CVE-2026-6069: Nasm Netwide Assembler Buffer Overflow

CVE-2026-6069 Overview

CVE-2026-6069 is a stack-based buffer overflow vulnerability in the Netwide Assembler (NASM), specifically affecting the disasm() function. The vulnerability occurs when formatting disassembly output, where an attacker can trigger an out-of-bounds write when the slen parameter exceeds the allocated buffer capacity. This flaw is classified as CWE-787 (Out-of-bounds Write) and can lead to denial of service conditions when processing maliciously crafted input.

Critical Impact

Processing specially crafted assembly files with NASM can trigger a stack-based buffer overflow, potentially crashing the application and causing denial of service.

Affected Products

• NASM Netwide Assembler version 3.02 RC5
• NASM Netwide Assembler versions with vulnerable disasm() implementation

Discovery Timeline

• 2026-04-10 - CVE-2026-6069 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-6069

Vulnerability Analysis

The vulnerability resides in NASM's disasm() function, which is responsible for formatting disassembly output into human-readable text. During the formatting process, the function writes output to a stack-allocated buffer without adequately validating that the slen variable (representing string length) remains within the buffer's boundaries.

When processing certain inputs, the slen value can exceed the buffer capacity, causing the function to write data beyond the allocated stack memory region. This out-of-bounds write corrupts adjacent stack data, which can result in application crashes and denial of service. The vulnerability is particularly concerning because it can be triggered remotely through maliciously crafted assembly source files or object files processed by NASM.

Root Cause

The root cause is insufficient bounds checking in the disasm() function when calculating and using the slen variable during output formatting operations. The function fails to verify that the cumulative length of the formatted disassembly string does not exceed the fixed-size stack buffer before writing additional data. This missing validation allows the write operation to overflow into adjacent stack memory.

Attack Vector

An attacker can exploit this vulnerability by crafting a malicious assembly file that, when disassembled by NASM, causes the disasm() function to generate output exceeding the buffer capacity. The attack can be delivered through:

1. Direct file processing - Convincing a user or automated build system to process a malicious .asm or object file
2. Build pipeline compromise - Submitting malicious assembly code to projects that use NASM in their build process
3. Web-based compilation services - Targeting online assemblers or educational platforms that use NASM

The vulnerability is exploitable over a network vector when NASM processes remotely-sourced files, requires no authentication or privileges, and needs no user interaction beyond normal file processing.

The vulnerability occurs in the disassembly formatting logic where the slen variable tracking output length can grow beyond the stack buffer's allocated size. When the accumulated string length exceeds buffer boundaries, subsequent write operations corrupt adjacent stack memory. Technical details and discussion can be found in the GitHub Issue Discussion.

Detection Methods for CVE-2026-6069

Indicators of Compromise

• Unexpected NASM process crashes during assembly or disassembly operations
• Segmentation fault errors in NASM process logs referencing the disasm() function
• Abnormal assembly files with unusually long or complex instruction sequences in monitored directories
• Core dumps from NASM showing stack corruption patterns

Detection Strategies

• Monitor for NASM process crashes and abnormal terminations in build environments
• Implement file integrity monitoring on assembly source files in critical build pipelines
• Deploy application crash analysis tools to detect stack overflow patterns in NASM
• Use static analysis tools to scan incoming assembly files for potentially malicious constructs

Monitoring Recommendations

• Configure centralized logging for all NASM invocations in CI/CD pipelines
• Set up alerts for repeated NASM process failures that could indicate exploitation attempts
• Monitor system resources for unusual memory consumption during assembly operations
• Track and audit the source of all assembly files processed by NASM in production environments

How to Mitigate CVE-2026-6069

Immediate Actions Required

• Update NASM to the latest patched version that addresses the buffer overflow in disasm()
• Audit and validate all assembly files before processing with NASM
• Restrict NASM execution to trusted input sources only
• Consider running NASM in a sandboxed environment to limit impact of potential crashes

Patch Information

Users should monitor the official NASM project for security updates addressing this vulnerability. Additional details about the issue and potential fixes are available in the GitHub Issue Discussion. Update to the latest stable release once a patched version is available.

Workarounds

• Run NASM in a containerized or sandboxed environment to contain potential crashes
• Implement input validation to reject unusually large or malformed assembly files before processing
• Limit NASM disassembly features usage if not required for your workflow
• Use alternative assemblers temporarily while awaiting an official patch

# Example: Running NASM in a sandboxed environment
# Using firejail to contain potential crashes
firejail --private --net=none nasm -f elf64 input.asm -o output.o

# Alternative: Use Docker for isolation
docker run --rm -v $(pwd):/work:ro nasm-sandbox nasm -f elf64 /work/input.asm -o /work/output.o