CVE-2026-6042: musl libc DOS Vulnerability

CVE-2026-6042 Overview

A security flaw has been discovered in musl libc up to version 1.2.6. The vulnerability affects the iconv function within the file src/locale/iconv.c, specifically in the GB18030 4-byte Decoder component. Exploitation of this vulnerability through manipulation of input data results in inefficient algorithmic complexity, which can lead to denial of service conditions. The attack must be initiated from a local position, requiring an attacker to have local access to the target system.

Critical Impact

Local attackers can exploit inefficient algorithmic complexity in the musl libc iconv function to cause resource exhaustion and denial of service conditions on systems using the GB18030 character encoding decoder.

Affected Products

• musl libc versions up to 1.2.6
• Systems utilizing the GB18030 4-byte Decoder functionality
• Applications using iconv character encoding conversion with musl libc

Discovery Timeline

• April 10, 2026 - CVE-2026-6042 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6042

Vulnerability Analysis

This vulnerability is classified under CWE-404 (Improper Resource Shutdown or Release), indicating issues with how resources are managed during the character encoding conversion process. The flaw exists in the iconv function implementation within musl libc's GB18030 4-byte decoder component.

The GB18030 character encoding is a Chinese national standard that includes a complex multi-byte encoding scheme. When processing specially crafted input through the affected decoder, the algorithm exhibits inefficient complexity characteristics that can be exploited to consume excessive computational resources. This type of algorithmic complexity attack can cause significant performance degradation or complete denial of service on affected systems.

The local attack vector requirement means an attacker needs existing access to the system to exploit this vulnerability, which somewhat limits the attack surface but still poses a risk in multi-user environments or containers using musl libc.

Root Cause

The root cause lies in the algorithmic implementation of the GB18030 4-byte decoder within the iconv function in src/locale/iconv.c. The decoder does not properly handle certain input patterns, leading to excessive computational overhead when processing maliciously crafted data. This represents a classic algorithmic complexity vulnerability where specific inputs trigger worst-case performance scenarios.

Attack Vector

The attack requires local access to the target system. An attacker with local privileges can craft malicious input data that, when processed through the iconv function with GB18030 encoding, triggers the inefficient code paths. This can result in:

• CPU resource exhaustion
• Application hangs or unresponsiveness
• Denial of service for legitimate users

The vulnerability exploits the conversion pathway by providing specifically crafted 4-byte sequences that cause the decoder to perform significantly more operations than expected for normal input data.

Detection Methods for CVE-2026-6042

Indicators of Compromise

• Unusual CPU consumption by processes performing character encoding conversion
• Applications using iconv with GB18030 encoding exhibiting unexpected slowdowns
• System logs showing repeated calls to iconv with GB18030-related parameters
• Process monitoring alerts for resource exhaustion in musl libc-based applications

Detection Strategies

• Monitor for abnormal CPU usage patterns in applications that utilize musl libc's iconv functionality
• Implement resource consumption thresholds for character encoding conversion operations
• Review application logs for repeated character encoding conversion failures or timeouts
• Deploy SentinelOne's behavioral analysis to detect resource exhaustion attack patterns

Monitoring Recommendations

• Configure system monitoring to alert on sustained high CPU usage by iconv-related operations
• Implement resource limits (cgroups, ulimits) for processes that perform character encoding conversion
• Monitor for applications processing unusually large amounts of GB18030-encoded data
• Enable auditing of character encoding conversion operations in security-sensitive environments

How to Mitigate CVE-2026-6042

Immediate Actions Required

• Identify all systems running musl libc version 1.2.6 or earlier
• Prioritize patching for systems that actively use GB18030 character encoding conversion
• Implement resource limits for processes using iconv functionality as a temporary mitigation
• Monitor affected systems for signs of exploitation until patches can be applied

Patch Information

To fix this issue, it is recommended to deploy the security patch addressing this vulnerability. System administrators should update musl libc to the latest patched version that resolves the algorithmic complexity issue in the GB18030 4-byte decoder. For additional technical details and patch information, refer to the OpenWall OSS Security discussions and VulDB vulnerability entry.

Workarounds

• Implement process resource limits using cgroups or ulimit to prevent complete resource exhaustion
• Consider disabling or restricting GB18030 encoding support if not required by applications
• Use application-level input validation to limit the size and complexity of data processed through iconv
• Deploy network segmentation to limit local access to affected systems

# Configuration example - Limit resources for affected processes
# Set CPU and memory limits using cgroups
cgcreate -g cpu,memory:/iconv_limited
echo 50000 > /sys/fs/cgroup/cpu/iconv_limited/cpu.cfs_quota_us
echo 100M > /sys/fs/cgroup/memory/iconv_limited/memory.limit_in_bytes

# Or use ulimit for process-level restrictions
ulimit -t 60  # Limit CPU time to 60 seconds
ulimit -v 262144  # Limit virtual memory to 256MB