CVE-2026-4111: libarchive RAR5 DoS Vulnerability

CVE-2026-4111 Overview

A flaw was identified in the RAR5 archive decompression logic of the libarchive library, specifically within the archive_read_data() processing path. When a specially crafted RAR5 archive is processed, the decompression routine may enter a state where internal logic prevents forward progress. This condition results in an infinite loop that continuously consumes CPU resources. Because the archive passes checksum validation and appears structurally valid, affected applications cannot detect the issue before processing. This can allow attackers to cause persistent denial-of-service conditions in services that automatically process archives.

Critical Impact

Attackers can cause persistent denial-of-service conditions by submitting specially crafted RAR5 archives that trigger infinite loops in the decompression logic, exhausting CPU resources on affected systems.

Affected Products

• libarchive library (RAR5 decompression component)
• Applications and services using libarchive for archive processing
• Linux distributions packaging libarchive (Red Hat, Fedora, etc.)

Discovery Timeline

• 2026-03-13 - CVE-2026-4111 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-4111

Vulnerability Analysis

This vulnerability is classified as CWE-835 (Loop with Unreachable Exit Condition), commonly known as an infinite loop vulnerability. The flaw resides in the RAR5 archive decompression logic within the libarchive library. When processing certain malformed RAR5 archives, the decompression routine enters a state where the exit conditions for the processing loop can never be satisfied, causing the function to execute indefinitely.

The insidious nature of this vulnerability lies in the fact that the malicious archive passes standard validation checks, including checksum verification and structural validation. This means applications relying on libarchive cannot distinguish between legitimate and malicious archives prior to decompression, making filtering ineffective without additional safeguards.

Root Cause

The root cause is a logic error in the archive_read_data() processing path for RAR5 archives. Specific archive structures can manipulate the internal state of the decompression routine in a way that prevents forward progress through the data stream. The decompression loop continues to execute without advancing through the archive data or reaching a termination condition, resulting in indefinite execution and CPU resource consumption.

Attack Vector

The attack can be executed remotely over a network by submitting a specially crafted RAR5 archive to any service that processes archives using the vulnerable libarchive library. Attack scenarios include:

• Email attachments processed by mail servers or security gateways
• File upload endpoints that accept compressed archives
• Automated backup or archive processing systems
• CI/CD pipelines that extract dependencies from archives
• Cloud storage services with archive preview or extraction features

The attack requires no authentication or user interaction since the malicious payload is processed automatically when the archive is decompressed.

The vulnerability manifests when a crafted RAR5 archive triggers specific internal state conditions in the decompression routine. The archive is constructed to pass checksum and structural validation while containing data that causes the processing loop to never terminate. For technical details on the fix implementation, refer to GitHub Pull Request #2877.

Detection Methods for CVE-2026-4111

Indicators of Compromise

• Sustained high CPU utilization by processes using libarchive
• Processes running archive_read_data() calls that never complete
• Stalled or unresponsive services that handle archive extraction
• RAR5 archives with unusual internal structures or abnormal decompression times

Detection Strategies

• Monitor for processes exhibiting prolonged CPU spikes during archive processing operations
• Implement timeout mechanisms for archive decompression tasks to detect hung processes
• Track archive processing duration metrics and alert on anomalous processing times
• Deploy application-level monitoring to detect stalled libarchive function calls

Monitoring Recommendations

• Configure resource limits and timeouts for archive processing services
• Implement process monitoring to detect and terminate runaway decompression tasks
• Set up alerting for archive processing queues that stop making progress
• Monitor CPU utilization patterns for services handling RAR5 archives

How to Mitigate CVE-2026-4111

Immediate Actions Required

• Update libarchive to the patched version addressing CVE-2026-4111
• Apply Red Hat security updates RHSA-2026:5063 and RHSA-2026:5080 for affected systems
• Implement processing timeouts for archive extraction operations
• Consider temporarily disabling RAR5 archive support if updates cannot be applied immediately

Patch Information

Security patches are available through vendor channels. Red Hat has released security advisories RHSA-2026:5063 and RHSA-2026:5080 addressing this vulnerability. The upstream fix is available in GitHub Pull Request #2877. Additional technical details can be found in the Red Hat CVE Details and Red Hat Bugzilla Report #2446453.

Workarounds

• Implement strict timeout limits on archive extraction operations to prevent infinite loops from consuming resources indefinitely
• Use process isolation (containers, sandboxing) for archive processing to limit the impact of CPU exhaustion
• Deploy resource control mechanisms (cgroups, ulimits) to cap CPU time for archive processing tasks
• Consider pre-screening archives with alternative tools before processing with libarchive

# Configuration example - Set CPU time limits for archive processing
# Using ulimit to restrict CPU time (in seconds)
ulimit -t 300

# Using timeout command to limit extraction duration
timeout 60s bsdtar -xf archive.rar

# Using cgroups to limit CPU usage (requires root)
cgcreate -g cpu:/archive_processing
cgset -r cpu.cfs_quota_us=50000 archive_processing
cgexec -g cpu:archive_processing bsdtar -xf archive.rar