HTML injection in email and account expiry notifications

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2021-21333: 1.35% estimated exploitation activity in the next 30 days; percentile 71.64 (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 notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. Patches This issue is fixed in #9200. Workarounds For the missed messages notifications: The notif.html, notifmail.html, and room.html templates can be overridden with custom templates that manually escapes the variables using JInja2's escape filter. See the email.templatedir setting. For the account expiry notifications: 1. Account expiry can be disabled via the accountvalidity.enabled setting. 2. The noticeexpiry.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.templatedir setting.

Published by the source

26 Mar 2021, 19:53 UTC

Last modified by the source

8 Jul 2026, 06:27 UTC

Source last checked

26 Oct 2026, 13:21 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-c5f8-35qr-q4fm

2. Revision 2 · amended · 24 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-c5f8-35qr-q4fm

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# Patch Next advisory brief: CVE-2021-21333

Advisory: HTML injection in email and account expiry notifications
IDs: CVE-2021-21333, GHSA-c5f8-35qr-q4fm
Record: https://patchnext.com/advisories/cve-2021-21333 (revision 3, source checked 26 Oct 2026, 13:21 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.27.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.27.0; fixed in 1.27.0

## 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-c5f8-35qr-q4fm (OSV record GHSA-c5f8-35qr-q4fm lists affected ranges: PyPI matrix-synapse >=0 <1.27.0 (fixed))
- Verified: https://github.com/advisories/GHSA-c5f8-35qr-q4fm (GitHub-reviewed advisory GHSA-c5f8-35qr-q4fm lists affected ranges: PyPI matrix-synapse >=0 <1.27.0)

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