Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N · score source

EPSS

• CVE-2020-26891: 1.92% estimated exploitation activity in the next 30 days; percentile 79.95 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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

Published by the source

16 Oct 2020, 16:56 UTC

Last modified by the source

8 Jul 2026, 06:30 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3x8c-fmpc-5rmq

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-3x8c-fmpc-5rmq

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# Patch Next advisory brief: CVE-2020-26891

Advisory: Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint
IDs: CVE-2020-26891, GHSA-3x8c-fmpc-5rmq
Record: https://patchnext.com/advisories/cve-2020-26891 (revision 3, source checked 26 Oct 2026, 13:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.21.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.21.0; fixed in 1.21.0

## Suggested inspection (read-only)
- `python -m pip show matrix-synapse`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on matrix-synapse.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-3x8c-fmpc-5rmq (OSV record GHSA-3x8c-fmpc-5rmq lists affected ranges: PyPI matrix-synapse >=0 <1.21.0 (fixed))
- Verified: https://github.com/advisories/GHSA-3x8c-fmpc-5rmq (GitHub-reviewed advisory GHSA-3x8c-fmpc-5rmq lists affected ranges: PyPI matrix-synapse >=0 <1.21.0)

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