CVE-2026-38755 Overview
CVE-2026-38755 is a heap overflow vulnerability in the evalcommand() function located in shell/ash.c of BusyBox v1.38.0. Attackers can trigger the flaw by supplying crafted input to the ash shell, causing memory corruption on the heap. Successful exploitation results in a Denial of Service (DoS) condition against the affected process. BusyBox is embedded in countless Linux-based systems including routers, IoT devices, containers, and minimal Linux distributions, making the impact broad across embedded and cloud-native environments.
Critical Impact
A crafted shell input processed by BusyBox ash can corrupt the heap in evalcommand(), terminating the shell process and disrupting services that rely on it.
Affected Products
- BusyBox v1.38.0
- Linux-based embedded systems bundling BusyBox v1.38.0
- Container base images shipping BusyBox v1.38.0 (for example, minimal Alpine or scratch-based images)
Discovery Timeline
- 2026-07-15 - CVE-2026-38755 published to the National Vulnerability Database (NVD)
- 2026-07-16 - Last updated in NVD database
Technical Details for CVE-2026-38755
Vulnerability Analysis
The vulnerability resides in the evalcommand() function of the BusyBox ash shell implementation, specifically within shell/ash.c. This function is responsible for evaluating and executing shell commands parsed from user or script input. A heap overflow condition occurs when the function processes specially crafted input, writing beyond the bounds of a heap-allocated buffer. The out-of-bounds write corrupts adjacent heap metadata and structures, which the allocator later dereferences during subsequent operations.
BusyBox is widely deployed as a single-binary replacement for common Unix utilities in embedded devices, initramfs images, and minimal container images. Any workflow that pipes untrusted content through the ash shell can trigger the flaw. See the BusyBox mailing list archive for the upstream discussion.
Root Cause
The root cause is missing or insufficient bounds validation on input processed inside evalcommand(). When the function copies or manipulates command tokens, argument lists, or expansion buffers, it fails to verify that the destination heap allocation can hold the resulting data. This classifies as a heap-based buffer overflow ([CWE-122] category).
Attack Vector
An attacker delivers a crafted shell script, command string, or standard input to a BusyBox ash instance. Delivery paths include scripts executed at boot, CI/CD pipelines that shell out through BusyBox, web interfaces on embedded devices that pass input to shell commands, and container entrypoints. The immediate outcome documented in the advisory is a Denial of Service through process termination. Because the corruption occurs on the heap, memory safety impact beyond DoS depends on heap layout and cannot be excluded without further analysis.
No verified public proof-of-concept exploit code is available at the time of publication. Refer to the BusyBox official website for source references.
Detection Methods for CVE-2026-38755
Indicators of Compromise
- Unexpected termination or crash signals (SIGSEGV, SIGABRT) from BusyBox ash or sh processes.
- Heap corruption diagnostics emitted by glibc or musl (for example, malloc(): corrupted top size) attributed to BusyBox.
- Repeated restarts of services or init scripts that invoke BusyBox shells on embedded devices.
Detection Strategies
- Inventory all systems, containers, and firmware images to identify BusyBox v1.38.0 binaries using busybox --help version checks or SBOM queries.
- Monitor process exit codes and core dumps for the BusyBox binary across production endpoints and container hosts.
- Deploy runtime application self-protection or endpoint detection tooling capable of flagging heap corruption events in shell processes.
Monitoring Recommendations
- Aggregate kernel and systemd journal entries referencing BusyBox segfaults into a centralized log platform for correlation.
- Alert on anomalous shell input patterns delivered to embedded management interfaces, especially oversized argument strings.
- Track container image scans over time to detect the reintroduction of vulnerable BusyBox versions through base image updates.
How to Mitigate CVE-2026-38755
Immediate Actions Required
- Identify every deployment of BusyBox v1.38.0 across servers, containers, embedded devices, and firmware images.
- Restrict who can supply input to BusyBox ash shells, particularly on network-exposed management interfaces.
- Rebuild container images against a patched BusyBox release once available from the upstream project.
Patch Information
At the time of NVD publication, monitor the BusyBox mailing list archive and the BusyBox official website for a fixed release beyond v1.38.0. Vendors shipping BusyBox in firmware or container base images should track their upstream advisories and rebuild affected artifacts once the patched source is tagged.
Workarounds
- Avoid passing untrusted input to BusyBox ash; route scripts through a hardened shell such as bash where feasible.
- Downgrade to a prior BusyBox release not affected by the evalcommand() regression, after validating no other advisories apply.
- Apply strict input length and character validation at the application layer before invoking shell commands on embedded devices.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
