---
id: CVE-2021-21274
aliases:
  - GHSA-2hwx-mjrm-v3g8
  - PYSEC-2021-132
title: Denial of service attack via .well-known lookups
summary: Denial of service attack via .well-known lookups
severity: medium
cvss: 4.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L'
vendor: matrix-synapse
product: matrix-synapse
ecosystem: pip
affected:
  - 'matrix-synapse >= 0.99.0, < 1.25.0'
patched:
  - matrix-synapse 1.25.0
published: '2021-03-01'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:49:14.211839612Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-2hwx-mjrm-v3g8'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-21274'
  - url: 'https://github.com/matrix-org/synapse/pull/8950'
  - url: >-
      https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
  - url: 'https://github.com/matrix-org/synapse'
  - url: 'https://github.com/matrix-org/synapse/releases/tag/v1.25.0'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-132.yaml
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
tags:
  - osv
  - pip
epss: 0.02164
epssPercentile: 0.81463
ingestedAt: '2026-09-12T03:13:01.637Z'
---

## Overview

### Impact
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.

### Patches
Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.

### Workarounds
The `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.

## Affected packages

- `matrix-synapse >= 0.99.0, < 1.25.0`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.25.0`