published a PoC exploit stripped of the code required to achieve a full System shell. “The PoC will create an arbitrary memory section object in any directory object writeable by System,” Chaotic Eclipse noted, explaining that it could be used to manipulate various Windows services, including kernel-mode drivers. “Even with limitations around the current proof-of-concept, any path toward System-level privileges deserves close scrutiny. If fully exploited, that kind of escalation could allow attackers to disable protections, manipulate trusted processes, deploy malware, or use the compromised machine as a stepping stone into the broader environment,” Swimlane principal security solution architect Joshua Roback said. According to Corsica Technologies CISO Ross Filipek, the newly released PoC code could allow attackers to quickly weaponize their own exploits and start targeting the zero-day flaws in the wild. “Public zero-day releases always change the risk equation because they shrink the window between discovery and exploitation. In this case, YellowKey and GreenPlasma expose two different but connected concerns: access to protected data and the potential for privilege escalation. Even when an exploit has limitations, proof-of-concept code gives attackers a starting point they can test, modify, and fold into broader intrusion chains,” Filipek said. SecurityWeek has emailed Microsoft for a statement on the zero-day exploits and will update this article if the company responds. In early April, Chaotic Eclipse published PoC exploit code targeting BlueHammer, a Windows Defender security defect patched by Microsoft on April Patch Tuesday. Threat actors started exploiting it four days before the fixes were rolled out. Related: Microsoft Patches Critical Zero-Click Outlook Vulnerability Threatening Enterprises Related: Microsoft Patches 137 Vulnerabilities Related: Google Detects First AI-Generated Zero-Day Exploit Related: Is the SOC Obsolete, and We Just Haven’t Admitted It Yet? WRITTEN BY Ionut Arghire Ionut Arghire is an international correspondent for SecurityWeek. 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