CVE-2021-45256 in NASM
Summary
by MITRE • 12/22/2021
A Null Pointer Dereference vulnerability existfs in nasm 2.16rc0 via asm/preproc.c.
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Analysis
by VulDB Data Team • 12/25/2021
The vulnerability identified as CVE-2021-45256 represents a critical null pointer dereference flaw within the netwide assembler nasm version 2.16rc0. This issue originates from the preprocessor component of the assembler, specifically within the asm/preproc.c source file, where improper input validation leads to a situation where a null pointer is dereferenced during the processing of malformed assembly code. The vulnerability manifests when the preprocessor encounters specific patterns in assembly source files that cause it to attempt accessing memory through a null pointer reference, resulting in an application crash or potential denial of service condition.
The technical exploitation of this vulnerability occurs during the preprocessing phase of assembly code compilation, where the nasm tool processes directives and macros before passing the code to the main assembler. When the preprocessor encounters malformed input that triggers the null pointer dereference, the application terminates abruptly without proper error handling, creating a denial of service scenario for legitimate users attempting to compile assembly code. This flaw operates at the boundary between input parsing and memory management, where insufficient validation of preprocessor directive parameters allows malicious or malformed input to propagate into a critical runtime error condition. The vulnerability aligns with CWE-476 which specifically addresses null pointer dereference issues, and represents a classic example of how improper input validation can lead to application instability.
From an operational perspective, this vulnerability poses significant risks to development environments that rely on nasm for assembly code compilation, particularly in automated build systems and continuous integration pipelines. Attackers could potentially exploit this weakness to disrupt development workflows by causing compilation failures or system crashes, effectively creating a denial of service condition. The impact extends beyond simple service interruption as it could be leveraged in a broader attack chain where an attacker might attempt to cause repeated crashes in development environments, potentially leading to more serious consequences in production systems where assembly code compilation is part of the build process. Organizations using nasm in their toolchains face potential disruption of their development processes, and the vulnerability could be particularly problematic in environments where assembly code is frequently compiled and tested.
Mitigation strategies for CVE-2021-45256 should prioritize immediate patching of the nasm tool to version 2.16 or later where the null pointer dereference has been addressed through proper input validation and error handling mechanisms. System administrators should implement monitoring solutions to detect unusual compilation failures that might indicate exploitation attempts, while developers should ensure their build environments are regularly updated with the latest security patches. The fix implemented by the nasm maintainers typically involves adding null checks before pointer dereference operations within the preprocessor logic, ensuring that all input parameters are properly validated before being processed. Additionally, organizations should consider implementing sandboxing or containerization of assembly compilation processes to limit the potential impact of such vulnerabilities, and should follow ATT&CK framework tactics related to defensive measures against code execution vulnerabilities in development tools. Regular security assessments of development toolchains should include verification of nasm versions and patch status to prevent exploitation of this and similar preprocessor vulnerabilities.