What is CVE-2019-18835?

CVE-2019-18835 is a critical-severity vulnerability in Matrix Synapse, classified under Insufficient Verification of Data Authenticity. CVSS score: 9.8/10. Published 2019-11-08.

How severe is CVE-2019-18835?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Vulnerability in Matrix Synapse

CVE-2019-18835

Matrix Synapse before 1.5.0 mishandles signature checking on some federation APIs. Events sent over /send_join, /send_leave, and /invite may not be correctly signed, or may not come from the expected servers.

Published 2019-11-08 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.009 (57.0th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 57th percentile — 57.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: 0.000

Affected products

Matrix Synapse

Weakness classification (CWE)

CWE-345 — Insufficient Verification of Data Authenticity

References

github.com/matrix-org/synapse/pull/6262 (Patch, Third Party Advisory)
github.com/matrix-org/synapse/releases/tag/v1.5.0 (Third Party Advisory, Release Notes)

Frequently asked questions

What is CVE-2019-18835?: CVE-2019-18835 is a critical-severity vulnerability in Matrix Synapse, classified under Insufficient Verification of Data Authenticity. CVSS score: 9.8/10. Published 2019-11-08.
How severe is CVE-2019-18835?: Critical severity. CVSS v3 base score is 9.8 out of 10.