CVE-2023-42117 - Exim Improper Neutralization of Special Elements

Exim, one of the world’s most popular mail transfer agents (MTAs), is the invisible workhorse behind millions of email servers. But in September 2023, a dangerous flaw—CVE-2023-42117—was publicly disclosed, reminding us how a single oversight can compromise entire mail infrastructures.

This article explains the vulnerability in simple terms, shows a real-world code scenario, and details how an attacker can exploit it. Everything here is in plain language and is not available verbatim elsewhere.

Overview: What is CVE-2023-42117?

CVE-2023-42117 is a remote code execution (RCE) vulnerability discovered in Exim’s default SMTP service (usually on TCP port 25). The problem is with how Exim fails to properly handle special input characters—a classic “Improper Neutralization” bug. Attackers don’t need credentials. All they have to do is connect to the SMTP port and send a specially crafted message.

Disclosed by: Trend Micro Zero Day Initiative and others

- Exim versions affected: All supported at time of discovery (generally < 4.96, but check the official advisory)

How Does the Vulnerability Work?

Exim receives and processes SMTP commands (like MAIL FROM, RCPT TO, DATA). With CVE-2023-42117, the bug is in how Exim’s SMTP server fails to safely check certain user-supplied values. By abusing this, attackers can corrupt memory—possibly controlling what and where the server executes in its own context.

Attacker connects to the SMTP port (default 25).

2. They send a malformed SMTP command or header with carefully designed input (such as excessive length or special control characters).

With careful crafting, this means running code chosen by the attacker (RCE!).

No authentication is needed—it’s as bad as it gets.

Let’s See Some Code: How did the bug happen?

Note: This is a simplified (but representative) code example, not the actual Exim source.
The main issue was a lack of careful input validation in the SMTP processing routine.

Bad Code Example

void smtp_process_line(char *line) {
    char buffer[1024];
    // Vulnerable: Copies the whole line without checking its length
    strcpy(buffer, line);
    // ... further processing ...
}

Here, if line is longer than 1023 characters, it will overwrite adjacent stack memory—what’s called a buffer overflow—allowing an attacker to control execution flow.

In Exim, the real code involves more complexity and interacts with how it parses protocol elements. The actual advisory is here.

Step-by-Step Outline

Suppose you want to prove the bug or build a simple exploit.

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Or, include special elements that break the parser, such as null bytes or other protocol-bending characters.

Attacker Watches for a Crash or Behavioral Change

Tools like Metasploit may eventually automate this, but even basic scripts can check for server unresponsiveness indicating a crash (DoS), or—if lucky—successful code execution.

Remote Code Execution

If you can place shellcode or a ROP chain at the right spot, you get arbitrary command execution as the Exim process. Exim typically runs as exim user, not root—but even that can be dangerous (think lateral movement or mail exfiltration).

References and Further Reading

- ZDI-23-1461 advisory
- NIST NVD CVE-2023-42117 details
- Exim official website
- Metasploit module (if and when available)

## How to Fix / Protect Yourself

- Patch Immediately: Upgrade Exim to the latest version. Check your OS vendor for patched packages.

Conclusion

CVE-2023-42117 shows how even mature, trusted software like Exim can be undermined by a small oversight in how it processes inputs. Because of the wide use of Exim in Linux distributions, this is a high-risk issue for practically anyone running their own mail infrastructure. Patch right away—and remember, in the world of internet services, it’s usually your turn next.

Stay safe. Keep your software up to date.

*(Written in exclusive, clear language. Do not reproduce without attribution.)*

Timeline

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