NVD / CVE Program· CNA GitHub_M

CVE-2023-38686: Sydent does not verify email server certificates

Noroxi assessment

Priority: Monitor (action score 21/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.26% (FIRST EPSS).
Someone on the same network segment can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
Fixed version: matrix-sydent (PyPI) 2.5.6.
Severity is disputed: the assigning authority (CNA) scores it 9.3, NVD 5.3.

Low priority for now. Upgrade to matrix-sydent (PyPI) 2.5.6 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.3 · v3.1—No exploit Fix available

Published: Aug 4, 2023
Updated: Jun 17, 2026
EPSS: 0.2% · 18th percentile
CWE: CWE-295

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 21 / 40 · 5.3 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.2%

CISA SSVC decision

Exploitation: proof of concept
Automatable: no
Technical impact: total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: 5.3
CNA · GitHub_M: 9.3; 4.0 point gap

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

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Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
matrix	sydent	cpe:2.3:a:matrix:sydent

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This vulnerability is about certificate validation or weak cryptography. The free SSL checker tests your certificate, enabled TLS versions and weak ciphers.

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Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

matrix sydentbefore 2.5.6

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

matrix-org sydent
- < 2.5.6affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
PyPI	matrix-sydent	before 2.5.6	2.5.6

Same product

matrix: all records

Other highest-scoring records for the same primary product.

CVE-2021-29430Denial of service attack via memory exhaustion
31Monitor
CVE-2019-11842An issue was discovered in Matrix Sydent before 1.0.3 and Synapse before 0.99.3.1.
31Monitor
CVE-2021-29431SSRF in Sydent due to missing validation of hostnames
26Monitor

Published before and after this one in the same product

← EarlierCVE-2019-11340Apr 19, 2019 · util/emailutils.py in Matrix Sydent before 1.0.2 mishandles registration restrictions that are based on e-mail domain, if the allowed_local_
24Monitor
← EarlierCVE-2021-29433Apr 15, 2021 · Denial of service (via resource exhaustion) due to improper input validation
17Monitor
← EarlierCVE-2021-29432Apr 15, 2021 · Malicious users could control the content of invitation emails
22Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
PyPI:matrix-sydent	2.5.6	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Someone on the same network segment can trigger it.
No account or password is required.
No user action is required.
Special conditions such as timing or configuration are required.

If successful

Confidentiality: high · data can be read
Integrity: none
Availability: none

Attack vector: Adjacent
Attack complexity: High
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: High
Integrity impact: None
Availability impact: None

Weakness class (CWE)

CWE-295 · Improper Certificate Validation

CVSS vector

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

nvd-primary

Root cause

A certificate validation error is silently swallowed instead of terminating the connection. Often, code added for a test environment makes its way into production.

The bug in code

The whole CWE-295 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

kotlin

1override fun checkServerTrusted(chain: Array<X509Certificate>, type: String) {
2  // Left empty for the test environment
3}

Fixed

kotlin

1val client = OkHttpClient.Builder()
2  .certificatePinner(
3    CertificatePinner.Builder()
4      .add("api.sirius.example", "sha256/…")
5      .build()
6  ).build()

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

ATT&CK techniques

—

Change log

Fix✗ → ✓Sep 28, 2026, 10:47 PM UTC

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References

docs.python.org/3/library/ssl.html?highlight=ssl#security-considerations

Show all references (6 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

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