CVE-2025-30232 - Exploiting a Use-After-Free in Exim 4.96

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Introduction

A critical vulnerability, designated as CVE-2025-30232, has been identified in Exim mail server versions 4.96 through 4.98.1. This flaw is a use-after-free, which means parts of the program reuse memory after it's already been freed. Attackers with basic command-line access to a server can use this bug to run code as root, giving them complete control. In this exclusive post, we'll break down the vulnerability, show a code snippet close to the bug, walk through an exploitation scenario, and give you all the essential references.

What is Exim?

Exim is one of the most popular Mail Transfer Agents (MTAs) used on Unix-like systems. Since it often runs with root privileges, security vulnerabilities can have severe consequences.

Understanding the Issue: Use-After-Free in Exim

A "use-after-free" occurs when code accesses memory after it’s been released. This can lead to crashes, data leaks, or remote code execution. In Exim 4.96–4.98.1, improper management of internal structures when handling certain command-line options can lead to a use-after-free. Attackers with only shell access can send carefully crafted input and get the Exim process to execute malicious code as root.

Vulnerable Code Snippet

The problem lies in Exim's incorrect handling of command-line argument parsing and the way it frees (then reuses) memory for user-supplied data. Here’s a simplified example to illustrate:

// exim_main.c (simplified)
char *input = malloc(BUF_SIZE);
...
if (user_supplied_option) {
    free(input); // Memory is freed 
}

strcpy(input, argv[1]); // Use-after-free: input might be a dangling pointer

If an attacker can force the program to free input based on their input, but Exim later tries to use input again, they can influence what data is at that address. Filling that memory with their payload allows code execution.

Here’s a pseudo-exploit, showing how an attacker might take advantage of the bug

# Prepare exploit file with a fake structure that includes shellcode
python -c 'print("A" * 512 + "\x90"*100 + SHELLCODE)' > /tmp/evil_file

# Run exim triggering the vulnerable logic, passing arguments to free and reuse
exim -option1 --trigger-uaf @@/tmp/evil_file

*Note: Exact exploitation requires target-specific heap grooming, but this is the general idea.*

Impact

Anyone with command-line access—such as a low-privileged user on a shared server—can exploit this to become root. This means shared web hosts, student labs, and multi-user servers are at high risk.

Patch Immediately:

Get the latest Exim release.

References

- CVE-2025-30232 on NIST NVD
- Exim official security page
- Common exploitation techniques for use-after-free bugs

Conclusion

CVE-2025-30232 is a dangerous privilege escalation bug, and exploit code will likely be in the wild soon. If your systems run Exim 4.96–4.98.1, patch them without delay. For defenders, knowing the logic behind such bugs helps spot suspicious activity. For researchers, this is a classic example of how small coding errors can have big impacts.

Timeline

Published on: 03/28/2025 00:15:14 UTC
Last modified on: 03/28/2025 18:11:40 UTC