Microsoft has recently addressed a critical privilege escalation vulnerability in Windows 11 version 23H2, tracked as CVE-2024-30085. The flaw is found in the Cloud Files Mini Filter Driver (cldflt.sys), which improperly handles user-supplied data when parsing reparse points. Specifically, the vulnerability arises due to inadequate validation of the data’s size before copying it into a fixed-length heap-based buffer. This allows an attacker to overwrite memory and execute malicious code, potentially elevating their privileges to SYSTEM-level access, which could lead to full control of the compromised system.
The vulnerability was first brought to light by Alex Birnberg, a security researcher who demonstrated the exploit during the prestigious TyphoonPWN 2024 cybersecurity competition. His demonstration earned him third place, highlighting the complexity and severity of the vulnerability. By exploiting this flaw, attackers could target systems where low-privileged code is already executed, greatly escalating the risk in environments with limited user access, making it easier to carry out attacks without requiring full system control.
The root cause of the vulnerability lies in the improper handling of reparse point bitmaps in the Cloud Files Mini Filter Driver, particularly in the HsmIBitmapNORMALOpen function. The system fails to verify the length of the reparse data under specific conditions during file operations. This oversight allows attackers to bypass critical checks and inject malicious data, which can lead to memory corruption and privilege escalation. Birnberg’s demonstration showcased the precise method to exploit this weakness, creating a specially crafted reparse point to trigger SYSTEM-level privileges.
Following the disclosure of the vulnerability, Microsoft released a patch to mitigate the risk associated with CVE-2024-30085. Users are strongly advised to update their systems promptly to ensure protection against potential exploits. In addition to applying the security patch, organizations should implement best practices such as restricting administrative access, regularly updating systems, monitoring unusual system behavior, and using intrusion detection systems (IDS) to detect signs of potential attacks. This incident underscores the importance of proactive security measures and staying vigilant in the face of constantly evolving cyber threats.
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