A heap-based buffer overflow vulnerability (CVE-2025-21180) has been recently discovered in the Windows exFAT file system. This vulnerability allows an unauthorized attacker to execute code locally on the affected system, potentially leading to arbitrary code execution, system crashes, data corruption, or even full system compromise. In this article, we walk you through the exploit details, provide code snippets to demonstrate the vulnerability, and link to original references for further study and understanding.

Vulnerability Details

The vulnerability is triggered by a specially crafted exFAT file system image that contains malformed entries. When the vulnerable code processes these malformed entries, it fails to ensure proper bounds checking on user-supplied input data, leading to a heap-based buffer overflow. This overflow then allows an attacker to overwrite memory structures and execute arbitrary code with the privilege level of the affected system component.

Exploit Scenario

An attacker can exploit this vulnerability by crafting a malicious exFAT file system image and enticing a user to mount the image on a vulnerable system. This can be achieved, for example, through phishing emails, removable media (e.g., USB drives), or file-sharing services. Once the user mounts the malicious file system image, the attacker's code is executed, and the system is compromised.

Code Snippet

Here's a code snippet that demonstrates the vulnerable code's behavior. Note that this is not the original code but a simplified example to showcase the vulnerability:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_ENTRIES 100
#define ENTRY_SIZE 30

void process_entry(char *entry_data, size_t entry_len) {
    char buffer[ENTRY_SIZE]; // Vulnerable heap-based buffer

    // Copy entry_data without proper bounds checking
    memcpy(buffer, entry_data, entry_len);

    // Process the entry in the following code
}

int main(int argc, char *argv[]) {
    FILE *input_file = fopen("malicious_exfat_image", "rb");
    if (input_file == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }
    
    char exfat_entry[MAX_ENTRIES][ENTRY_SIZE];
    size_t entry_len = ;

    // Read MAX_ENTRIES from the file
    for (int i = ; i < MAX_ENTRIES; i++) {
        entry_len = fread(exfat_entry[i], 1, ENTRY_SIZE, input_file);
        process_entry(exfat_entry[i], entry_len);
    }

    fclose(input_file);
    return ;
}

In this example, the process_entry function copies the entry_data to a heap-based buffer without proper bounds checking, resulting in a heap-based buffer overflow.

To better understand and mitigate this vulnerability, refer to the following original sources

1. exFAT Specification: https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification
2. Vulnerability Details and Mitigations: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-21180
3. Article on Buffer Overflows: https://www.owasp.org/index.php/Buffer_overflow_attack

Conclusion

Heap-based buffer overflow vulnerabilities, like the one discussed in this post (CVE-2025-21180), can be challenging to detect, but they're also dangerous and have the potential to grant attackers significant control over an affected system. Developers, system administrators, and users should stay informed about this and similar vulnerabilities, apply recommended patches, and implement best practices to minimize the risk of exploitation.

Timeline

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