---
id: CVE-2021-29430
aliases:
  - GHSA-wmg4-8cp2-hpg9
  - PYSEC-2021-21
title: Sydent vulnerable to denial of service attack via memory exhaustion
summary: Sydent vulnerable to denial of service attack via memory exhaustion
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
vendor: matrix-sydent
product: matrix-sydent
ecosystem: pip
affected:
  - matrix-sydent < 2.3.0
patched:
  - matrix-sydent 2.3.0
published: '2021-04-19'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:49:26.904044812Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-wmg4-8cp2-hpg9'
references:
  - url: >-
      https://github.com/matrix-org/sydent/security/advisories/GHSA-wmg4-8cp2-hpg9
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-29430'
  - url: >-
      https://github.com/matrix-org/sydent/commit/0523511d2fb40f2738f8a8549868f44b96e5dab7
  - url: >-
      https://github.com/matrix-org/sydent/commit/89071a1a754c69a50deac89e6bb74002d4cda19d
  - url: >-
      https://github.com/matrix-org/sydent/commit/f56eee315b6c44fdd9f6aa785cc2ec744a594428
  - url: 'https://github.com/matrix-org/sydent'
  - url: 'https://github.com/matrix-org/sydent/releases/tag/v2.3.0'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-sydent/PYSEC-2021-21.yaml
  - url: 'https://pypi.org/project/matrix-sydent'
tags:
  - osv
  - pip
epss: 0.01833
epssPercentile: 0.78794
ingestedAt: '2026-09-12T03:13:01.736Z'
---

## Overview

### Impact

Sydent does not limit the size of requests it receives from HTTP clients. A malicious user could send an HTTP request with a very large body, leading to disk space exhaustion and denial of service.

Sydent also does not limit response size for requests it makes to remote Matrix homeservers. A malicious homeserver could return a very large response, again leading to memory exhaustion and denial of service.

This affects any server which accepts registration requests from untrusted clients.

### Patches

Patched by 89071a1, 0523511, f56eee3.

### Workarounds

Request sizes can be limited in an HTTP reverse-proxy.

There are no known workarounds for the problem with overlarge responses.

### For more information

If you have any questions or comments about this advisory, email us at security@matrix.org.

## Affected packages

- `matrix-sydent < 2.3.0`

## Remediation

Upgrade to a patched release:

- `matrix-sydent 2.3.0`