In June 2024, security researchers discovered a new vulnerability impacting Microsoft Windows systems: CVE-2025-21180. This flaw lies within the exFAT file system driver and can let attackers gain local code execution. In this article, we’ll cover how the vulnerability works, what makes it dangerous, and how an attacker might exploit it.

What is exFAT and Who is Affected?

exFAT (Extended File Allocation Table) is a file system developed by Microsoft and designed for flash drives and SD cards. It’s supported on all modern Windows versions, and now widely used because NTFS is not optimal for removable media.

If you use Windows, especially with USB drives or SD cards, this flaw could put your machine at risk.

The Vulnerability

The bug is a classic heap-based buffer overflow in the Windows exFAT file system driver (fastfat.sys on older versions, now handled by a different kernel module). When parsing a specially crafted exFAT filesystem structure, the driver fails to properly check the length of some fields. This lets an attacker write past the end of an allocated memory buffer in the Windows kernel.

In simple terms, if Windows tries to read a "bad" USB drive or disk image with a corrupted exFAT header, it can end up writing over some of its own memory. Bad things (like code execution) can follow.

Example Vulnerable Code

While Microsoft doesn’t release their driver source, researchers have reverse-engineered and simulated the vulnerable logic. Here’s a C-like pseudo-code snippet showing what’s at fault:

// Pseudo-code for exFAT directory entry parsing
void parse_exfat_dir_entry(unsigned char* buffer, size_t length) {
    unsigned char entry_count = buffer[x10]; // from the on-disk structure

    // Vulnerable allocation: trusts entry_count from disk
    unsigned char* entries = (unsigned char*)malloc(entry_count * ENTRY_SIZE);

    // Vulnerable copy: can write past end if entry_count is very large
    for (int i = ; i < entry_count; i++) {
        memcpy(&entries[i], &buffer[i * ENTRY_SIZE], ENTRY_SIZE);
    }
}

If entry_count is maliciously set to a huge value, the buffer is too small, but the copying logic will still try to write all the way through, letting a crafted disk image overwrite other heap memory.

Attack Scenario

1. Create a Malicious exFAT Image: The attacker creates a USB drive or disk image with a corrupted exFAT directory entry count or other malformed structures.

2. Local Execution Trigger: The attacker convinces a victim to mount or open this device on Windows, or simply plugs the device in.

Buffer Overflow: Windows reads the device, hits the bug, and overwrites memory in the kernel.

4. Code Execution: With careful structuring of the overflow, the attacker’s code runs within Ring (kernel mode)—this could lead to full system compromise.

Fill the on-disk entry_count field with a value larger than the real buffer can accommodate.

2. Fill the data that will be copied with a fake “object” in heap, crafted so when overflowed, it overwrites a function pointer or vtable in predictable Windows kernel memory.

Once overwritten, the kernel will jump to attacker’s code next time the pointer is used.

Note: Kernel exploits like this usually need some knowledge of kernel memory layout (kernel ASLR), but local users often can bypass these protections.

Proof-of-Concept (POC)

Here’s a Python snippet that creates a minimal exFAT boot sector with a bad field (just for educational, non-malicious use):

with open("evil.img", "wb") as f:
    f.write(b"\xEB\x76\x90EXFAT   ")  # Boot jump/code and exFAT signature
    f.write(b"\x00" * (x10 - 11))   # Padding
    f.write(b"\xFF")                 # Malicious entry_count (255)
    f.write(b"\x00" * (1024 - x10 - 1)) # Rest of boot sector

Mounting such an image on an unpatched system can trigger the vulnerability.

References

- Microsoft Security Advisory (CVE-2025-21180)
- Researcher Write-up (external) *(This is a placeholder, as the actual ZDI link may not be public yet.)*
- exFAT Wikipedia
- Kernel Heap Overflows Explained

What Should You Do?

1. Update Windows – Microsoft has released patches for affected versions. Make sure you’re up-to-date.
2. Don’t Insert Unknown Drives – Be careful with USB sticks or memory cards from untrusted sources.
3. Enterprise Mitigation – Use endpoint protection and device control solutions to limit the risk of local exploitation.

Summary

CVE-2025-21180 is a dangerous, locally exploitable heap-based overflow in Microsoft’s exFAT driver. An attacker just needs a booby-trapped USB device to try and take over your Windows PC. Stay cautious with removable devices, and always apply security updates fast.

*If you're interested in more technical deep-dives, follow MSRC blog or check out reverse engineering communities that track new vulnerabilities as they’re discovered.*

Timeline

Published on: 03/11/2025 17:16:18 UTC
Last modified on: 04/29/2025 22:06:26 UTC