CVE-2025-57851: Multicluster Engine Privilege Escalation

CVE-2025-57851 Overview

A container privilege escalation vulnerability has been discovered in certain Multicluster Engine for Kubernetes images. This security flaw originates from improper file permissions during the container build process, where the /etc/passwd file is created with group-writable permissions. Under specific conditions, an attacker who can execute commands within an affected container—even as a non-root user—can exploit their membership in the root group to modify the /etc/passwd file. This manipulation could allow the attacker to add a new user with an arbitrary UID, including UID 0, ultimately granting full root privileges within the container.

Critical Impact

Attackers with command execution capability inside affected containers can escalate to root privileges by exploiting misconfigured /etc/passwd file permissions, potentially compromising the entire container environment.

Affected Products

• Multicluster Engine for Kubernetes (specific affected images)
• Red Hat container images with group-writable /etc/passwd
• Kubernetes deployments using vulnerable Multicluster Engine images

Discovery Timeline

• 2026-04-08 - CVE-2025-57851 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2025-57851

Vulnerability Analysis

This vulnerability is classified as CWE-276 (Incorrect Default Permissions), a common security weakness where file system resources are created with overly permissive access controls. The flaw exists because the container build process sets the /etc/passwd file with permissions that allow members of the root group to write to it. While the intent may have been to facilitate certain container operations, this permission model creates a significant security gap.

In containerized environments, users are often added to the root group for legitimate operational reasons. However, this privilege can be abused when combined with write access to critical system files like /etc/passwd. An attacker who has gained initial access to the container—even with limited non-root privileges—can leverage their root group membership to inject malicious entries into the passwd file.

The attack requires local access and high privileges (root group membership), which constrains the attack surface. However, once successfully exploited, the attacker gains complete control over the container's root account, potentially enabling further lateral movement or data exfiltration.

Root Cause

The root cause is improper file permission configuration during the container image build process. The /etc/passwd file is created with group-writable permissions (likely mode 0664 or similar), allowing any user in the root group to modify its contents. This violates the principle of least privilege and security best practices for system file management, where /etc/passwd should typically have mode 0644 (owner-writable, group and world read-only).

Attack Vector

The attack requires local access to the affected container environment. An attacker must first gain command execution capability within the container, potentially through application vulnerabilities, supply chain attacks, or compromised credentials. Once inside, the attacker can check their group membership and, if part of the root group, directly modify /etc/passwd to create a privileged user account.

The exploitation sequence involves:

1. Gaining initial access to the container with command execution capability
2. Verifying membership in the root group using commands like id or groups
3. Appending a new user entry to /etc/passwd with UID 0
4. Switching to the newly created privileged account to obtain full root access

Since no verified code examples are available, administrators should consult the Red Hat CVE-2025-57851 Advisory for detailed technical information on exploitation patterns and detection guidance.

Detection Methods for CVE-2025-57851

Indicators of Compromise

• Unexpected modifications to /etc/passwd file within container environments
• New user entries with UID 0 that were not part of the original container image
• Unusual process execution patterns indicating privilege escalation attempts
• Audit log entries showing write operations to /etc/passwd by non-root processes

Detection Strategies

• Implement file integrity monitoring (FIM) on critical system files including /etc/passwd within containers
• Configure container runtime security tools to alert on passwd file modifications
• Use Kubernetes admission controllers to scan container images for improper file permissions before deployment
• Deploy runtime security agents to detect and block unauthorized file system changes

Monitoring Recommendations

• Enable detailed audit logging for file operations within containerized workloads
• Monitor for su or sudo commands executed after passwd file modifications
• Implement behavioral analysis to detect privilege escalation patterns
• Review container security posture regularly using vulnerability scanning tools

How to Mitigate CVE-2025-57851

Immediate Actions Required

• Audit all Multicluster Engine for Kubernetes container images for group-writable /etc/passwd files
• Restrict container execution to trusted workloads and implement network segmentation
• Review and minimize root group membership assignments within containers
• Consider running containers with read-only root filesystems where operationally feasible

Patch Information

Red Hat has acknowledged this vulnerability and is tracking it under Bug Report #2391104. Administrators should monitor the Red Hat CVE-2025-57851 Advisory for official patch releases and updated container images. Apply security updates as soon as they become available from the vendor.

Workarounds

• Manually correct the /etc/passwd file permissions to 0644 in custom container builds
• Use container security policies to enforce proper file permissions at deployment time
• Implement seccomp profiles or AppArmor/SELinux policies to restrict file modification capabilities
• Deploy containers with non-root users that are not members of the root group

# Configuration example: Fix /etc/passwd permissions in Dockerfile
# Add this line after any user creation steps in your Dockerfile
RUN chmod 644 /etc/passwd

# Verify correct permissions during container runtime
# Expected output: -rw-r--r-- for /etc/passwd
ls -la /etc/passwd