CVE-2021-21274

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 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.

At a glance

Exploitability

Can be exploited over the network without logging in, but needs a user to act. Limited direct impact.

Exploit odds

2.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-02-26 18:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 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: 2.2% in the next 30 days (EPSS).
• Severity: 4.3 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-02-26 18:15 UTC

Last modified

2026-06-17 03:35 UTC

Exploit probability

2.2% · EPSS, next 30 days

Weakness

CWE-400 · Uncontrolled resource consumption

Record status

Modified

Affected

• Matrix Synapse < 1.25.0
• Fedoraproject Fedora 34

Attack profile CVSS 3.1 · 4.3

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

Required

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

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-2021-21274.md and follow it. Check whether this project is affected by CVE-2021-21274. 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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