How severe is CVE-2021-21671?

CVE-2021-21671 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21671?

CVE-2021-21671 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-2021-21671?

Update org.jenkins-ci.main:jenkins-core to 2.300 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21671 is resolved across your whole dependency graph.

How do I detect CVE-2021-21671 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-2021-21671 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-2021-21671 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21671 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-2021-21671?

CVE-2021-21671 has an EPSS (Exploit Prediction Scoring System) score of 1.8%, placing it in the 78th 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.

When was CVE-2021-21671 published, and has it been updated?

CVE-2021-21671 was published on June 30, 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

HIGH severity

CVE-2021-21671 — jenkins-core

HIGHFix: jenkinsci/jenkins@25a42f3

CVE-2021-21671 is a high-severity (CVSS 7.5) 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.

Session fixation vulnerability in Jenkins

Also known asBIT-jenkins-2021-21671GHSA-4wr9-2xc6-jmg5

Published

Jun 30, 2021

Updated

Oct 24, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 1, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.8%probability of exploitation in next 30 days

Lower Risk+0.05%

Lower risk than most CVEs78th percentile — riskier than 78% 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

This vulnerability was introduced in Jenkins 2.266 and LTS 2.277.1.

Jenkins 2.300, LTS 2.289.2 invalidates the previous session on login.

In case of problems, administrators can choose a different implementation by setting the Java system property hudson.security.SecurityRealm.sessionFixationProtectionMode to 2, or disable the fix entirely by setting that system property to 0.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.jenkins-ci.main:jenkins-core`	≥ 2.292&&< 2.300	2.300`org.jenkins-ci.main:jenkins-core:2.300`
☕Maven	`org.jenkins-ci.main:jenkins-core`	all versions	2.289.2`org.jenkins-ci.main:jenkins-core:2.289.2`

Affected Products

1 product · 2 configurations

Application

jenkinsjenkins

≥ 2.277.1 && < 2.289.2

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.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 2.300 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21671 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.

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 HatModerate

Product	Fixed in	Advisory
Red Hat OpenShift Container Platform 4.6	`jenkins-0:2.289.2.1629437819-1.el8`	RHBA-2021:3396
Red Hat OpenShift Container Platform 4.7	`jenkins-0:2.289.2.1628252553-1.el8`	RHBA-2021:3033
Red Hat OpenShift Container Platform 4.8	`jenkins-0:2.289.3.1633554819-1.el8`	RHSA-2021:3820

Frequently Asked Questions

Jenkins 2.299 and earlier, LTS 2.289.1 and earlier does not invalidate the previous session on login. This allows attackers to use social engineering techniques to gain administrator access to Jenkins. This vulnerability was introduced in Jenkins 2.266 and LTS 2.277.1. Jenkins 2.300, LTS 2.289.2 invalidates the previous session on login. In case of problems, administrators can choose a different implementation by setting the [Java system property `hudson.security.SecurityRealm.sessionFixationProtectionMode`](https://www.jenkins.io/doc/book/managing/system-properties/#hudson-security-securit

O3 Security · Impact-Aware SCA

Is CVE-2021-21671 in your dependencies?

Find it across Maven, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

7.5/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

Required

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Raw vector

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

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

Jun 30, 2021

Last updated

Oct 24, 2026

Identifiers

CVE-2021-21671

GHSA-4wr9-2xc6-jmg5

BIT-jenkins-2021-21671

References

Advisoryopenwall.com Advisoryjenkins.io · 2021 06 30 Advisorynvd.nist.gov · CVE 2021 21671 Webgithub.com · 25a42f3942fd9f8bd768c887c679dbc796b4fcd5 Packagegithub.com · jenkins

Related vulnerabilities

This CVE is high severity; 2 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

CVE-2021-21671 — jenkins-core

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-21671 in your dependencies?