User content sandbox can be confused into opening arbitrary documents

Exploitation

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

CVSS

2.6 · CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:L/A:N

EPSS

• CVE-2021-21320: 0.92% estimated exploitation activity in the next 30 days; percentile 60.37 (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 user content sandbox can be abused to trick users into opening unexpected documents after several user interactions. The content can be opened with a blob origin from the Matrix client, so it is possible for a malicious document to access user messages and secrets. Patches This has been fixed by https://github.com/matrix-org/matrix-react-sdk/pull/5657, which is included in 3.15.0. Workarounds There are no known workarounds.

Published by the source

3 Mar 2021, 02:23 UTC

Last modified by the source

8 Jul 2026, 06:28 UTC

Source last checked

26 Oct 2026, 12:20 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-52mq-6jcv-j79x

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-52mq-6jcv-j79x

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# Patch Next advisory brief: CVE-2021-21320

Advisory: User content sandbox can be confused into opening arbitrary documents
IDs: CVE-2021-21320, GHSA-52mq-6jcv-j79x
Record: https://patchnext.com/advisories/cve-2021-21320 (revision 2, source checked 26 Oct 2026, 12:20 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-react-sdk 3.15.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm matrix-react-sdk: affected < 3.15.0; fixed in 3.15.0

## Suggested inspection (read-only)
- `npm ls matrix-react-sdk --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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-react-sdk.
- 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-52mq-6jcv-j79x (OSV record GHSA-52mq-6jcv-j79x lists affected ranges: npm matrix-react-sdk >=0 <3.15.0 (fixed))
- Verified: https://github.com/advisories/GHSA-52mq-6jcv-j79x (GitHub-reviewed advisory GHSA-52mq-6jcv-j79x lists affected ranges: npm matrix-react-sdk >=0 <3.15.0)

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