How severe is CVE-2019-18835?

CVE-2019-18835 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2019-18835 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (54th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2019-18835?

CVE-2019-18835 affects Matrix Synapse. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2019-18835?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2019-18835 published?

CVE-2019-18835 was published on 2019-11-08 and last updated on 2026-06-17.

CVE-2019-18835

CVE-2019-18835 is a critical-severity vulnerability in Matrix Synapse with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-345.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 1% (54th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-345
Affected product: Matrix Synapse
Published: 2019-11-08
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is 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.
How severe is CVE-2019-18835?: CVE-2019-18835 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2019-18835 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (54th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2019-18835?: CVE-2019-18835 affects Matrix Synapse. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2019-18835?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2019-18835 published?: CVE-2019-18835 was published on 2019-11-08 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*

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