CVE-2021-30139: Alpine Linux Apk-tools Buffer Overflow

CVE-2021-30139 Overview

CVE-2021-30139 is an Out-of-Bounds Read vulnerability affecting Alpine Linux apk-tools before version 2.12.5. The vulnerability exists in the tarball parser component, which allows a buffer overflow condition that can lead to application crashes. This flaw can be triggered by processing specially crafted tarball archives, potentially enabling remote attackers to cause denial of service conditions on vulnerable Alpine Linux systems.

Critical Impact

Remote attackers can exploit this buffer overflow in the apk-tools tarball parser to crash package management operations, disrupting system updates and potentially leaving systems in vulnerable states.

Affected Products

• Alpine Linux apk-tools versions prior to 2.12.5
• Alpine Linux distributions using vulnerable apk-tools packages
• Container images and systems based on Alpine Linux with affected apk-tools versions

Discovery Timeline

• 2021-04-21 - CVE-2021-30139 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-30139

Vulnerability Analysis

The vulnerability resides in the tarball parsing functionality of Alpine Linux's apk-tools package manager. When processing malformed or maliciously crafted tarball archives, the parser fails to properly validate buffer boundaries, resulting in an out-of-bounds read condition (CWE-125). This memory safety issue can be exploited remotely without authentication by providing a specially crafted package or tarball to systems running vulnerable versions of apk-tools.

The attack can be launched over the network and requires no user interaction or special privileges, making it particularly dangerous in automated package update scenarios. While the vulnerability does not directly compromise confidentiality or integrity, it poses a significant availability risk by enabling attackers to crash the package manager.

Root Cause

The root cause is improper bounds checking in the tarball parser implementation within apk-tools. When parsing tar archive headers or file contents, the code reads beyond allocated buffer boundaries when encountering unexpected or malformed data structures. This out-of-bounds read (CWE-125) occurs because the parser does not adequately validate the size fields or structure boundaries within tarball entries before accessing memory.

Attack Vector

The attack vector is network-based and can be exploited through several scenarios:

1. Malicious Package Repository: An attacker could host or inject a crafted package into a repository that users connect to
2. Man-in-the-Middle Attack: Intercepting package downloads and replacing them with malicious tarballs
3. Local Exploitation: Processing a locally downloaded malicious tarball with apk-tools

When apk-tools attempts to parse the crafted tarball, the buffer overflow is triggered, causing the application to crash. This denial of service can prevent critical system updates from being applied, potentially leaving the system vulnerable to other known exploits.

The vulnerability mechanism involves malformed tarball headers that cause the parser to read past buffer boundaries. For technical details on the specific parsing flaw, refer to the Alpine Linux APK Tools Issue and the Alpine Linux Aports Issue.

Detection Methods for CVE-2021-30139

Indicators of Compromise

• Unexpected crashes or segmentation faults in apk processes during package operations
• Core dumps generated by apk-tools with stack traces pointing to tarball parsing functions
• Failed package installations or updates with abnormal termination signals
• Presence of malformed tarball files in package cache directories

Detection Strategies

• Monitor system logs for apk-tools crash events, particularly SIGSEGV or SIGABRT signals during package operations
• Implement file integrity monitoring on package cache directories to detect suspicious tarball files
• Deploy endpoint detection solutions capable of identifying buffer overflow exploitation attempts
• Audit package repository configurations to ensure only trusted sources are configured

Monitoring Recommendations

• Set up centralized logging for package manager operations across Alpine Linux hosts
• Configure alerts for abnormal termination of apk processes in container orchestration environments
• Monitor network traffic for unexpected package download sources or modified package content
• Implement hash verification for downloaded packages before processing

How to Mitigate CVE-2021-30139

Immediate Actions Required

• Upgrade apk-tools to version 2.12.5 or later immediately
• Audit all Alpine Linux systems and containers to identify vulnerable apk-tools versions
• Review package repository configurations and restrict to trusted, official Alpine mirrors
• Consider temporarily disabling automated package updates until systems are patched

Patch Information

Alpine Linux has addressed this vulnerability in apk-tools version 2.12.5 and later releases. System administrators should update their Alpine Linux installations using the following approach:

First, verify your current apk-tools version to determine if your system is vulnerable. If running a version prior to 2.12.5, proceed with the upgrade immediately. For containerized environments, rebuild container images using updated Alpine base images that include the patched apk-tools version.

For detailed patch information and discussion, refer to the Alpine Linux APK Tools Issue.

Workarounds

• Restrict network access to package repositories to trusted, authenticated connections only
• Implement network segmentation to limit exposure of vulnerable systems to untrusted package sources
• Use hash verification and signature checking for all downloaded packages before installation
• Consider using alternative package sources or mirrors that implement additional integrity checks

# Upgrade apk-tools to patched version
apk update
apk upgrade apk-tools

# Verify the installed version
apk version apk-tools
# Ensure version is 2.12.5 or later

# For Docker containers, rebuild with updated base image
# In Dockerfile, use a fixed Alpine version with patched apk-tools
# FROM alpine:3.13.5 or later