How severe is CVE-2021-42567?

CVE-2021-42567 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-42567?

CVE-2021-42567 affects the following packages: org.apereo.cas:cas-server-core-web (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-42567?

Update org.apereo.cas:cas-server-core-web to 6.4.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-42567 is resolved across your whole dependency graph.

How do I detect CVE-2021-42567 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.apereo.cas:cas-server-core-web, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-42567 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-2021-42567 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-42567 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-2021-42567?

CVE-2021-42567 has an EPSS (Exploit Prediction Scoring System) score of 8.2%, placing it in the 95th 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-2021-42567?

CVE-2021-42567 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.

Is there exploit tooling available for CVE-2021-42567?

A Nuclei detection template exists for CVE-2021-42567, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-42567 published, and has it been updated?

CVE-2021-42567 was published on December 7, 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.

☕

☕ Maven

Not in CISA KEV

MEDIUM severity

CVE-2021-42567 — cas-server-core-web

MEDIUMFix: apereo/cas@376bf08

CVE-2021-42567 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in org.apereo.cas:cas-server-core-web. A fix is available for org.apereo.cas:cas-server-core-web — see the affected versions and patch details below.

Cross-site Scripting in Apereo CAS

Also known asGHSA-gfhx-jjwq-63gv

Published

Dec 7, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

8.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs95th percentile — riskier than 95% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-42567 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

☕org.apereo.cas:cas-server-core-web

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

Description

Apereo CAS through 6.4.1 allows XSS via POST requests sent to the REST API endpoints.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.apereo.cas:cas-server-core-web`	all versions	6.4.2`org.apereo.cas:cas-server-core-web:6.4.2`

Affected Products

1 product · 2 configurations

Application

central authentication serviceapereo

≥ 6.4.0 && < 6.4.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.apereo.cas:cas-server-core-web, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.apereo.cas:cas-server-core-web to 6.4.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-42567 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.

How to detect CVE-2021-42567

A community-maintained Nuclei template exists for this CVE. You can scan for it directly:

nuclei -id cve-2021-42567 -u https://target

Template: Apereo CAS Cross-Site Scripting
Severity: medium
Impact: Successful exploitation of this vulnerability could allow an attacker to execute malicious scripts in the context of a victim's browser, potentially leading to session hijacking, data theft, or defacement.
Remediation: Apply the latest security patches or updates provided by the vendor to mitigate this vulnerability.

Template by ProjectDiscovery nuclei-templates (pdteam), MIT licensed. View the full template. Scan only systems you are authorised to test.

Frequently Asked Questions

Apereo CAS through 6.4.1 allows XSS via POST requests sent to the REST API endpoints.

O3 Security · Impact-Aware SCA

Is CVE-2021-42567 in your dependencies?

Find it across Maven, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

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

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

OpenSSF Scorecard

Package health score

cas

5.1/10

Maintained10/10

Code-Review0/10

Branch-ProtectionN/A

Assessed Oct 5, 2026

Timeline

Disclosed

Dec 7, 2021

Last updated

Jul 9, 2026

Identifiers

CVE-2021-42567

GHSA-gfhx-jjwq-63gv

References

Advisorygithub.com Advisorynvd.nist.gov Fixapereo.github.io Webgithub.com Webapereo.github.io Packagegithub.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