CVE-2023-51764: Medium severity Postfix Postfix vulnerability

Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpddatarestrictions=rejectunauthpipelining and smtpddiscardehlokeywords=chunking (or certain other options that exist in recent versions). Remote attackers can use a published exploitation technique to inject e-mail messages with a spoofed MAIL FROM address, allowing bypass of an SPF protection mechanism. This occurs because Postfix supports <LF>.<CR><LF> but some other popular e-mail servers do not. To prevent attack variants (by always disallowing <LF> without <CR>), a different solution is required, such as the smtpdforbidbarenewline=yes option with a Postfix minimum version of 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9.

— Launchpad

Summary: Depending on how SMTP servers/software are configured to interpret the end-of-data sequence (e.g., <CR><LF>.<CR><LF>), an attacker can break out of the message data to "smuggle" SMTP commands to send spoofed emails that pass SPF checks.

Description: By exploiting interpretation differences of the SMTP protocol, it is possible to smuggle/send spoofed e-mails - hence SMTP smuggling - while still passing SPF alignment checks. During this research, two types of SMTP smuggling, outbound and inbound, were discovered. These allowed sending spoofed e-mails from millions of domains (e.g., admin[@]outlook.com) to millions of receiving SMTP servers (e.g., Amazon, PayPal, eBay). Identified vulnerabilities in Microsoft and GMX were quickly fixed, however, SEC Consult urges companies using the also affected Cisco Secure Email product to manually update their vulnerable default configuration.

References: https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ https://www.mail-archive.com/postfix-users@postfix.org/msg100901.html https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1059230

— Red Hat