CVE-2017-15042

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.

Notes