CVE-2021-21394

5.3 MEDIUM 2% exploit odds Fix listed

What it is

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

At a glance

Exploitability: Can be exploited over the network with a low-privilege account. High risk of service disruption.
Exploit odds: 1.5% chance of exploitation in the next 30 days (EPSS).
Fix: A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

2021-04-12 21:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2026-06-17 03:35 UTC Last updatedMost recent change to the record

Why it ranks here

Exploited in the wild: no, CISA does not list it.
Exploit odds: 1.5% in the next 30 days (EPSS).
Severity: 5.3 MEDIUM (CVSS).
Fix: a patch or advisory is linked.

Details

Published: 2021-04-12 21:15 UTC
Last modified: 2026-06-17 03:35 UTC
Exploit probability: 1.5% · EPSS, next 30 days
Weakness: CWE-20 · Improper input validation
Record status: Modified

Affected

Matrix Synapse < 1.28.0
Fedoraproject Fedora 34

Attack profile CVSS 3.1 · 5.3

Attack vector: Network
Complexity: High
Privileges needed: Low
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Raw vector

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

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-21394.md and follow it. Check whether this project is affected by CVE-2021-21394. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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Fix and advisories

References

NVD record · CVE.org record