How severe is CVE-2021-21603?

CVE-2021-21603 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-2021-21603?

CVE-2021-21603 affects the following packages: org.jenkins-ci.main:jenkins-core (Maven), org.jenkins-ci.main:jenkins-core (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-21603?

Update org.jenkins-ci.main:jenkins-core to 2.275 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21603 is resolved across your whole dependency graph.

How do I detect CVE-2021-21603 in my Maven dependencies?

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

How do I mitigate CVE-2021-21603 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-2021-21603 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21603 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-2021-21603?

CVE-2021-21603 has an EPSS (Exploit Prediction Scoring System) score of 1.0%, placing it in the 64th 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-2021-21603?

CVE-2021-21603 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-2021-21603 published, and has it been updated?

CVE-2021-21603 was published on January 13, 2021 and was last updated on October 24, 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-2021-21603 — jenkins-core

MEDIUMFix: jenkinsci/jenkins@f5d9842

CVE-2021-21603 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in org.jenkins-ci.main:jenkins-core. A fix is available for org.jenkins-ci.main:jenkins-core — see the affected versions and patch details below.

XSS vulnerability in Jenkins notification bar

Also known asBIT-jenkins-2021-21603GHSA-98gq-6hxg-52r6

Published

Jan 13, 2021

Updated

Oct 24, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.0%probability of exploitation in next 30 days

Lower Risk+0.01%

Lower risk than most CVEs64th percentile — riskier than 64% 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

2 pkgs affected

☕org.jenkins-ci.main:jenkins-core☕org.jenkins-ci.main:jenkins-core

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

Jenkins 2.274 and earlier, LTS 2.263.1 and earlier does not escape notification bar response contents (typically shown after form submissions via Apply button).

This results in a cross-site scripting (XSS) vulnerability exploitable by attackers able to influence notification bar contents.

Jenkins 2.275, LTS 2.263.2 escapes the content shown in notification bars.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.jenkins-ci.main:jenkins-core`	all versions	2.275`org.jenkins-ci.main:jenkins-core:2.275`
☕Maven	`org.jenkins-ci.main:jenkins-core`	≥ 2.263.2&&< 2.275	2.275`org.jenkins-ci.main:jenkins-core:2.275`

Affected Products

1 product · 2 configurations

Application

jenkinsjenkins

≤ 2.274

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.jenkins-ci.main:jenkins-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.jenkins-ci.main:jenkins-core to 2.275 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21603 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.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Product	Fixed in	Advisory
Red Hat OpenShift Container Platform 3.11	`jenkins-0:2.263.3.1612433584-1.el7`	RHSA-2021:0637
Red Hat OpenShift Container Platform 4.5	`conmon-2:2.0.21-1.rhaos4.5.el7`	RHSA-2021:0429
Red Hat OpenShift Container Platform 4.6	`jenkins-0:2.263.3.1612434510-1.el8`	RHSA-2021:0423
Red Hat OpenShift Container Platform 4.8	`atomic-openshift-service-idler-0:4.8.0-202106281541.p0.git.39cfc66.assembly.stream.el8`	RHSA-2021:2437

Frequently Asked Questions

Jenkins 2.274 and earlier, LTS 2.263.1 and earlier does not escape notification bar response contents (typically shown after form submissions via Apply button). This results in a cross-site scripting (XSS) vulnerability exploitable by attackers able to influence notification bar contents. Jenkins 2.275, LTS 2.263.2 escapes the content shown in notification bars.

O3 Security · Impact-Aware SCA

Is CVE-2021-21603 in your dependencies?

Find it across Maven, 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 Nuclei template, Metasploit module or public proof-of-concept found.

OpenSSF Scorecard

Package health score

jenkins

6.4/10

Code-Review6/10

Maintained10/10

Branch-Protection3/10

Assessed Oct 26, 2026

Timeline

Disclosed

Jan 13, 2021

Last updated

Oct 24, 2026

Identifiers

CVE-2021-21603

GHSA-98gq-6hxg-52r6

BIT-jenkins-2021-21603

References

Advisoryjenkins.io · 2021 01 13 Advisorynvd.nist.gov · CVE 2021 21603 Webgithub.com · f5d98421604e44f398e7de9d222b191a705608af Packagegithub.com · jenkins

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs affecting jenkins-core are rated as high or higher (top: high).

CVE-2026-53436CVE-2026-53437CVE-2026-53440CVE-2026-53441CVE-2026-53439CVE-2026-53435CVE-2026-53442CVE-2026-53438

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 24, 2026