CVE-2023-38686 is a critical-severity (CVSS 9.3) CWE-295 vulnerability in matrix-sydent. A fix is available for matrix-sydent — see the affected versions and patch details below.
Sydent does not verify email server certificates
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-38686.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2023-38686 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
matrix-sydentReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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
- 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.
- 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.
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. If you are using self-signed certificates, make sure to copy your Certification Authority certificate, or your self signed certificate if using only one, to the trust store of your operating system.
Workarounds
One can ensure Sydent's emails fail to send by setting the configured SMTP server to a loopback or non-routable address under your control which does not have a listening SMTP server. For example:
email:
smtphost: "localhost" # Assuming there is no SMTP server listening on localhost
References
- https://github.com/matrix-org/sydent/pull/574 implements the fix.
- https://github.com/matrix-org/sydent/releases/tag/v2.5.6 is the release including this fix.
- https://docs.python.org/3/library/ssl.html?highlight=ssl#security-considerations details the best-practice advice on how to use the standard library
smtpmodule safely. - https://peps.python.org/pep-0476/ (accepted) proposed enabling TLS certificate verification by default in standard library HTTP clients.
- https://github.com/python/cpython/issues/91826 discusses enabling TLS certificate verification by default in the Python standard library, for SMTP and other protocols.
For more information
If you have any questions or comments about this advisory, e-mail us at [email protected].
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | matrix-sydent | all versions | 2.5.6pip install --upgrade 'matrix-sydent==2.5.6' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-sydent, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-sydent to 2.5.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-38686 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2023-38686 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-38686. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2023-38686 in your dependencies?
O3 Security finds CVE-2023-38686 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.