In this long-read post, we will be discussing CVE-2025-21191, a critical vulnerability discovered in the Windows Local Security Authority (LSA) subsystem. This vulnerability is classified as a Time-of-Check Time-of-Use (TOCTOU) race condition and can be exploited by an authorized attacker to escalate their privileges on a local computer system. We will evaluate the exploit details, how it works, and possible mitigation strategies while providing relevant code snippets and references to original sources.

Exploit Details

Before diving into the exploit details, let us understand what LSA is and why this vulnerability is a concern. The Local Security Authority (LSA) is a critical component in the Windows operating system responsible for managing security policies and user authentication. If an attacker successfully exploits this vulnerability, they could unlawfully gain access to system resources and execute further malicious actions, such as installing malware or stealing data.

The vulnerability CVE-2025-21191 relies on exploiting a TOCTOU race condition. TOCTOU race conditions generally occur when a program checks the state or attributes of a resource (such as a file) and then performs an operation based on that information. If an attacker can change the resource's state between the check and the operation, they can provoke undesired behavior in the system, leading to security breaches.

In the case of CVE-2025-21191, an attacker with local access to a Windows computer can exploit this vulnerability and significantly elevate their privileges. Although the attacker must have valid login credentials, they do not need to possess administrative rights to exploit this vulnerability, making it a dangerous attack vector.

Code Snippet

The code snippet below demonstrates an example of a Time-of-Check Time-of-Use race condition vulnerability in a simplified scenario:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char *argv[]) {
    struct stat check_file;

    if (argc != 2) {
        printf("Usage: %s <file>\n", argv[]);
        return 1;
    }

    if (lstat(argv[1], &check_file) != ) {
        perror("lstat error");
        return 1;
    }

    if (!S_ISREG(check_file.st_mode)) { // Check if regular file
        printf("%s is not a regular file\n", argv[1]);
        return 1;
    }

    sleep(5); // Simulate time gap between check and use

    FILE *fileptr = fopen(argv[1], "r"); // Open file
    if (fileptr == NULL) {
        perror("fopen error");
        return 1;
    }

    printf("Opened %s successfully\n", argv[1]);
    fclose(fileptr);
}

In this example, we have a simple C program that checks if a given file is a regular file (time-of-check) and then opens it (time-of-use). An attacker exploiting this TOCTOU race condition could replace the file after the check and before the program opens it, resulting in unintended behavior.

Original References

- Microsoft Security Bulletin
- Common Vulnerabilities and Exposures (CVE)

Restrict user privileges and reduce the number of users with administrative rights on the computer.

3. Implement strong access control mechanisms to prevent unauthorized users from accessing sensitive files and resources.

Conclusion

CVE-2025-21191 is a significant security vulnerability in the Windows Local Security Authority (LSA) subsystem that can allow an attacker to escalate their privileges on a local Windows computer. By understanding the importance of secure software design and keeping systems updated, users can mitigate the risk posed by this and other similar vulnerabilities.

Timeline

Published on: 04/08/2025 18:15:44 UTC
Last modified on: 04/30/2025 17:14:00 UTC