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EUVD-2026-193911

fkie_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.

GHSA-p6hw-wm59-3g5g — Sydent does not verify email server certificates

Affected: PyPI: matrix-sydent

## 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](https://en.wikipedia.org/wiki/Man-in-the-middle_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](https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=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&version=3.1)

_Reported by Martin Schobert, [Pentagrid AG](https://pentagrid.ch/)._

### Details

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

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

in its config file. Alternatively it can be configured to use [Opportunistic TLS](https://en.wikipedia.org/wiki/Opportunistic_TLS) by setting

```yaml 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.

Therefor…

gsd-2023-38686

PYSEC-2023-139

Affected: PyPI: matrix-sydent

UBUNTU-CVE-2023-38686

Affected: Ubuntu:22.04:LTS: matrix-sydent, Ubuntu:24.04:LTS: matrix-sydent