Security Bulletin: IBM Cloud Kubernetes Service is affected by a Kubernetes kubelet security vulnerability (CVE-2021-25741)

Summary

IBM Cloud Kubernetes Service is affected by a Kubernetes kubelet security vulnerability that could allow host path like access without use of the hostPath feature, thus bypassing the restriction (CVE-2021-25741).

Vulnerability Details

CVEID: CVE-2021-25741
Description: Kubernetes could allow a remote authenticated attacker to bypass security restrictions, caused by a symlink exchange flaw in kubelet. By sending a specially-crafted request, an attacker could exploit this vulnerability to create a container with subpath volume mounts to access files and directories outside of the volume.
CVSS Base Score: 8.8
CVSS Temporal Score: <https://exchange.xforce.ibmcloud.com/vulnerabilities/209533&gt; for more information
CVSS Vector: (CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

Affected Products and Versions

IBM Cloud Kubernetes Service 1.21.0-1.21.4_1530
IBM Cloud Kubernetes Service 1.20.0-1.20.10_1552
IBM Cloud Kubernetes Service 1.19.0-1.19.14_1559
IBM Cloud Kubernetes Service 1.5-1.18

Remediation/Fixes

Updates for IBM Cloud Kubernetes Service cluster worker nodes are available that fix this vulnerability. Customers must update worker nodes created before the fix was available to address the vulnerability. See updating worker nodes for details on updating worker nodes. To verify your cluster worker nodes have been updated, use the following IBM Cloud CLI command to confirm the currently running versions:

ibmcloud ks workers --cluster <cluster name or ID>

If the versions are at one of the following patch levels or later, the cluster worker nodes have the fix:
1.22.2_1523
1.21.5_1532
1.20.11_1554
1.19.15_1561

Customers running IBM Cloud Kubernetes Service clusters at version 1.17 or 1.18 must upgrade their clusters to version 1.19 by October 31, 2021. Please review the documentation before starting an upgrade since additional actions may be required.

Customers running IBM Cloud Kubernetes Service clusters at version 1.16 or earlier must create a new cluster and deploy their apps to the new cluster.

IBM Cloud Kubernetes Service versions 1.17 and earlier are no longer supported, and version 1.18 is deprecated. See the IBM Cloud Kubernetes Service version information and update actions documentation for more information about Kubernetes versions and version support policies.

Workarounds and Mitigations

You can partially mitigate this vulnerability by configuring pod security policies to restrict users from running containers as root as is detailed in the Kubernetes security advisory.

Monitor IBM Cloud Status for Future Security Bulletins

Monitor the security notifications on the IBM Cloud Status page to be advised of future security bulletins.

References

Complete CVSS v3 Guide
On-line Calculator v3

Off

Kubernetes Security Announcement for CVE-2021-25741

Related Information

IBM Secure Engineering Web Portal
IBM Product Security Incident Response Blog

*The CVSS Environment Score is customer environment specific and will ultimately impact the Overall CVSS Score. Customers can evaluate the impact of this vulnerability in their environments by accessing the links in the Reference section of this Security Bulletin.

Disclaimer

Review the IBM security bulletin disclaimer and definitions regarding your responsibilities for assessing potential impact of security vulnerabilities to your environment.

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