CVE-2025-57849: Fuse Container Privilege Escalation Flaw

CVE-2025-57849 Overview

A container privilege escalation vulnerability has been identified in certain Fuse images. This security flaw originates from improper file permission configuration during the container build process, where the /etc/passwd file is created with group-writable permissions. Under specific conditions, an attacker who gains command execution capabilities 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 enable the attacker to add a new user account with arbitrary UID values, including UID 0, ultimately achieving full root privileges within the container environment.

Critical Impact

Attackers with limited container access can escalate to full root privileges by exploiting insecure file permissions on /etc/passwd, potentially leading to complete container compromise and lateral movement opportunities.

Affected Products

• Fuse container images with group-writable /etc/passwd file permissions
• Container environments where non-root users are members of the root group
• Red Hat Fuse-based container deployments

Discovery Timeline

• 2026-03-13 - CVE-2025-57849 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2025-57849

Vulnerability Analysis

This privilege escalation vulnerability (CWE-276: Incorrect Default Permissions) exploits a fundamental misconfiguration in how the /etc/passwd file permissions are set during the container image build process. The vulnerability requires local access to the container and elevated privileges to exploit effectively, but the conditions for exploitation can exist in multi-tenant container environments or where container breakout is a concern.

The attack exploits a race between the expected security model (where /etc/passwd should be read-only for non-root users) and the actual file permissions that allow group write access. When a user is part of the root group (GID 0), they inherit write permissions to the /etc/passwd file, enabling unauthorized account creation.

The security impact is significant because successful exploitation grants complete control over the container's user authentication mechanism, allowing attackers to create backdoor accounts, escalate privileges, and potentially use the compromised container as a pivot point for further attacks within the container orchestration environment.

Root Cause

The root cause of this vulnerability is an insecure default permission configuration (CWE-276) applied to the /etc/passwd file during the Fuse container image build process. Instead of the standard permission mode of 644 (read-write for owner, read-only for group and others), the file is created with group-writable permissions (664 or similar). This configuration oversight allows any process running with root group membership to modify the authentication database.

Attack Vector

The attack requires local access to the container and follows this exploitation path:

1. Initial Access: Attacker gains command execution within the affected container, even as a non-root user
2. Group Membership Check: Attacker verifies membership in the root group (GID 0)
3. Permission Verification: Attacker confirms /etc/passwd has group-writable permissions
4. Account Injection: Attacker appends a new user entry with UID 0 to /etc/passwd
5. Privilege Escalation: Attacker switches to the newly created root-equivalent account

The exploitation mechanism involves appending a crafted line to /etc/passwd that creates a user with UID 0. Once this entry exists, the attacker can use su or other authentication mechanisms to assume root identity within the container.

For detailed technical analysis, refer to the Red Hat CVE-2025-57849 Advisory and Red Hat Bugzilla #2391100.

Detection Methods for CVE-2025-57849

Indicators of Compromise

• Unexpected new user entries in /etc/passwd files, particularly those with UID 0
• User accounts with non-standard usernames appearing in container authentication logs
• Modification timestamps on /etc/passwd that don't correspond to authorized configuration changes
• Process execution by users that didn't exist in the original container image

Detection Strategies

• Implement file integrity monitoring (FIM) on /etc/passwd within container environments to detect unauthorized modifications
• Configure container runtime security tools to alert on privilege escalation attempts and unexpected root process spawning
• Deploy SentinelOne Singularity for real-time detection of privilege escalation behaviors and suspicious file modifications in containerized workloads
• Audit container images during CI/CD pipelines to identify insecure file permissions before deployment

Monitoring Recommendations

• Enable audit logging for all file modifications to /etc/passwd and /etc/shadow within containers
• Monitor for su or sudo commands executed by users not present in the original container image
• Track changes to user and group membership configurations across container deployments
• Implement runtime security monitoring to detect privilege escalation patterns characteristic of this attack

How to Mitigate CVE-2025-57849

Immediate Actions Required

• Audit all Fuse-based container images for insecure /etc/passwd permissions using ls -la /etc/passwd
• Rebuild affected container images with corrected file permissions (chmod 644 /etc/passwd)
• Review container user configurations to minimize root group membership for application accounts
• Implement read-only filesystem mounts for sensitive system files where operationally feasible

Patch Information

Organizations should consult the Red Hat CVE-2025-57849 Advisory for official patch information and updated container images. Monitor the Red Hat Bugzilla #2391100 tracking issue for remediation guidance and updated image releases.

Workarounds

• Correct the file permissions manually in running containers using chmod 644 /etc/passwd (temporary measure)
• Remove unnecessary users from the root group within container configurations
• Mount /etc/passwd as read-only using container runtime options where application requirements permit
• Implement security contexts that drop group-write capabilities for sensitive system files

# Configuration example
# Verify and correct /etc/passwd permissions in container images
# Check current permissions
ls -la /etc/passwd

# Correct insecure permissions
chmod 644 /etc/passwd

# Verify correction applied
stat -c '%a %U:%G %n' /etc/passwd
# Expected output: 644 root:root /etc/passwd

# For Dockerfile remediation, add this layer:
# RUN chmod 644 /etc/passwd