Security fixes in SMTP_SSL and SMTP_TLS strategies

  - The SMTP_SSL and SMTP_TLS transport strategies now validate certificates
    by setting JavaMail's `mail.<protocol>.ssl.checkserveridentity` property
    to true.

    Previously, no identity validation was performed, leaving SMTPS and
    STARTTLS connections vulnerable to man-in-the-middle attacks. Without
    identity validation, JavaMail accepts _any_ certificate issued by a
    JVM-trusted CA, regardless of the identity encoded in the certificate.

  - The SMTP_TLS transport strategy now requires STARTTLS support by setting
    JavaMail's `mail.smtp.starttls.required` property to true.

    Previously, STARTTLS support was not required, enabling a man-in-the-middle
    attack whereby an attacker could strip the STARTTLS request from an SMTP
    connection, causing JavaMail to fall back to plaintext SMTP for
    authentication and email transport.

src/main/java/org/simplejavamail/mailer/config/TransportStrategy.java

@@ -64,7 +64,8 @@ public String propertyNameAuthenticate() {
 	},
 	/**
 	 * SMTPS / SSL transport strategy, that returns the ".smtps." variation of the SMTP_PLAIN version. Additionally the transport protocol is
-	 * explicitly set to smtps. Finally, he property "mail.smtps.quitwait" is set to false, to get rid of a strange SSL exception:<br>
+	 * explicitly set to smtps, and strict certificate validation is enabled by setting "mail.smtps.ssl.checkserveridentity" to true. Finally,
+	 * the property "mail.smtps.quitwait" is set to false, to get rid of a strange SSL exception:<br>
 	 * <p>
 	 * <pre>
 	 * javax.mail.MessagingException: Exception reading response;
@@ -79,14 +80,16 @@ public String propertyNameAuthenticate() {
 	 */
 	SMTP_SSL {
 		/**
-		 * Here protocol "mail.transport.protocol" is set to "smtps" and the quitwait property "mail.smtps.quitwait" is set to "false".
+		 * Here protocol "mail.transport.protocol" is set to "smtps", certificate validation "mail.smtps.ssl.checkserveridentity" is set to "true", and
+		 * the quitwait property "mail.smtps.quitwait" is set to "false".
 		 *
 		 * @see TransportStrategy#SMTP_SSL
 		 */
 		@Override
 		public Properties generateProperties() {
 			final Properties properties = super.generateProperties();
 			properties.put("mail.transport.protocol", "smtps");
+			properties.put("mail.smtps.ssl.checkserveridentity", "true");
 			properties.put("mail.smtps.quitwait", "false");
 			return properties;
 		}
@@ -127,11 +130,13 @@ public String propertyNameAuthenticate() {
 	 * <strong>NOTE: this code is in untested beta state</strong>
 	 * <p>
 	 * Uses standard ".smtp." property names (like {@link TransportStrategy#SMTP_PLAIN}). Additionally the transport protocol is explicitly set to
-	 * smtp. Finally, the property "mail.smtp.starttls.enable" is being set to true.
+	 * smtp. STARTTLS is enabled by setting "mail.smtp.starttls.enable" and "mail.smtp.starttls.required" to true, and strict certificate validation
+	 * is enabled by setting "mail.smtp.ssl.checkserveridentity" to true.
 	 */
 	SMTP_TLS {
 		/**
-		 * Here protocol "mail.transport.protocol" is set to "smtp" and the tls property "mail.smtp.starttls.enable" is set to "true".
+		 * Here protocol "mail.transport.protocol" is set to "smtp", STARTTLS properties "mail.smtp.starttls.enable" and "mail.smtp.starttls.required"
+		 * are set to "true", and certificate validation "mail.smtp.ssl.checkserveridentity" is set to "true".
 		 *
 		 * @see TransportStrategy#SMTP_TLS
 		 */
@@ -140,6 +145,8 @@ public Properties generateProperties() {
 			final Properties props = super.generateProperties();
 			props.put("mail.transport.protocol", "smtp");
 			props.put("mail.smtp.starttls.enable", "true");
+			props.put("mail.smtp.starttls.required", "true");
+			props.put("mail.smtp.ssl.checkserveridentity", "true");
 			return props;
 		}
 

src/test/java/org/simplejavamail/mailer/MailerTest.java

@@ -90,6 +90,8 @@ public void createMailSession_AnonymousProxyConstructor_WithoutConfig()
 		assertThat(session.getProperty("mail.smtp.port")).isEqualTo("25");
 		assertThat(session.getProperty("mail.transport.protocol")).isEqualTo("smtp");
 		assertThat(session.getProperty("mail.smtp.starttls.enable")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.starttls.required")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.ssl.checkserveridentity")).isEqualTo("true");
 		assertThat(session.getProperty("mail.smtp.username")).isEqualTo("username smtp");
 		assertThat(session.getProperty("mail.smtp.auth")).isEqualTo("true");
 		assertThat(session.getProperty("mail.smtp.socks.host")).isEqualTo("proxy host");
@@ -112,6 +114,8 @@ public void createMailSession_MaximumConstructor_WithoutConfig()
 		assertThat(session.getProperty("mail.smtp.port")).isEqualTo("25");
 		assertThat(session.getProperty("mail.transport.protocol")).isEqualTo("smtp");
 		assertThat(session.getProperty("mail.smtp.starttls.enable")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.starttls.required")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.ssl.checkserveridentity")).isEqualTo("true");
 		assertThat(session.getProperty("mail.smtp.username")).isEqualTo("username smtp");
 		assertThat(session.getProperty("mail.smtp.auth")).isEqualTo("true");
 		// the following two are because authentication is needed, otherwise proxy would be straightworward
@@ -131,6 +135,8 @@ public void createMailSession_MinimalConstructor_WithConfig() {
 		assertThat(session.getProperty("mail.smtp.port")).isEqualTo("25");
 		assertThat(session.getProperty("mail.transport.protocol")).isEqualTo("smtp");
 		assertThat(session.getProperty("mail.smtp.starttls.enable")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.starttls.required")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.ssl.checkserveridentity")).isEqualTo("true");
 		assertThat(session.getProperty("mail.smtp.username")).isEqualTo("username smtp");
 		assertThat(session.getProperty("mail.smtp.auth")).isEqualTo("true");
 		// the following two are because authentication is needed, otherwise proxy would be straightworward
@@ -150,6 +156,8 @@ public void createMailSession_MaximumConstructor_WithConfig()
 		assertThat(session.getProperty("mail.smtp.port")).isEqualTo("25");
 		assertThat(session.getProperty("mail.transport.protocol")).isEqualTo("smtp");
 		assertThat(session.getProperty("mail.smtp.starttls.enable")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.starttls.required")).isEqualTo("true");
+		assertThat(session.getProperty("mail.smtp.ssl.checkserveridentity")).isEqualTo("true");
 		assertThat(session.getProperty("mail.smtp.username")).isEqualTo("overridden username smtp");
 		assertThat(session.getProperty("mail.smtp.auth")).isEqualTo("true");
 		// the following two are because authentication is needed, otherwise proxy would be straightworward