How severe is CVE-2023-38686?

CVE-2023-38686 has a CVSS score of 9.3/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2023-38686?

CVE-2023-38686 affects the following packages: matrix-sydent (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2023-38686?

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.

How do I detect CVE-2023-38686 in my PyPI dependencies?

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.

How do I mitigate CVE-2023-38686 if there is no patch (or I can't update yet)?

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.

Is CVE-2023-38686 actively exploited in the wild?

No public exploit code has been indexed for CVE-2023-38686 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2023-38686?

CVE-2023-38686 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 18th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2023-38686?

CVE-2023-38686 is classified as CWE-295 (CWE-295). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2023-38686 published, and has it been updated?

CVE-2023-38686 was published on August 4, 2023 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

CRITICAL severity

CVE-2023-38686 — matrix-sydent

CRITICALFix: matrix-org/sydent@1cd7483

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

Also known asGHSA-p6hw-wm59-3g5gPYSEC-2023-139

Published

Aug 4, 2023

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

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

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs18th percentile — riskier than 18% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍matrix-sydent

Real-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 smtp module 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

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-sydent`	all versions	2.5.6`pip install --upgrade 'matrix-sydent==2.5.6'`

Affected Products

1 product · 1 configurations

Application

sydentmatrix

< 2.5.6

range

Detection & mitigation playbook

Open-source dependency

Detect
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.

Frequently Asked Questions

## 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/