Add a setting to disable TLS for sending email (#10546)

This is mostly useful in case the server offers TLS, but doesn't present a valid certificate.

3 files changed, 91 insertions, 23 deletions

diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index 8d8f166e9b..42526502f0 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -80,6 +80,12 @@ class EmailConfig(Config):
         self.require_transport_security = email_config.get(
             "require_transport_security", False
         )
+        self.enable_smtp_tls = email_config.get("enable_tls", True)
+        if self.require_transport_security and not self.enable_smtp_tls:
+            raise ConfigError(
+                "email.require_transport_security requires email.enable_tls to be true"
+            )
+
         if "app_name" in email_config:
             self.email_app_name = email_config["app_name"]
         else:
@@ -368,6 +374,14 @@ class EmailConfig(Config):
           #
           #require_transport_security: true
 
+          # Uncomment the following to disable TLS for SMTP.
+          #
+          # By default, if the server supports TLS, it will be used, and the server
+          # must present a certificate that is valid for 'smtp_host'. If this option
+          # is set to false, TLS will not be used.
+          #
+          #enable_tls: false
+
           # notif_from defines the "From" address to use when sending emails.
           # It must be set if email sending is enabled.
           #
diff --git a/synapse/handlers/send_email.py b/synapse/handlers/send_email.py
index e9f6aef06f..dda9659c11 100644
--- a/synapse/handlers/send_email.py
+++ b/synapse/handlers/send_email.py
@@ -16,7 +16,12 @@ import email.utils
 import logging
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
-from typing import TYPE_CHECKING
+from io import BytesIO
+from typing import TYPE_CHECKING, Optional
+
+from twisted.internet.defer import Deferred
+from twisted.internet.interfaces import IReactorTCP
+from twisted.mail.smtp import ESMTPSenderFactory
 
 from synapse.logging.context import make_deferred_yieldable
 
@@ -26,19 +31,75 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+async def _sendmail(
+    reactor: IReactorTCP,
+    smtphost: str,
+    smtpport: int,
+    from_addr: str,
+    to_addr: str,
+    msg_bytes: bytes,
+    username: Optional[bytes] = None,
+    password: Optional[bytes] = None,
+    require_auth: bool = False,
+    require_tls: bool = False,
+    tls_hostname: Optional[str] = None,
+) -> None:
+    """A simple wrapper around ESMTPSenderFactory, to allow substitution in tests
+
+    Params:
+        reactor: reactor to use to make the outbound connection
+        smtphost: hostname to connect to
+        smtpport: port to connect to
+        from_addr: "From" address for email
+        to_addr: "To" address for email
+        msg_bytes: Message content
+        username: username to authenticate with, if auth is enabled
+        password: password to give when authenticating
+        require_auth: if auth is not offered, fail the request
+        require_tls: if TLS is not offered, fail the reqest
+        tls_hostname: TLS hostname to check for. None to disable TLS.
+    """
+    msg = BytesIO(msg_bytes)
+
+    d: "Deferred[object]" = Deferred()
+
+    factory = ESMTPSenderFactory(
+        username,
+        password,
+        from_addr,
+        to_addr,
+        msg,
+        d,
+        heloFallback=True,
+        requireAuthentication=require_auth,
+        requireTransportSecurity=require_tls,
+        hostname=tls_hostname,
+    )
+
+    # the IReactorTCP interface claims host has to be a bytes, which seems to be wrong
+    reactor.connectTCP(smtphost, smtpport, factory, timeout=30, bindAddress=None)  # type: ignore[arg-type]
+
+    await make_deferred_yieldable(d)
+
+
 class SendEmailHandler:
     def __init__(self, hs: "HomeServer"):
         self.hs = hs
 
-        self._sendmail = hs.get_sendmail()
         self._reactor = hs.get_reactor()
 
         self._from = hs.config.email.email_notif_from
         self._smtp_host = hs.config.email.email_smtp_host
         self._smtp_port = hs.config.email.email_smtp_port
-        self._smtp_user = hs.config.email.email_smtp_user
-        self._smtp_pass = hs.config.email.email_smtp_pass
+
+        user = hs.config.email.email_smtp_user
+        self._smtp_user = user.encode("utf-8") if user is not None else None
+        passwd = hs.config.email.email_smtp_pass
+        self._smtp_pass = passwd.encode("utf-8") if passwd is not None else None
         self._require_transport_security = hs.config.email.require_transport_security
+        self._enable_tls = hs.config.email.enable_smtp_tls
+
+        self._sendmail = _sendmail
 
     async def send_email(
         self,
@@ -82,17 +143,16 @@ class SendEmailHandler:
 
         logger.info("Sending email to %s" % email_address)
 
-        await make_deferred_yieldable(
-            self._sendmail(
-                self._smtp_host,
-                raw_from,
-                raw_to,
-                multipart_msg.as_string().encode("utf8"),
-                reactor=self._reactor,
-                port=self._smtp_port,
-                requireAuthentication=self._smtp_user is not None,
-                username=self._smtp_user,
-                password=self._smtp_pass,
-                requireTransportSecurity=self._require_transport_security,
-            )
+        await self._sendmail(
+            self._reactor,
+            self._smtp_host,
+            self._smtp_port,
+            raw_from,
+            raw_to,
+            multipart_msg.as_string().encode("utf8"),
+            username=self._smtp_user,
+            password=self._smtp_pass,
+            require_auth=self._smtp_user is not None,
+            require_tls=self._require_transport_security,
+            tls_hostname=self._smtp_host if self._enable_tls else None,
         )
diff --git a/synapse/server.py b/synapse/server.py
index 095dba9ad0..6c867f0f47 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -34,8 +34,6 @@ from typing import (
 )
 
 import twisted.internet.tcp
-from twisted.internet import defer
-from twisted.mail.smtp import sendmail
 from twisted.web.iweb import IPolicyForHTTPS
 from twisted.web.resource import IResource
 
@@ -443,10 +441,6 @@ class HomeServer(metaclass=abc.ABCMeta):
         return RoomShutdownHandler(self)
 
     @cache_in_self
-    def get_sendmail(self) -> Callable[..., defer.Deferred]:
-        return sendmail
-
-    @cache_in_self
     def get_state_handler(self) -> StateHandler:
         return StateHandler(self)