CVE-2021-28831: BusyBox Buffer Overflow Vulnerability

CVE-2021-28831 Overview

CVE-2021-28831 is a memory corruption vulnerability in BusyBox through version 1.32.1 affecting the decompress_gunzip.c component. The vulnerability stems from improper handling of the error bit on the huft_build result pointer, which can lead to an invalid free or segmentation fault when processing malformed gzip data. This flaw enables remote attackers to cause a denial of service condition by supplying specially crafted gzip archives.

Critical Impact

Remote attackers can trigger denial of service conditions on systems running vulnerable BusyBox versions by providing malformed gzip data, potentially disrupting embedded systems, IoT devices, and Linux distributions that rely on BusyBox utilities.

Affected Products

• BusyBox through version 1.32.1
• Fedora 32, 33, and 34
• Debian Linux 9.0

Discovery Timeline

• 2021-03-19 - CVE-2021-28831 published to NVD
• 2025-12-17 - Last updated in NVD database

Technical Details for CVE-2021-28831

Vulnerability Analysis

The vulnerability exists within the gzip decompression functionality in BusyBox, specifically in the decompress_gunzip.c source file. The core issue relates to improper exception handling (CWE-755) where the error bit on the huft_build function's result pointer is mishandled during the Huffman table construction process.

When processing gzip-compressed data, the huft_build function constructs Huffman decoding tables. If this function encounters an error condition while building the table, it sets an error indicator in the result pointer. The vulnerable code fails to properly check this error condition before proceeding with memory operations, leading to attempts to free memory that was never properly allocated or has already been freed.

This improper error handling manifests in two potential outcomes: an invalid free operation (attempting to free a pointer that doesn't point to valid heap memory) or a segmentation fault (accessing memory outside the process's allocated address space). Both conditions result in immediate process termination, causing denial of service.

Root Cause

The root cause is classified as CWE-755 (Improper Handling of Exceptional Conditions). The decompress_gunzip.c code does not adequately validate the state of the huft_build result pointer before performing subsequent memory management operations. When malformed gzip data triggers an error during Huffman table construction, the error bit in the result pointer is set, but subsequent code paths fail to check this bit, leading to undefined behavior when memory deallocation is attempted.

Attack Vector

The attack can be executed over the network without requiring any privileges or user interaction. An attacker crafts a malicious gzip archive containing data structures that will cause the huft_build function to fail during decompression. When a vulnerable BusyBox instance attempts to decompress this malformed archive—whether through gunzip, zcat, or any utility that processes gzip data—the improper error handling triggers the vulnerability.

The exploitation flow involves:

1. Attacker creates a gzip file with malformed Huffman code definitions
2. Victim system processes the malicious gzip data using BusyBox decompression utilities
3. The huft_build function fails to construct valid Huffman tables
4. Error bit is set but not properly checked by subsequent code
5. Invalid free or segmentation fault occurs, crashing the process

This is particularly concerning for embedded systems and IoT devices where BusyBox is commonly deployed, as service disruption could have significant operational impacts.

Detection Methods for CVE-2021-28831

Indicators of Compromise

• Unexpected crashes or segmentation faults in BusyBox processes handling gzip decompression
• Abnormal process terminations when processing gzip archives from untrusted sources
• Core dumps generated by BusyBox utilities such as gunzip, zcat, or tar with gzip compression
• System logs indicating SIGSEGV signals in BusyBox-related processes

Detection Strategies

• Monitor for abnormal termination of BusyBox decompression processes using process monitoring tools
• Implement file integrity monitoring on systems where BusyBox is critical infrastructure
• Deploy network-based intrusion detection rules to identify malformed gzip headers in incoming traffic
• Review system logs for patterns of repeated crashes when processing compressed archives

Monitoring Recommendations

• Configure alerting for process crashes involving BusyBox utilities in production environments
• Implement application-level logging for gzip decompression operations to capture failure patterns
• Monitor resource usage patterns that may indicate repeated crash-restart cycles
• Deploy endpoint detection and response solutions capable of identifying exploitation attempts

How to Mitigate CVE-2021-28831

Immediate Actions Required

• Upgrade BusyBox to a version containing the fix (commit f25d254dfd4243698c31a4f3153d4ac72aa9e9bd)
• Apply distribution-specific security updates for Fedora, Debian, and other affected platforms
• Restrict processing of gzip archives from untrusted sources until patching is complete
• Consider implementing input validation on gzip data before passing to BusyBox utilities

Patch Information

The vulnerability has been addressed in the official BusyBox repository. The fix is available in BusyBox commit f25d254dfd4243698c31a4f3153d4ac72aa9e9bd. Distribution-specific patches are available through:

• Debian LTS Security Announcement
• Fedora Package Announcements
• Gentoo GLSA 202105-09
• NetApp Security Advisory

Workarounds

• Avoid using BusyBox gzip decompression utilities for processing data from untrusted sources
• Use alternative decompression tools (such as GNU gzip) where available until patches can be applied
• Implement network-level filtering to block potentially malicious gzip archives
• Deploy application sandboxing to limit the impact of BusyBox process crashes on system stability

# Check current BusyBox version
busybox | head -1

# On Debian-based systems, update BusyBox
apt-get update && apt-get upgrade busybox

# On Fedora systems, update BusyBox
dnf update busybox