CVE-2021-21392

6.3 MEDIUM 0.9% 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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.

At a glance

Exploitability: Can be exploited over the network with a low-privilege account, but needs a user to act. High risk of data tampering.
Exploit odds: 0.9% 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 22:15 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: 0.9% in the next 30 days (EPSS).
Severity: 6.3 MEDIUM (CVSS).
Fix: a patch or advisory is linked.

Details

Published: 2021-04-12 22:15 UTC
Last modified: 2026-06-17 03:35 UTC
Exploit probability: 0.9% · EPSS, next 30 days
Weakness: CWE-601
Record status: Modified

Affected

Matrix Synapse < 1.28.0
Fedoraproject Fedora 34

Attack profile CVSS 3.1 · 6.3

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

Raw vector

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

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-2021-21392.md and follow it. Check whether this project is affected by CVE-2021-21392. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

· More options · Status badge

Download Square Reddit

Fix and advisories

References

NVD record · CVE.org record