---
id: CVE-2020-26891
aliases:
  - GHSA-3x8c-fmpc-5rmq
  - PYSEC-2020-238
title: >-
  Cross-site scripting (XSS) vulnerability in the fallback authentication
  endpoint
summary: >-
  Cross-site scripting (XSS) vulnerability in the fallback authentication
  endpoint
severity: medium
cvss: 6.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N'
vendor: matrix-synapse
product: matrix-synapse
ecosystem: pip
affected:
  - matrix-synapse < 1.21.0
patched:
  - matrix-synapse 1.21.0
published: '2020-10-16'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-3x8c-fmpc-5rmq'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2020-26891'
  - url: 'https://github.com/matrix-org/synapse/pull/8444'
  - url: 'https://github.com/matrix-org/synapse'
  - url: 'https://github.com/matrix-org/synapse/releases'
  - url: 'https://github.com/matrix-org/synapse/releases/tag/v1.21.2'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-238.yaml
  - url: >-
      https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory
tags:
  - osv
  - pip
epss: 0.01931
epssPercentile: 0.79163
ingestedAt: '2026-07-08T18:25:45.209Z'
---

## Overview

### Impact
The fallback authentication endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains.

### Patches
This is fixed by #8444, which is included in Synapse v1.21.0.

### Workarounds
If the homeserver is not configured to use reCAPTCHA, consent (terms of service), or single sign-on then the affected endpoint can be blocked at a reverse proxy:

* `/_matrix/client/r0/auth/.*/fallback/web`
* `/_matrix/client/unstable/auth/.*/fallback/web`

## Affected packages

- `matrix-synapse < 1.21.0`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.21.0`