Researchers warned that hackers may snoop on email communications by attacking a flaw in the underlying technology used by most of the email servers that run the Internet Message Access Protocol or known as IMAP. 

The flaw was initially reported in August 2020 and was fixed on 21st June 2021. According to the Open Email Survey, it is linked to the email server software Dovecot, which is used by nearly three-quarters of IMAP servers. 

According to a paper by researchers Fabian Ising and Damian Poddebniak of Münster University of Applied Sciences in Germany, the vulnerability allows for a meddle-in-the-middle (MITM) attack. 

In accordance with research linked to a bug bounty page, dated August 2020, “the vulnerability allows a MITM attacker between a mail client and Dovecot to inject unencrypted commands into the encrypted TLS context, redirecting user credentials and mails to the attacker.” 

Dovecot version v2.3.14.1, a patch for the vulnerability is rated -severity by the vendor and critical by the third-party security firm Tenable, is available for download. According to a technical analysis provided by Anubisnetworks, the flaw revolves around the execution of the START-TLS email instruction, which is a command issued between an email program and a server that is used to protect the delivery of email messages. 

“We found that Dovecot is affected by a command injection issue in START-TLS. This bug allows [an attacker] to bypass security features of SMTP such as the blocking of plaintext logins. Furthermore, it allows [an attacker] to mount a session fixation attack, which possibly results in stealing of credentials such as the SMTP username and password,” researchers stated. 

According to an OWASP description, a session fixation attack permits an adversary to take over a client-server connection once the user logs in. As per researchers, due to a START-TLS implementation issue in Dovecot, the intruder can log in to the session and transfer the entire TSL traffic from the targeted victim's SMTP server as part of its own session. 

“The attacker obtains the full credentials from its own inbox. At no point was TLS broken or certificates compromised,” the researchers wrote. 

For Dovecot operating on Ubuntu, a Linux version based on Debian, a fix for the issue, dubbed CVE-2021-33515, is now available. Ising and Poddebniak have provided workaround fixes for the vulnerability. Disabling START-TLS and configuring Dovecot to accept only “pure TLS connections” on port 993/465/995 is one solution. 

The researchers stated, “Note that it is not sufficient to reconfigure a mail client to not use START-TLS. The attack must be mitigated on the server, as any TLS connection is equally affected.”