CVE-2017-15042 in Google
Summary
by MITRE
An unintended cleartext issue exists in Go before 1.8.4 and 1.9.x before 1.9.1. RFC 4954 requires that, during SMTP, the PLAIN auth scheme must only be used on network connections secured with TLS. The original implementation of smtp.PlainAuth in Go 1.0 enforced this requirement, and it was documented to do so. In 2013, upstream issue #5184, this was changed so that the server may decide whether PLAIN is acceptable. The result is that if you set up a man-in-the-middle SMTP server that doesn't advertise STARTTLS and does advertise that PLAIN auth is OK, the smtp.PlainAuth implementation sends the username and password.
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Analysis
by VulDB Data Team • 12/30/2022
The vulnerability CVE-2017-15042 represents a critical security flaw in the go programming language's standard library implementation of SMTP authentication. This issue affects versions prior to 1.8.4 and 1.9.1, where the smtp.PlainAuth function fails to properly enforce the security requirements defined in RFC 4954. The fundamental problem lies in the modification of the authentication mechanism's behavior between Go 1.0 and 2013, when the implementation was changed to allow PLAIN authentication even on non-TLS connections. This change was introduced to address upstream issue #5184 but inadvertently created a significant security risk by removing the mandatory TLS requirement for PLAIN authentication.
The technical flaw stems from the violation of RFC 4954 standards which explicitly mandate that the PLAIN authentication mechanism must only be used over secure network connections protected by TLS encryption. The original Go implementation correctly enforced this requirement, but the 2013 modification removed this enforcement, allowing authentication credentials to be transmitted in cleartext over unencrypted connections. When an attacker sets up a man-in-the-middle SMTP server that advertises PLAIN authentication without offering STARTTLS capabilities, the Go smtp.PlainAuth function will proceed to send username and password credentials in cleartext, completely undermining the security model designed to protect authentication information.
The operational impact of this vulnerability is severe and far-reaching, particularly for applications that rely on Go's standard library for email authentication. Any system using smtp.PlainAuth without proper TLS verification becomes vulnerable to credential interception attacks, where attackers can capture authentication credentials during email transmission. This vulnerability affects a wide range of applications including web applications, email clients, and server applications that use Go's standard library for SMTP communication. The risk is particularly high in environments where network security cannot be guaranteed, such as public Wi-Fi networks or untrusted network segments where attackers can easily position themselves to intercept communication traffic.
Organizations should immediately upgrade to Go versions 1.8.4 or later, or 1.9.1 and subsequent releases to remediate this vulnerability. Additionally, security practitioners should implement network monitoring to detect and prevent man-in-the-middle attacks targeting SMTP services, and ensure that all SMTP connections properly negotiate TLS encryption before authentication occurs. The vulnerability demonstrates the importance of maintaining strict adherence to security standards and the potential risks introduced by seemingly benign API changes that inadvertently weaken security controls. This issue aligns with CWE-310 and ATT&CK techniques related to credential access and network protocol manipulation, highlighting the critical need for proper authentication security enforcement in network programming libraries.