---
id: CVE-2020-26890
aliases:
  - GHSA-4mp3-385r-v63f
  - PYSEC-2020-237
title: Denial of service attack due to invalid JSON
summary: Denial of service attack due to invalid JSON
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-synapse
product: matrix-synapse
ecosystem: pip
affected:
  - matrix-synapse < 1.20.0
patched:
  - matrix-synapse 1.20.0
published: '2020-11-24'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-4mp3-385r-v63f'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-4mp3-385r-v63f
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2020-26890'
  - url: 'https://github.com/matrix-org/synapse'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-237.yaml
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/G7YXMMYQP46PYL664JQUXCA3LPBJU7DQ
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/U34DPP4ZLOEDUY2ZCWOHQPU5GA5LYNUQ
  - url: 'https://pypi.org/project/matrix-synapse'
tags:
  - osv
  - pip
epss: 0.03004
epssPercentile: 0.86828
ingestedAt: '2026-07-08T18:25:45.508Z'
---

## Overview

### Impact
A denial of service attack against Matrix clients can be exploited by sending an event including invalid JSON data to Synapse. Synapse would relay the data to clients which could crash or hang. Impact is long-lasting if the event is made part of the room state.

### Patches
At a minimum #8106 and #8291 must be applied. #7372 and #8124 include additional checks.

### Workarounds
There are no known workarounds.

### Upgrading notes
If an invalid event is accepted by an earlier Synapse it can become part of the room state and will not be fixed by upgrading Synapse. Redacting the invalid event should avoid clients receiving the invalid event.

## Affected packages

- `matrix-synapse < 1.20.0`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.20.0`