In the realm of computer security, even subtle, seemingly innocuous issues can become significant vulnerabilities with the potential to cause harm. CVE-2021-3899 is one such example where a race condition in replaced executable detection may let attackers execute arbitrary code with root privileges, thereby compromising the security of the system.

In this long read, we'll take a closer look at CVE-2021-3899, analyze specific code snippets, delve into the details of the exploit, and provide links to the original references. By the end of this post, you'll have a solid understanding of this vulnerability and will be better equipped to consider its impact on your systems.

Vulnerability Summary

The CVE-2021-3899 vulnerability stems from a race condition that occurs during the replaced executable detection process. By exploiting this issue, a local attacker can execute arbitrary code as root with the right configuration. Essentially, the race condition allows the attacker to manipulate the timing of events, enabling them to replace binaries or files without raising suspicion.

Code Snippet

Here is a sample code snippet which demonstrates how the replaced executable detection is affected by a race condition:

// This function performs checks on the executable being replaced
void check_executable(char* file_path) {
    struct stat stat_original, stat_replaced;

    // First check of the executable
    if (lstat(file_path, &stat_original) == -1) {

    // Some operations and checks...

    // Second check of the executable
    if (lstat(file_path, &stat_replaced) == -1) {

    // If the first check differs from the second check,
    // we have a race condition vulnerability
    if (stat_original.st_ino != stat_replaced.st_ino ||
        stat_original.st_dev != stat_replaced.st_dev) {
        fprintf(stderr, "Executable replaced during execution!\n");


In this code, the 'lstat()' function is being used to check the status of a file. The problem occurs because the same function is called twice, with some operations being performed in between the two calls. An attacker can exploit this time gap to manipulate the file, which may eventually lead to arbitrary code execution.

When the script detects an interaction, quickly replace the target with a malicious file.

4. If successful, the malicious file is executed with root privileges, allowing the attacker access to the system.

It is important to note that exploiting this vulnerability also depends on the attacker's ability to time the replacement of the target executable accurately.

For more details on the CVE-2021-3899 vulnerability, you can refer to the following resources

1. Official CVE Details page
2. National Vulnerability Database Entry
3. Race Condition Exploit Article


CVE-2021-3899 is an interesting vulnerability caused by a race condition in replaced executable detection. Although the attacker requires specific local configuration and precise timing to execute the exploit, the potential for causing harm should not be overlooked. Understanding the details of the exploit, being aware of the code patterns that lead to such vulnerabilities, and proactively scanning for such issues are crucial steps to strengthen your system's security.


Published on: 06/03/2024 19:15:08 UTC
Last modified on: 06/03/2024 19:23:17 UTC