Recent findings indicate that Apache James email servers prior to versions 3.8.1 and 3.7.5 are potentially vulnerable to SMTP smuggling. The issue stems from a lenient behavior in line delimiter handling, which could result in differences in interpretation between the sender and receiver. When exploited by an attacker, this vulnerability could be utilized to forge an SMTP envelope, ultimately allowing for bypassing SPF (Sender Policy Framework) checks.

Exploit Details

SMTP smuggling in Apache James, tagged as CVE-2023-51747, takes advantage of an inconsistency in line delimiter handling to bypass SPF checks. To further understand this vulnerability, let's examine a sample email message and how an attacker might exploit the flaw:

msg = "Subject: Test email\r\n" \
      "To: user@example.com\r\n" \
      "From: attacker@example.org\r\n" \
      "\r\n" \
      "This is a test email."

When sending this message, the email server expects CRLF (carriage return and line feed) as the line delimiter. However, in older versions of Apache James, the server would be lenient and allow CR or LF alone. It might create a scenario where the sender and receiver interpret the message differently. By exploiting this discrepancy, an attacker could craft an SMTP envelope that bypasses SPF checks, such as:

exploit_msg = "Subject: Test email\r\n" \
              "To: user@example.com\n" \
              "From: attacker@example.org\r\n" \
              "\r\n" \
              "This is a test email."

In the exploit message above, the attacker has replaced CRLF with a solitary LF ("\n") in the "To" header. This difference in interpretation between the sender and receiver might allow an attacker to successfully smuggle the message and bypass SPF checks.

Patch and Mitigation

The patch implemented by the Apache James team enforces the use of CRLF as the line delimiter in the DATA transaction. This change in delimiter handling resolves the vulnerability and eliminates the possibility of SMTP smuggling. To mitigate this vulnerability, it is highly recommended that users upgrade to non-vulnerable versions of Apache James. The following are the fixed software versions:

Apache James 3.7.5

For more information about the issue, you can refer to the official CVE page and the Apache James Changelog.

Conclusion

SMTP smuggling is a critical vulnerability that could allow an attacker to bypass SPF checks and deliver potentially harmful emails. By upgrading your Apache James server to fixed versions (3.8.1 or 3.7.5), you can effectively protect your system from CVE-2023-51747 and ensure the security of your email infrastructure.

Timeline

Published on: 02/27/2024 14:15:27 UTC
Last modified on: 02/27/2024 15:15:07 UTC