CVE-2020-13163

Root

Cause

The vulnerability in em-imap 0.5 lies in its failure to validate the hostname in the TLS server certificate when establishing IMAP connections over SSL. The library uses EventMachine in an insecure manner, omitting hostname verification, which allows an attacker to present a valid certificate for a different domain and be trusted by the client [1][3].

Exploitation

An attacker can exploit this by setting up a malicious server with a valid TLS certificate on an arbitrary domain, then intercepting traffic (e.g., through DNS spoofing or ARP poisoning) to redirect the IMAP client to the attacker's server. Since the client does not verify that the certificate's hostname matches the intended server, the connection proceeds, allowing the attacker to perform a man-in-the-middle attack [3][4].

Impact

Successful exploitation gives the attacker the ability to intercept, read, and modify all IMAP traffic, including login credentials and email content. This compromises the confidentiality and integrity of email communications [4].

Mitigation

The em-imap library is unmaintained and no official fix has been released. Users are advised to stop using the library or implement hostname validation themselves. The GitHub repository explicitly warns that the gem is insecure and should not be used [1].