CVE-2026-6067: Nasm Netwide Assembler RCE Vulnerability

CVE-2026-6067 Overview

A heap buffer overflow vulnerability exists in the Netwide Assembler (NASM) due to a lack of bounds checking in the obj_directive() function. This vulnerability can be exploited by a user assembling a malicious .asm file, potentially leading to heap memory corruption, denial of service (crash), and arbitrary code execution.

Critical Impact

Attackers can craft malicious assembly files that trigger heap memory corruption when processed by NASM, potentially enabling arbitrary code execution or causing application crashes.

Affected Products

• NASM Netwide Assembler version 3.02 rc5
• NASM Netwide Assembler (additional versions may be affected)

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-6067 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-6067

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), a memory safety issue that occurs when the obj_directive() function in NASM fails to properly validate input boundaries before writing data to heap-allocated memory. When processing specially crafted assembly source files, the function can write beyond the allocated buffer boundaries, corrupting adjacent heap memory structures.

The exploitation scenario requires a user to assemble a maliciously crafted .asm file using a vulnerable version of NASM. While this requires local interaction with a malicious file, the consequences can be severe in build pipelines, automated assembly processes, or development environments where untrusted code may be compiled.

Root Cause

The root cause is insufficient bounds checking in the obj_directive() function within NASM's object file generation code. When parsing certain directive constructs in assembly source files, the function allocates a heap buffer but does not adequately verify that subsequent write operations remain within the buffer's boundaries. This allows an attacker to craft input that causes writes past the end of the allocated memory region.

Attack Vector

The attack requires a victim to process a malicious assembly file using a vulnerable version of NASM. The attack vector is classified as network-based since malicious .asm files could be distributed through various means including:

• Compromised source code repositories
• Supply chain attacks on build systems
• Phishing campaigns targeting developers
• Malicious pull requests in open-source projects

The vulnerability manifests when NASM parses object file directives in the malicious assembly source. For technical details regarding the specific triggering conditions, refer to the GitHub Issue Discussion where the vulnerability is documented.

Detection Methods for CVE-2026-6067

Indicators of Compromise

• Unexpected NASM crashes or segmentation faults during assembly operations
• Anomalous memory usage patterns when processing assembly files
• Core dumps or crash reports indicating heap corruption in NASM processes
• Build failures with memory-related error messages

Detection Strategies

• Monitor build systems and CI/CD pipelines for unexpected NASM process terminations
• Implement file integrity monitoring for assembly source files in development environments
• Use memory sanitizers (ASan, MSan) when running NASM in development or testing contexts
• Deploy endpoint detection solutions that can identify heap corruption exploitation attempts

Monitoring Recommendations

• Enable crash reporting and centralized logging for build infrastructure
• Monitor for unusual file access patterns involving .asm files from untrusted sources
• Implement application whitelisting to control which assembler versions are permitted in production environments
• Review source code submissions for suspicious assembly directive patterns

How to Mitigate CVE-2026-6067

Immediate Actions Required

• Audit systems for vulnerable NASM version 3.02 rc5 and earlier potentially affected versions
• Restrict processing of assembly files to trusted sources only
• Implement input validation for assembly files before processing in automated pipelines
• Consider sandboxing or containerizing NASM execution environments

Patch Information

Organizations should monitor the NASM project for security updates addressing this vulnerability. For the latest information on patches and fixes, refer to the GitHub Issue Discussion tracking this vulnerability.

Workarounds

• Avoid processing assembly files from untrusted or unverified sources
• Run NASM in isolated environments such as containers or virtual machines with limited permissions
• Implement strict input validation and sanitization for assembly files before processing
• Consider using alternative assemblers if a patched version of NASM is not available

# Configuration example - Run NASM in a restricted container environment
docker run --rm --read-only --security-opt=no-new-privileges \
  -v /path/to/trusted/source:/src:ro \
  -v /path/to/output:/out \
  nasm-container nasm -f elf64 /src/trusted_file.asm -o /out/output.o