Rancher has downstream cluster privilege escalation through cluster and project role template binding (CRTB/PRTB)

Impact

An issue was discovered in Rancher versions up to and including 2.5.15 and 2.6.6 where a flaw with authorization logic allows privilege escalation through cluster role template binding (CRTB) and project role template binding (PRTB). This issue does not affect the local cluster, it affects only downstream clusters.

The vulnerability can be exploited by any user who has permissions to create/edit cluster role template bindings or project role template bindings (such as cluster-owner, manage cluster members, project-owner and manage project members) to gain owner permission in another project in the same cluster or in another project on a different downstream cluster.

• The user must have kubectl access in the local cluster to exploit this scenario.

• This can only be abused to gain cluster-owner permission on a different downstream cluster if the user is already cluster-owner on at least one downstream cluster.

• Example of a modified CRTB (note: the clusterName points to the cluster ID of the cluster that the privileges will be escalated and namespace points to the current cluster ID that the user has permissions):

kubectl edit clusterroletemplatebindings crtb-<crtb-ID> -n c-<cluster-ID>
---
apiVersion: management.cattle.io/v3
clusterName: <ID-of-the-cluster-to-escalate>
kind: ClusterRoleTemplateBinding
metadata:
  annotations:
    <omitted> 
  finalizers:
  - <omitted> 
  generateName: crtb-
  labels:
    <omitted> 
    cattle.io/creator: norman
  name: crtb-<crtb-ID>
  namespace: c-<current-cluster-ID>
roleTemplateName: cluster-owner
userName: u-<user-ID>
userPrincipalName: local://u-<user-ID>

An artifact to flag the exploitation of this issue is that the namespace of the CRTB/PRTB will not match the cluster name (clusterName) of the CRTB/PRTB. For example, every CRTB in the c-123xyz namespace should have a cluster name of c-123xyz. If instead, the cluster name is c-abc567, for example, this is likely a result of a user exploiting this flaw.

For more information about cluster and project roles, please consult Rancher's documentation.

Patches

Patched versions include releases 2.5.16, 2.6.7 and later versions.

Workarounds

Limit access in Rancher to trusted users. There is not a direct mitigation besides upgrading to the patched Rancher versions.

Important:

• It is highly advised to check the local and downstream clusters for potential unrecognized CRTBs (kubectl get clusterroletemplatebindings -A) and PRTBs (kubectl get projectroletemplatebindings -A) assignments.
• The ability to add other users to projects and clusters is a highly-privileged permission which may result in users being able to operate beyond their explicitly specified RBAC. It is recommended that this permission be granted selectively.

The following script can be used as a helper to detect possible deviations of CRTBs and PRTBs that do not match the expected value. Further investigation is required to determine if the flagged objects were maliciously modified or not. The script requires kubectl access to the local cluster and the jq command.

#!/usr/bin/env bash

echo "CRTBs that don't match cluster:"
kubectl get clusterroletemplatebindings -A -o=jsonpath="{range .items[?(@.clusterName!=@.metadata.namespace)]}{.metadata.name}{'\n'}{end}"

echo "PRTBs that don't match project:"
kubectl get projectroletemplatebindings -A -ojson | jq -r '.items[]|.metadata as $m|select(.projectName|endswith($m.namespace)|not)|.metadata.name'

For more information

If you have any questions or comments about this advisory:

• Reach out to SUSE Rancher Security team for security related inquiries.
• Open an issue in Rancher repository.
• Verify our support matrix and product support lifecycle.