Sydent does not verify email server certificates

Impact

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.

CVSS 3.1 overall score: 3.3 - AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N/CR:L/IR:L/AR:X/MAV:A/MAC:H/MPR:N/MUI:N/MS:C/MC:L/MI:L/MA:N

Reported by Martin Schobert, Pentagrid AG.

Details

Sydent can be configured to send emails over a TLS-encrypted socket by setting

email:
    tlsmode: "TLS"  # or the legacy value "SSL"

in its config file. Alternatively it can be configured to use Opportunistic TLS by setting

email:
    tlsmode: "STARTTLS"

In both situations, Sydent will encrypt its communication with the SMTP server when sending emails. In affected versions, Sydent will not verify the destination server's certificate.

Vulnerability

Sydent sends email for two purposes:

• to inform a third party that they have been invited to a Matrix room by their email address; and
• to verify that a given Matrix user controls an email address.

Therefore, attackers capable of running a MITM attack can

1. Intercept room invitations sent to an email address. The invitation includes
   • the room ID and its avatar, and
   • the inviter's username, displayname and their avatar, and
   • credentials for a guest Matrix account on the inviter's homeserver.
2. Intercept address ownership confirmation emails. This would allow the attacker to falsely claim ownership of the indented recipient's Matrix account, if that account was permitted to log in using an email address and no other authentication factors.

Patches

This is patched in Sydent 2.5.6, see PR https://github.com/matrix-org/sydent/pull/574.