Remote denial of service in Matrix Synapse

1) Input validation error

EUVDB-ID: #VU106284

Risk: High

CVSSv4.0: 4.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/U:Amber]

CVE-ID: CVE-2025-30355

CWE-ID: CWE-20 - Improper input validation

Exploit availability: No

Description

The vulnerability allows a remote user to perform a denial of service (DoS) attack.

The vulnerability exists due to insufficient validation of user-supplied input. A remote user can pass specially crafted events to the application and prevent it from federating with other servers.

Note, the vulnerability is being exploited in the wild.

Mitigation

Install updates from vendor's website.

Vulnerable software versions

Synapse: 1.0.0 - 1.127.0

CPE2.3

• cpe:2.3:a:matrix_org:synapse:1.0.0:*:*:*:*:*:*:*
• cpe:2.3:a:matrix_org:synapse:1.1.0:*:*:*:*:*:*:*
• cpe:2.3:a:matrix_org:synapse:1.1.1:*:*:*:*:*:*:*

External links

https://github.com/element-hq/synapse/commit/2277df2a1eb685f85040ef98fa21d41aa4cdd389
https://github.com/element-hq/synapse/releases/tag/v1.127.1
https://github.com/element-hq/synapse/security/advisories/GHSA-v56r-hwv5-mxg6

Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote authenticated user via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.