CVE-2023-38686

Sydent is an identity server for the Matrix communications protocol. Prior
to version 2.5.6, if configured to send emails using TLS, Sydent does not
verify SMTP servers’ certificates. This makes Sydent’s emails vulnerable to
interception via a man-in-the-middle (MITM) attack. Attackers with
privileged access to the network can intercept room invitations and address
confirmation emails. This is patched in Sydent 2.5.6. When patching, make
sure that Sydent trusts the certificate of the server it is connecting to.
This should happen automatically when using properly issued certificates.
Those who use self-signed certificates should make sure to copy their
Certification Authority certificate, or their self signed certificate if
using only one, to the trust store of your operating system. As a
workaround, one can ensure Sydent’s emails fail to send by setting the
configured SMTP server to a loopback or non-routable address under one’s
control which does not have a listening SMTP server.

Bugs

• <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1043162&gt;