Introduction
Attackers gaining root access to Windows nodes in Kubernetes clusters can lead to full cluster compromise, data exfiltration, and lateral movement. A recent vulnerability in Kubernetes Image Builder, tracked as CVE-2025-7342, makes this risk real for any organization using Nutanix or OVA providers to build Windows VM images without overriding default credentials.
Kubernetes is the dominant open source container orchestration platform, powering cloud-native infrastructure for enterprises worldwide. The Image Builder project is a key automation tool for creating consistent VM images across providers. Its security directly impacts the integrity of Kubernetes clusters in production environments.
Technical Information
CVE-2025-7342 arises from the Kubernetes Image Builder's failure to enforce secure credential handling when building Windows VM images with Nutanix or OVA providers. In all versions up to v0.1.44, if the user does not explicitly set the Windows Administrator password (via the WINDOWS_ADMIN_PASSWORD environment variable or admin_password JSON parameter), the build process leaves the default credentials active in the resulting image. This creates a predictable and easily exploitable authentication vector.
The vulnerability's root cause is the lack of enforced credential randomization or mandatory user-supplied credentials during image creation. As a result, attackers who obtain access to a Windows node built from a vulnerable image can authenticate using known default credentials. This grants full administrative privileges, allowing for remote code execution, privilege escalation, and persistence.
The attack surface includes any remote access protocol enabled on the Windows node, such as RDP or WinRM. If SSH is enabled, it also becomes a viable entry point. The risk is highest in environments where these nodes are exposed to untrusted networks or where network segmentation is weak.
This issue is classified under CWE-798 (Use of Hard-coded Credentials), highlighting the architectural flaw in the credential management logic of the affected Image Builder versions.
Patch Information
In response to the security vulnerabilities identified in Kubernetes Image Builder versions up to v0.1.37, particularly concerning the use of default credentials during the image build process, the development team has implemented critical patches in subsequent releases to enhance security.
Addressing Default Credentials in Proxmox Provider (CVE-2024-9486):
The Proxmox provider previously retained default credentials in the final virtual machine images, posing a significant security risk. To mitigate this, the patch introduced in version v0.1.38 ensures that the 'builder' user account is properly removed upon completion of the image build process. This change prevents unauthorized access by eliminating residual default credentials.
Implementing Randomized SSH Passwords (CVE-2024-9594):
For images built using the Nutanix, OVA, QEMU, or raw providers, default SSH passwords were set during the build process, which could be exploited if an attacker gained access during this phase. The patch addresses this by generating a random SSH password for the 'builder' user during the auto-installation process. This approach significantly reduces the risk of unauthorized access during the build phase.
Code Implementation:
# Generate a random password BUILDER_PASSWORD=$(openssl rand -base64 32) # Set the password for the 'builder' user echo "builder:$BUILDER_PASSWORD" | chpasswd # Lock the 'builder' user account after the build process usermod -L builder
By integrating these patches, Kubernetes Image Builder enhances the security of virtual machine images by eliminating default credentials and introducing randomized passwords during the build process. Users are strongly encouraged to upgrade to version v0.1.38 or later to benefit from these security improvements.
Patch sources:
- https://github.com/kubernetes/kubernetes/issues/133115
- https://github.com/kubernetes-sigs/image-builder/releases/tag/v0.1.45
Detection Methods
Detecting the presence of default credentials in VM images built with Kubernetes Image Builder's Nutanix or OVA providers requires a proactive approach. Since these images may inadvertently include default credentials if not explicitly overridden, it's crucial to implement detection mechanisms to identify and mitigate potential security risks.
1. Review Image Build Configurations:
Begin by examining the build configurations used during the image creation process. Ensure that the parameters for setting credentials are explicitly defined and not left to default values. This step helps in identifying any oversight where default credentials might have been unintentionally included.
2. Analyze Image Contents:
Utilize tools that can inspect the contents of VM images to detect the presence of default or hardcoded credentials. By scanning the filesystem and configuration files within the image, you can identify and address any embedded credentials that pose a security risk.
3. Implement Automated Scanning:
Incorporate automated scanning tools into your CI/CD pipeline to detect default credentials during the image build process. These tools can flag instances where credentials have not been set or have been left to default values, allowing for immediate remediation before deployment.
4. Monitor Deployment Logs:
After deploying VM images, monitor system and application logs for any indications of unauthorized access or anomalies that could suggest the exploitation of default credentials. Setting up alerts for unusual login attempts or access patterns can help in early detection of security incidents.
5. Regularly Update and Patch:
Ensure that all VM images are regularly updated and patched to mitigate vulnerabilities associated with default credentials. Staying current with security updates reduces the risk of exploitation due to known issues.
Detection source:
Affected Systems and Versions
- Kubernetes Image Builder versions up to and including v0.1.44 are affected when building Windows VM images with Nutanix or OVA providers.
- All Windows VM images created using these providers and versions are vulnerable if credentials were not explicitly overridden during the build process.
- The vulnerability does not affect images where the administrator password was manually set during build.
Vendor Security History
- Previous similar vulnerabilities have been reported in Kubernetes Image Builder, including CVE-2024-9486 (Proxmox provider) and CVE-2024-9594 (Nutanix, OVA, QEMU, raw providers).
- The Kubernetes community has demonstrated timely patching and coordinated disclosure practices for these issues.