Summary
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 to 2, or disable the fix entirely by setting that system property to 0.
Impact
CVE-2021-21671 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.300, 2.289.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
org.jenkins-ci.main:jenkins-core to 2.300 or later; org.jenkins-ci.main:jenkins-core to 2.289.2 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-21671? CVE-2021-21671 is a high-severity security vulnerability in org.jenkins-ci.main:jenkins-core (maven), affecting versions >= 2.292, <= 2.299. It is fixed in 2.300, 2.289.2.
- How severe is CVE-2021-21671? CVE-2021-21671 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of org.jenkins-ci.main:jenkins-core are affected by CVE-2021-21671? org.jenkins-ci.main:jenkins-core (maven) versions >= 2.292, <= 2.299 is affected.
- Is there a fix for CVE-2021-21671? Yes. CVE-2021-21671 is fixed in 2.300, 2.289.2. Upgrade to this version or later.
- Is CVE-2021-21671 exploitable, and should I be worried? Whether CVE-2021-21671 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-21671 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-21671?
- Upgrade
org.jenkins-ci.main:jenkins-coreto 2.300 or later - Upgrade
org.jenkins-ci.main:jenkins-coreto 2.289.2 or later
- Upgrade