How severe is CVE-2021-21365?

CVE-2021-21365 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-2021-21365?

CVE-2021-21365 affects the following packages: bk2k/bootstrap-package (Packagist), bk2k/bootstrap-package (Packagist), bk2k/bootstrap-package (Packagist), bk2k/bootstrap-package (Packagist), bk2k/bootstrap-package (Packagist), bk2k/bootstrap-package (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-21365?

Update bk2k/bootstrap-package to 7.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21365 is resolved across your whole dependency graph.

How do I detect CVE-2021-21365 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for bk2k/bootstrap-package, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21365 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-2021-21365 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2021-21365, including 2 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-21365?

CVE-2021-21365 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 60th 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-2021-21365?

CVE-2021-21365 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-2021-21365 published, and has it been updated?

CVE-2021-21365 was published on April 27, 2021 and was last updated on July 9, 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-2021-21365 — bootstrap-package

MEDIUMFix: benjaminkott/bootstrap_package@de3a568

CVE-2021-21365 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in bk2k/bootstrap-package. 2 public exploit references exist, so weaponization risk is real. A fix is available for bk2k/bootstrap-package — see the affected versions and patch details below.

Cross-Site Scripting in Bootstrap Package

Also known asBIT-typo3-2021-21365GHSA-p48w-vf3c-rqjx

Published

Apr 27, 2021

Updated

Jul 9, 2026

Affected

6 pkgs

Patched

6 / 6

Exploits

2 known

Exploitation data as of Oct 10, 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 CVEs60th percentile — riskier than 60% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-21365 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

6 pkgs affected

🐘bk2k/bootstrap-package🐘bk2k/bootstrap-package🐘bk2k/bootstrap-package🐘bk2k/bootstrap-package🐘bk2k/bootstrap-package🐘bk2k/bootstrap-package

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 rendering content in the website frontend is vulnerable to cross-site scripting. A valid backend user account is needed to exploit this vulnerability.

The following templates are affected by the vulnerability:

Resources/Private/Partials/ContentElements/Carousel/Item/CallToAction.html
Resources/Private/Partials/ContentElements/Carousel/Item/Header.html
Resources/Private/Partials/ContentElements/Carousel/Item/Text.html
Resources/Private/Partials/ContentElements/Carousel/Item/TextAndImage.html
Resources/Private/Partials/ContentElements/Header/SubHeader.html

Users of the extension, who have overwritten the affected templates with custom code must manually apply the security fix as shown in this Git commit.

Solution

Update to version 7.1.2, 8.0.8, 9.0.4, 9.1.3, 10.0.10 or 11.0.3 of the Bootstrap Package that fix the problem described.

Updated version are available from the TYPO3 extension manager, Packagist and at https://extensions.typo3.org/extension/download/bootstrap_package/.

Credits

Thanks to TYPO3 security team member Oliver Hader who reported and fixed the issue.

Affected Packages

6 total 6 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`bk2k/bootstrap-package`	≥ 7.1.0&&< 7.1.2	7.1.2`composer require bk2k/bootstrap-package:^7.1.2`
🐘Packagist	`bk2k/bootstrap-package`	≥ 8.0.0&&< 8.0.8	8.0.8`composer require bk2k/bootstrap-package:^8.0.8`
🐘Packagist	`bk2k/bootstrap-package`	≥ 9.0.0&&< 9.0.4	9.0.4`composer require bk2k/bootstrap-package:^9.0.4`
🐘Packagist	`bk2k/bootstrap-package`	≥ 9.1.0&&< 9.1.3	9.1.3`composer require bk2k/bootstrap-package:^9.1.3`
🐘Packagist	`bk2k/bootstrap-package`	≥ 10.0.0&&< 10.0.10	10.0.10`composer require bk2k/bootstrap-package:^10.0.10`
🐘Packagist	`bk2k/bootstrap-package`	≥ 11.0.0&&< 11.0.3	11.0.3`composer require bk2k/bootstrap-package:^11.0.3`

Affected Products

1 product · 6 configurations

Application

typo3typo3

≥ 11.0.0 && < 11.0.3

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Bootstrap Package is a theme for TYPO3. It has been discovered that rend…

github.comNVDMay 2021

Bootstrap Package is a theme for TYPO3. It has been discovered that rend…

typo3.orgNVDMay 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for bk2k/bootstrap-package, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update bk2k/bootstrap-package to 7.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21365 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 rendering content in the website frontend is vulnerable to cross-site scripting. A valid backend user account is needed to exploit this vulnerability. The following templates are affected by the vulnerability: * `Resources/Private/Partials/ContentElements/Carousel/Item/CallToAction.html` * `Resources/Private/Partials/ContentElements/Carousel/Item/Header.html` * `Resources/Private/Partials/ContentElements/Carousel/Item/Text.html` * `Resources/Private/Partials/ContentElements/Carousel/Item/TextAndImage.html` * `Resources/Private/Partials/ContentElements/

O3 Security · Impact-Aware SCA

Is CVE-2021-21365 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

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

bootstrap_package

4.5/10

Code-Review2/10

Maintained10/10

Branch-ProtectionN/A

Assessed Oct 5, 2026

Timeline

Disclosed

Apr 27, 2021

First exploit

May 7, 2021

Last updated

Jul 9, 2026

Identifiers

CVE-2021-21365

GHSA-p48w-vf3c-rqjx

BIT-typo3-2021-21365

References

Advisorynvd.nist.gov Fixgithub.com Fixtypo3.org Evidencegithub.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs with CWE-79 are rated as high or higher (top: high).

CVE-2026-91127CVE-2026-77606CVE-2026-84992CVE-2026-77608CVE-2026-77607GHSA-9rcc-pmj8-ffhrCVE-2026-77616CVE-2025-61682Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026