Summary
Access Restriction Bypass in kube-apiserver
A vulnerability in Kubernetes kube-apiserver could allow node updates to bypass a Validating Admission Webhook and allow unauthorized node updates. The information that is provided to the admission controller could contain old configurations that overwrite values used for validation. Since the overwriting takes place before the validation, this could lead the admission controller to accept requests that should be blocked. The vulnerability can be exploited when an update action on node resources is performed and an admission controller is in place and configured to validate the action.
Users are only affected by this vulnerability if they are running a Validating Admission Webhook for Nodes that denies admission based partially on the old state of the Node object. It only impacts validating admission plugins that rely on old values in certain fields and does not impact calls from kubelets that go through the built-in NodeRestriction admission plugin.
Impact
The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.
CVE-2021-25735 has a CVSS score of 6.5 (Medium). The vector is network-reachable, high privileges required, and no user interaction. 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 (1.20.6, 1.19.10, 1.18.18); 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.
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k8s.io/kubernetes to 1.20.6 or later; k8s.io/kubernetes to 1.19.10 or later; k8s.io/kubernetes to 1.18.18 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-25735? CVE-2021-25735 is a medium-severity incorrect authorization vulnerability in k8s.io/kubernetes (go), affecting versions >= 1.20.0, <= 1.20.5. It is fixed in 1.20.6, 1.19.10, 1.18.18. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
- How severe is CVE-2021-25735? CVE-2021-25735 has a CVSS score of 6.5 (Medium). 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 k8s.io/kubernetes are affected by CVE-2021-25735? k8s.io/kubernetes (go) versions >= 1.20.0, <= 1.20.5 is affected.
- Is there a fix for CVE-2021-25735? Yes. CVE-2021-25735 is fixed in 1.20.6, 1.19.10, 1.18.18. Upgrade to this version or later.
- Is CVE-2021-25735 exploitable, and should I be worried? Whether CVE-2021-25735 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-2021-25735 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-2021-25735?
- Upgrade
k8s.io/kubernetesto 1.20.6 or later - Upgrade
k8s.io/kubernetesto 1.19.10 or later - Upgrade
k8s.io/kubernetesto 1.18.18 or later
- Upgrade