A year-outdated evidence-of-principle attack that will allow an attacker to bypass TLS email protections to snoop on messages has been patched.
Scientists warn hackers can snoop on email messages by exploiting a bug in the underlying technology made use of by the bulk of email servers that run the Internet Message Obtain Protocol, frequently referred to as IMAP. The bug, first reported in August 2020 and patched Monday, is tied to the email server software package Dovecot, utilised by more than a few-quarters of IMAP servers, in accordance to Open up Email Survey.
The vulnerability opens the doorway to what is referred to as a meddle-in-the-middle (MITM) attack, according to a report by scientists Fabian Ising and Damian Poddebniak, with Münster University of Utilized Sciences, centered in Germany.
“The vulnerability permits a MITM attacker concerning a mail shopper and Dovecot to inject unencrypted commands into the encrypted TLS context, redirecting consumer credentials and mails to the attacker,” in accordance to research linked to from a bug bounty webpage and dated August 2020.
A patch for the vulnerability, rated by the seller as -severity and by the third-party security company Tenable as critical, is obtainable for download in the type of Dovecot version v18.104.22.168.
Bypassing TLS and Certificates
The flaw facilities all over the implementation of the email instruction named Start-TLS, a command issued in between an email plan and server that is made to safe the shipping and delivery of email messages, in accordance to a complex description by Anubisnetworks.
“We discovered that Dovecot is influenced by a command injection issue in Start off-TLS. This bug will allow [an attacker] to bypass security functions of SMTP this kind of as the blocking of plaintext logins. Additionally, it allows [an attacker] to mount a session fixation attack, which maybe effects in thieving of qualifications this kind of as the SMTP username and password,” researchers wrote.
A session fixation attack permits an adversary to hijack a consumer-server connection right after the user logs in, according to an OWASP description.
“In get to carry out the attack, an attacker 1st produces a legit account on a Dovecot server. They now wait for and [intercept] an encrypted link on port 465 from a victim’s email consumer,” researchers wrote. “As shortly as the shopper connects, the attacker initiates a different Start out-TLS link to Dovecot and injects their possess destructive prefix, e.g. a login command.”
Scientists say, thanks to the implementation flaw with Commence-TLS in Dovecot, the attacker can login to the session and forward the whole TSL site visitors from the qualified victim’s SMTP server as portion of its possess session.
“The attacker obtains the whole qualifications from its possess inbox. At no place was TLS damaged or certificates compromised,” the researchers wrote. The pair also outlined the bug in a evidence-of-thought attack.
A repair for the vulnerability, tracked as CVE-2021-33515, is available for Dovecot managing on Ubuntu, the Linux distribution based mostly on Debian. Dovecot variation v22.214.171.124 and afterwards mitigates the issue.
Workaround fixes have been readily available for the flaw and are outlined by Ising and Poddebniak. A single of them contains disabling Commence-TLS and configuring Dovecot to only settle for “pure TLS connections” on port 993/465/995.
“Note that it is not ample to reconfigure a mail customer to not use Get started-TLS. The attack have to be mitigated on the server, as any TLS link is similarly affected,” the scientists wrote.
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