How severe is CVE-2021-21320?

CVE-2021-21320 has a CVSS score of 4.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21320?

CVE-2021-21320 affects the following packages: matrix-react-sdk (npm). Ecosystems affected: npm.

How do I fix CVE-2021-21320?

Update matrix-react-sdk to 3.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21320 is resolved across your whole dependency graph.

How do I detect CVE-2021-21320 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-react-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21320 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21320 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21320 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21320?

CVE-2021-21320 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 61th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21320?

CVE-2021-21320 is classified as CWE-345 (CWE-345). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21320 published, and has it been updated?

CVE-2021-21320 was published on March 2, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

MEDIUM severity

CVE-2021-21320 — matrix-react-sdk

MEDIUMFix: matrix-org/matrix-react-sdk@b386f0c

CVE-2021-21320 is a medium-severity (CVSS 4.3) CWE-345 vulnerability in matrix-react-sdk. A fix is available for matrix-react-sdk — see the affected versions and patch details below.

User content sandbox can be confused into opening arbitrary documents

Also known asGHSA-52mq-6jcv-j79x

Published

Mar 2, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs61th percentile — riskier than 61% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

3other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

matrix-react-sdknpm

13Kdownloads / week

Description

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`matrix-react-sdk`	all versions	3.15.0`npm install matrix-react-sdk@3.15.0`

Affected Products

1 product · 1 configurations

Application

matrix-react-sdkmatrix-react-sdk_project

< 3.15.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-react-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-react-sdk to 3.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21320 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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.