CVE-2023-42118: Exim libspf2 Integer Underflow Remote Code Execution Vulnerability – A Comprehensive Guide

The cybersecurity world has recently witnessed the discovery of CVE-2023-42118, a critical vulnerability affecting Exim libspf2 - Sender Policy Framework (SPF) libraries. This post will provide an in-depth analysis of the vulnerability, including the code snippet, original references, and exploit details. CVE-2023-42118 allows network-adjacent attackers to execute arbitrary code on affected installations of Exim libspf2 without authentication. Let's dive into the specifics of this flaw and see how it impacts the security landscape.

Vulnerability Analysis

The core issue in this vulnerability lies within the parsing of SPF macros while handling user-supplied data. The process fails to validate this data correctly, leading to an integer underflow before writing to memory. Attackers can exploit this flaw to execute code in the service account's context.

Original Reference

This vulnerability was initially identified through the Zero Day Initiative (ZDI) with the reference ZDI-CAN-17578. More information about this vulnerability can be found on ZDI's website: ZDI-CAN-17578

Code Snippet

The vulnerability occurs due to improper validation of user-supplied data when processing SPF macros. The following code block demonstrates the affected part of the library:

void process_spf_macros(char *input, char *output) {
    int i = , j = ;

    for (; input[i] != '\'; i++) {
    if (input[i] == '%' && input[i + 1] != '%' && input[i + 1] != '{') {
        int macro_len = decode_spf_macro(input[i + 1]); // Macro length calculation
        if (j + macro_len > OUTPUT_BUFFER_SIZE) {
        // Handle buffer overflow
        break;
        }

        memcpy(&output[j], &input[i], macro_len); // Vulnerable memcpy
        i += macro_len;
    } else if (input[i] == '%' && input[i + 1] == '{') {
        // Other macro processing
        } else {
        output[j++] = input[i];
        }
    }

output[j] = '\';
}

Given below is a step-by-step guide on how an attacker can exploit this vulnerability

1. Craft a malicious SPF record containing a specially crafted macro that results in an integer underflow when parsed.
2. Set up a DNS server to respond with this malicious SPF record when queried by the target Exim libspf2 installation.

Send an email to the target Exim libspf2 installation, causing the system to perform an SPF check.

4. The SPF check will trigger the parsing of the malicious SPF record, causing the integer underflow and the subsequent execution of arbitrary code in the service account's context.

Mitigation

The Exim team has released a patch to mitigate this vulnerability in the latest version of the libspf2 library. Users are advised to update their installations to the latest version to protect themselves from this remote code execution vulnerability.

Conclusion

CVE-2023-42118 is a critical remote code execution vulnerability affecting Exim libspf2 installations. A thorough understanding of this flaw and its exploitation process is essential to protect your systems and maintain a strong security posture. Be sure to keep your Exim libspf2 installations up-to-date to defend against this and other potential threats.

Timeline

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