How severe is CVE-2021-21370?

CVE-2021-21370 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21370?

CVE-2021-21370 affects the following packages: typo3/cms-backend (Packagist), typo3/cms-backend (Packagist), typo3/cms-backend (Packagist), typo3/cms-backend (Packagist), typo3/cms-backend (Packagist), typo3/cms-core (Packagist), typo3/cms-core (Packagist), typo3/cms-core (Packagist), and 3 more. Ecosystems affected: Packagist.

How do I fix CVE-2021-21370?

Update typo3/cms-backend to 7.6.51 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21370 is resolved across your whole dependency graph.

How do I detect CVE-2021-21370 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for typo3/cms-backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21370 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-21370 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21370 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-2021-21370?

CVE-2021-21370 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 58th 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-2021-21370?

CVE-2021-21370 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-21370 published, and has it been updated?

CVE-2021-21370 was published on March 23, 2021 and was last updated on August 27, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2021-21370 — cms-backend

MEDIUM

CVE-2021-21370 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in typo3/cms-backend. A fix is available for typo3/cms-backend — see the affected versions and patch details below.

Cross-Site Scripting in Content Preview (CType menu)

Also known asBIT-typo3-2021-21370GHSA-x7hc-x7fm-f7qh

Published

Mar 23, 2021

Updated

Aug 27, 2026

Affected

11 pkgs

Patched

11 / 11

Exploits

None indexed

Exploitation data as of Oct 8, 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 CVEs58th percentile — riskier than 58% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-21370 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

11 pkgs affected

🐘typo3/cms-backend🐘typo3/cms-backend🐘typo3/cms-backend🐘typo3/cms-backend🐘typo3/cms-backend🐘typo3/cms-core🐘typo3/cms-core🐘typo3/cms-core+3 more

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

Problem

It has been discovered that content elements of type menu are vulnerable to cross-site scripting when their referenced items get previewed in the page module. A valid backend user account is needed to exploit this vulnerability.

Solution

Update to TYPO3 versions 7.6.51, 8.7.40, 9.5.25, 10.4.14, 11.1.1 that fix the problem described.

Credits

Thanks to TYPO3 contributor Oliver Bartsch who reported and fixed the issue.

References

TYPO3-CORE-SA-2021-008

Affected Packages

11 total 11 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`typo3/cms-backend`	≥ 7.0.0&&< 7.6.51	7.6.51`composer require typo3/cms-backend:^7.6.51`
🐘Packagist	`typo3/cms-backend`	≥ 8.0.0&&< 8.7.40	8.7.40`composer require typo3/cms-backend:^8.7.40`
🐘Packagist	`typo3/cms-backend`	≥ 9.0.0&&< 9.5.25	9.5.25`composer require typo3/cms-backend:^9.5.25`
🐘Packagist	`typo3/cms-backend`	≥ 10.0.0&&< 10.4.14	10.4.14`composer require typo3/cms-backend:^10.4.14`
🐘Packagist	`typo3/cms-backend`	≥ 11.0.0&&< 11.1.1	11.1.1`composer require typo3/cms-backend:^11.1.1`
🐘Packagist	`typo3/cms-core`	≥ 10.0.0&&< 10.4.14	10.4.14`composer require typo3/cms-core:^10.4.14`

Affected Products

1 product · 5 configurations

Application

typo3typo3

≥ 11.0.0 && < 11.1.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for typo3/cms-backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update typo3/cms-backend to 7.6.51 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21370 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Problem It has been discovered that content elements of type _menu_ are vulnerable to cross-site scripting when their referenced items get previewed in the page module. A valid backend user account is needed to exploit this vulnerability. ### Solution Update to TYPO3 versions 7.6.51, 8.7.40, 9.5.25, 10.4.14, 11.1.1 that fix the problem described. ### Credits Thanks to TYPO3 contributor Oliver Bartsch who reported and fixed the issue. ### References * [TYPO3-CORE-SA-2021-008](https://typo3.org/security/advisory/typo3-core-sa-2021-008)

O3 Security · Impact-Aware SCA

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CVSS Score

5.4/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

TYPO3.CMS

6.0/10

Code-Review10/10

Maintained10/10

Branch-Protection0/10

Assessed Oct 5, 2026

Timeline

Disclosed

Mar 23, 2021

Last updated

Aug 27, 2026

Identifiers

CVE-2021-21370

GHSA-x7hc-x7fm-f7qh

BIT-typo3-2021-21370

References

Advisorygithub.com Advisorypackagist.org Advisorytypo3.org Advisorynvd.nist.gov Webgithub.com Webgithub.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs affecting typo3/cms-backend are rated as high or higher (top: high).

CVE-2026-19418CVE-2026-47352CVE-2026-47351CVE-2026-6553CVE-2025-59020CVE-2025-59017CVE-2025-59019CVE-2025-59014

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 27, 2026

CVE-2021-21370 — cms-backend

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Problem

Solution

Credits

References

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-21370 in your dependencies?