CVE-2026-6068: Nasm Assembler Use-After-Free Vulnerability

CVE-2026-6068 Overview

A heap use after free vulnerability exists in NASM (Netwide Assembler) within the response file (-@) processing functionality. The vulnerability occurs when a dangling pointer to freed memory is stored in the global depend_file variable and subsequently dereferenced. This happens because the response-file buffer is freed before the pointer is used, potentially allowing for data corruption or unexpected application behavior.

Critical Impact

This use after free vulnerability in NASM's response file processing could lead to data corruption or unpredictable behavior when processing maliciously crafted response files.

Affected Products

• NASM Netwide Assembler version 3.02 RC5
• Potentially other versions utilizing the same response file parsing logic

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-6068 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-6068

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to reference memory after it has been freed. In NASM's case, the flaw manifests in how response files are processed when using the -@ command-line option.

The core issue stems from improper lifecycle management of memory allocated for processing response files. When NASM parses a response file, it allocates a buffer to store the file contents. A pointer from this buffer is assigned to the global depend_file variable. However, the buffer is deallocated before depend_file is subsequently accessed, creating a classic use after free condition where the pointer references memory that may have been reallocated or corrupted.

Root Cause

The root cause is a memory management error in the response file handling code path. The sequence of operations that leads to this vulnerability involves:

1. Allocation of a buffer for response file contents
2. Assignment of a pointer within this buffer to the global depend_file variable
3. Premature deallocation of the response file buffer
4. Later dereferencing of depend_file, which now points to freed memory

This ordering violation means that when depend_file is accessed after the buffer is freed, the program reads from memory that may contain arbitrary data, leading to undefined behavior.

Attack Vector

An attacker could exploit this vulnerability by crafting a malicious response file and tricking a user into assembling code that references it via the -@ flag. The network attack vector indicates that such malicious files could be distributed through various channels including:

• Malicious repositories containing poisoned build scripts
• Compromised development environments
• Supply chain attacks targeting software projects that use NASM

Since NASM is commonly used in build systems and development toolchains, the vulnerability could potentially be exploited in automated build pipelines where response files are fetched from external sources.

For detailed technical information about this vulnerability, see GitHub Issue #222.

Detection Methods for CVE-2026-6068

Indicators of Compromise

• Unexpected crashes or segmentation faults when running NASM with the -@ response file option
• Memory corruption warnings from address sanitizers (ASan) during NASM execution
• Anomalous NASM process behavior including unexpected memory access patterns

Detection Strategies

• Deploy AddressSanitizer (ASan) or similar memory debugging tools when running NASM in development environments to detect use after free conditions
• Monitor build systems for unusual NASM invocations, particularly those using the -@ flag with untrusted input sources
• Implement file integrity monitoring on response files used in build pipelines to detect tampering

Monitoring Recommendations

• Review and audit response files used in automated build systems before execution
• Implement logging for all NASM invocations in CI/CD pipelines, capturing command-line arguments including response file paths
• Consider sandboxing NASM execution in build environments to contain potential exploitation

How to Mitigate CVE-2026-6068

Immediate Actions Required

• Audit all projects using NASM to identify instances where response files (-@ option) are utilized
• Ensure response files come only from trusted, verified sources
• Consider temporarily avoiding the -@ response file option if possible, using direct command-line arguments instead
• Monitor the NASM GitHub repository for patch availability

Patch Information

As of the last update on 2026-04-16, users should monitor the official NASM project for security patches addressing this use after free vulnerability. Check the NASM GitHub repository for updates on the fix status.

Workarounds

• Avoid using the -@ response file option when processing untrusted input
• If response files must be used, validate their contents and ensure they originate from trusted sources only
• Run NASM in isolated or sandboxed environments to limit the impact of potential exploitation
• Use memory debugging tools like ASan during development to detect memory corruption issues early

# Alternative: Use direct command-line arguments instead of response files
# Instead of: nasm -@ response.txt source.asm
# Use direct arguments:
nasm -f elf64 -o output.o source.asm

# If response files are required, validate the source
# and run NASM in a restricted environment