net/smtp: allow PLAIN auth when advertised

The smtp package originally allowed PLAIN whenever, but then
the TLS check was added for paranoia, but it's too paranoid:
it prevents using PLAIN auth even from localhost to localhost
when the server advertises PLAIN support.

This CL also permits the client to send PLAIN if the server
advertises it.

Fixes #5184

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8279043

src/pkg/net/smtp/auth.go

@@ -54,7 +54,16 @@ func PlainAuth(identity, username, password, host string) Auth {
 
 func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 	if !server.TLS {
-		return "", nil, errors.New("unencrypted connection")
+		advertised := false
+		for _, mechanism := range server.Auth {
+			if mechanism == "PLAIN" {
+				advertised = true
+				break
+			}
+		}
+		if !advertised {
+			return "", nil, errors.New("unencrypted connection")
+		}
 	}
 	if server.Name != a.host {
 		return "", nil, errors.New("wrong host name")

src/pkg/net/smtp/smtp_test.go

@@ -57,6 +57,41 @@ testLoop:
 	}
 }
 
+func TestAuthPlain(t *testing.T) {
+	auth := PlainAuth("foo", "bar", "baz", "servername")
+
+	tests := []struct {
+		server *ServerInfo
+		err    string
+	}{
+		{
+			server: &ServerInfo{Name: "servername", TLS: true},
+		},
+		{
+			// Okay; explicitly advertised by server.
+			server: &ServerInfo{Name: "servername", Auth: []string{"PLAIN"}},
+		},
+		{
+			server: &ServerInfo{Name: "servername", Auth: []string{"CRAM-MD5"}},
+			err:    "unencrypted connection",
+		},
+		{
+			server: &ServerInfo{Name: "attacker", TLS: true},
+			err:    "wrong host name",
+		},
+	}
+	for i, tt := range tests {
+		_, _, err := auth.Start(tt.server)
+		got := ""
+		if err != nil {
+			got = err.Error()
+		}
+		if got != tt.err {
+			t.Errorf("%d. got error = %q; want %q", i, got, tt.err)
+		}
+	}
+}
+
 type faker struct {
 	io.ReadWriter
 }