CVE-2026-6042

What it actually does

CVE-2026-6042 is a local denial-of-service weakness in musl libc up to version 1.2.6, specifically in the GB18030 4-byte decoding path of iconv in src/locale/iconv.c. A local user or local process that can cause data to be converted through this decoder can supply crafted input that triggers inefficient algorithmic complexity, consuming excessive CPU time and reducing availability of the affected process. The impact is limited to availability; the provided scoring does not indicate confidentiality or integrity compromise, privilege escalation, or remote unauthenticated exploitation.

Why it matters to the business

Successful exploitation can make affected applications or services slow, unresponsive, or resource-starved when they process attacker-controlled character conversion input. The risk is lower than a remote code execution issue because exploitation requires local access or the ability to run a local workload, and current EPSS is low with no known KEV listing or ransomware association. However, musl is widely used in lightweight Linux distributions and containers, so organizations with Alpine-based containers, embedded Linux devices, or statically linked musl binaries may have broad exposure and should patch during normal maintenance windows, prioritizing multi-user systems and services that process untrusted text input.

How an attack unfolds

An attacker first obtains a local foothold or access to run code, submit jobs, or influence input to a local program that uses musl libc iconv for GB18030 character set conversion. The attacker then supplies specially crafted GB18030 4-byte sequences designed to trigger the inefficient decoding behavior in musl's iconv implementation. When the vulnerable conversion path processes this input, CPU usage can spike or processing can stall for an extended period. Repeated or concurrent invocations can degrade the availability of the targeted application, container, host workload, or shared service. Because the vulnerability is local and availability-only, it is most useful after initial access, in shared hosting or CI/build environments, or as part of resource-exhaustion abuse against applications that expose local conversion operations to less-trusted users.

How to fix it

Upgrade musl libc to a patched version newer than the affected range, or apply the vendor-provided patch for the GB18030 iconv decoder issue. Since no exact fixed version was provided in the input, follow the security advisories from the operating system, container base image, embedded Linux vendor, or musl package maintainer. Rebuild and redeploy statically linked applications and container images that bundle musl, because updating the host package alone may not remediate embedded copies. Where immediate patching is not possible, reduce exposure by limiting local user access, applying CPU and memory limits to containers and job runners, restricting untrusted use of character set conversion utilities, and disabling or filtering GB18030 conversion paths where operationally feasible.

How to confirm the fix worked

Confirm remediation by checking that musl libc packages, container base images, and statically linked binaries are rebuilt against a patched musl release or vendor backport. On Alpine-like systems, verify package versions with the platform package manager and confirm containers have been rebuilt from updated base images rather than only restarted. Inventory applications that call iconv or perform character set conversions and confirm they no longer use musl 1.2.6 or earlier unless the vendor has backported the fix. Functional validation can include processing representative GB18030 input under resource monitoring and confirming CPU usage and latency remain bounded; conduct any stress testing in a controlled environment to avoid service disruption.

Typical attack path

1. Gain local access to a system, container, shell, job runner, or application feature that processes attacker-controlled text through musl libc iconv.
2. Identify or trigger a code path that converts input using the GB18030 character set, particularly the 4-byte decoder path.
3. Submit crafted GB18030 input that causes inefficient algorithmic behavior in src/locale/iconv.c.
4. Repeat the operation or run concurrent conversions to increase CPU consumption and latency.
5. Cause the affected process or local workload to become slow, unresponsive, or unavailable.

How to detect it

Review endpoint, container, and application telemetry for unexplained CPU exhaustion tied to processes that perform character set conversion, especially iconv utilities or applications handling GB18030 input. Useful sources include EDR process telemetry, Linux audit logs, shell history where appropriate, container runtime logs, job scheduler logs, application performance logs, and system metrics showing sustained high CPU in a single process without matching workload. Look for repeated invocations of iconv with GB18030-related parameters, unusual local jobs feeding large or malformed text into conversion routines, crashes or timeouts around text decoding, and bursts of short-lived conversion processes. There are no unique public exploit strings or actor-specific indicators from the provided information, so detection should focus on behavioral signs of local resource exhaustion and suspicious use of GB18030 conversion paths.

• iconv -f GB18030
• iconv -t GB18030
• GB18030
• gb18030
• src/locale/iconv.c
• musl libc 1.2.6

What your security team should do

• Prioritize patching systems and containers that use musl libc and allow local users, untrusted jobs, CI workloads, or application-controlled text conversion.
• Inventory Alpine-based containers, embedded Linux images, statically linked Go/C/C++ components with musl linkage, and packages that expose iconv or GB18030 conversion functionality.
• Rebuild and redeploy container images and statically linked binaries after applying the musl fix; do not rely solely on host package updates.
• Apply resource controls such as cgroup CPU quotas, process limits, and job timeouts on shared systems to reduce denial-of-service impact.
• Monitor for unusual CPU spikes in processes performing character conversion, especially repeated iconv usage involving GB18030.
• Because EPSS is low, exploit maturity is listed as none, and the issue is not in CISA KEV, handle through risk-based patch management unless affected systems are multi-tenant or process untrusted text.