A newly discovered remote code execution vulnerability, CVE-2023-42115, has been found affecting Exim, the popular mail transfer agent (MTA) used in many Linux and Unix-based systems. This vulnerability permits remote attackers to execute arbitrary code on the affected Exim installations without requiring authentication.

In this post, we will delve deep into the exploit details, provide code snippets, and offer links to original references. We will also discuss potential remedies and preventive measures that can help secure your Exim MTA installation.

Exploit Details

CVE-2023-42115 is traced back to a flaw within the Simple Mail Transfer Protocol (SMTP), which by default listens on TCP port 25. Due to inadequate validation of user-supplied data, a buffer overflow can occur, leading to a write past the end of a buffer. Consequently, an attacker can leverage this vulnerability to execute code in the context of the service account. This critical flaw was earlier identified as ZDI-CAN-17434.

To better understand the vulnerability, let's take a look at a sample code snippet that demonstrates the issue:

// Sample code snippet illustrating the vulnerability
void smtp_auth(char *buf, int buflen) {
    // ... code omitted for brevity ...
    char user[64], pass[64];
    int user_len, pass_len;

    // ... code omitted for brevity ...
    if (sscanf(buf, "AUTH LOGIN %s", user) == 1) {
        // Decode the user string in base64.
        user_len = b64_decoded_size(user);
        b64_decode(user, user_len, user);

        // TODO: Perform proper validation of the decoded user data.
        // ... code omitted for brevity ...
    }

    // ... code omitted for brevity ...
}

In the above code snippet, the user-supplied data is decoded from base64 format, and afterward, the "user" variable's length is determined. However, proper validation of the decoded user data is missing, which could result in a buffer overflow and lead to remote code execution.

CVE-2023-42115 Details

- The National Vulnerability Database (NVD) - CVE-2023-42115
- CVE List - CVE-2023-42115

Exim MTA Official Announcement and Patch

- Exim Security Update
- Exim Git Repository - Patch

Update to the latest version of Exim that includes the patch for CVE-2023-42115.

2. If updating is not immediately possible, restrict access to the SMTP service to trusted IP addresses or networks by modifying the Exim configuration file.
3. Keep a close watch on your system, checking for any unusual activities or high resource usage that could indicate a potential exploit.
4. Regularly apply security patches and updates to your entire system to keep all software up-to-date and minimize vulnerabilities.

Conclusion

In this post, we have explored the remote code execution vulnerability CVE-2023-42115 affecting Exim installations. By understanding the code snippet and exploit details, system administrators can better comprehend the severity of this flaw and take appropriate measures to secure their Exim systems. Quick response and applying cited mitigation strategies are highly recommended to protect against unforeseen exploits.

Timeline

Published on: 05/03/2024 03:15:50 UTC