CVE-2021-36976: Libarchive Use-After-Free Vulnerability

CVE-2021-36976 Overview

CVE-2021-36976 is a use-after-free vulnerability affecting libarchive versions 3.4.1 through 3.5.1. The flaw exists in the copy_string function, which is called from do_uncompress_block and process_block during archive decompression operations. This memory corruption vulnerability can be triggered when processing specially crafted archive files, potentially leading to denial of service conditions.

Critical Impact

A use-after-free condition in libarchive's decompression routines can cause application crashes or unexpected behavior when processing malicious archive files, affecting a wide range of systems including Apple iOS, macOS, and enterprise software like Splunk Universal Forwarder.

Affected Products

• libarchive libarchive (versions 3.4.1 through 3.5.1)
• Apple iPadOS
• Apple iPhone OS
• Apple macOS
• Apple watchOS
• Fedora Project Fedora 35
• Splunk Universal Forwarder

Discovery Timeline

• 2021-07-20 - CVE-2021-36976 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-36976

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a dangerous memory corruption flaw that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of libarchive, this issue manifests during the decompression of archive data, specifically within the copy_string function that handles string operations during block processing.

The vulnerability was discovered through OSS-Fuzz automated fuzzing, as documented in OSS-Fuzz Bug Report #32375. The issue occurs when the decompression routines in do_uncompress_block and process_block invoke copy_string with references to memory that has already been deallocated. This can lead to accessing stale memory, causing crashes or potentially allowing an attacker to manipulate program execution.

The network attack vector indicates that exploitation requires user interaction—typically opening or extracting a maliciously crafted archive file. While the primary impact is availability (denial of service through application crash), use-after-free vulnerabilities are known to sometimes escalate to more severe consequences depending on memory layout and exploitation techniques.

Root Cause

The root cause lies in improper memory management within libarchive's LZW/LZMA decompression implementation. When processing compressed blocks, the copy_string function references string data that may be freed prematurely due to buffer reallocation or cleanup operations during the decompression process. The freed memory pointer is not properly nullified or checked before subsequent use, creating the use-after-free condition.

Attack Vector

The attack vector is network-based but requires user interaction. An attacker would need to craft a malicious archive file (such as tar, cpio, or other formats supported by libarchive) that triggers the vulnerable code path during extraction. The victim would need to download and attempt to extract or process this archive using an application that relies on the vulnerable libarchive versions.

Attack scenarios include:

1. Email-based delivery: Malicious archive attachments sent via email that crash applications when opened
2. Web download exploitation: Compromised or malicious websites hosting crafted archives
3. Supply chain attacks: Malicious archives distributed through software repositories or package managers
4. Automated processing systems: Exploitation of systems that automatically process uploaded archives

The vulnerability requires the attacker to entice a user to process a malicious archive file, but once triggered, it can cause immediate application crashes and potential data loss from interrupted operations.

Detection Methods for CVE-2021-36976

Indicators of Compromise

• Application crashes in processes using libarchive when extracting or processing archive files
• Crash dumps referencing copy_string, do_uncompress_block, or process_block functions
• Unusual archive files with malformed compression blocks in downloaded content
• Core dumps or segmentation faults in archive-handling utilities like tar or bsdtar

Detection Strategies

• Monitor for crash events in applications using libarchive, particularly during archive extraction operations
• Implement file integrity monitoring on systems processing archives to detect exploitation attempts
• Use memory sanitizer tools (AddressSanitizer) in development and testing environments to catch use-after-free conditions
• Deploy endpoint detection that monitors for suspicious process terminations in archive utilities

Monitoring Recommendations

• Enable crash reporting and analysis on systems processing user-supplied archive files
• Monitor application logs for repeated failures when processing archives from untrusted sources
• Track libarchive version usage across the environment to identify vulnerable deployments
• Review security advisories from Apple, Fedora, Gentoo, and Debian for patch availability

How to Mitigate CVE-2021-36976

Immediate Actions Required

• Update libarchive to version 3.5.2 or later, which contains the fix for this vulnerability
• Apply vendor-specific patches from Apple (macOS 12.3, iOS 15.4, iPadOS 15.4, watchOS 8.5), Fedora, Gentoo, or Debian
• Restrict processing of archive files from untrusted sources until patches are applied
• Implement input validation to reject malformed or suspicious archive files

Patch Information

Multiple vendors have released patches addressing this vulnerability:

• Apple: Security updates available as documented in Apple Security Advisory HT213182, HT213183, and HT213193
• Fedora: Patch available through Fedora Package Announcement
• Gentoo: Update available via Gentoo GLSA 2022-08-26
• Debian: Long-term support update documented in Debian LTS Announcement November 2024
• Splunk Universal Forwarder: Check Splunk security advisories for updated versions

Workarounds

• Isolate archive processing operations in sandboxed environments to contain potential crashes
• Implement strict file type validation before processing archives with libarchive
• Use alternative archive libraries for critical operations until patching is complete
• Deploy application restart mechanisms to recover from crash-based denial of service

# Check installed libarchive version
pkg-config --modversion libarchive

# On macOS, verify system library version
otool -L /usr/lib/libarchive.dylib

# On Linux, check package version
dpkg -l libarchive13 || rpm -q libarchive