CVE-2020-18974 in Netwide Assembler
Summary
by MITRE • 08/25/2021
Buffer Overflow in Netwide Assembler (NASM) v2.15.xx allows attackers to cause a denial of service via 'crc64i' in the component 'nasmlib/crc64'. This issue is different than CVE-2019-7147.
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Analysis
by VulDB Data Team • 08/29/2021
The vulnerability identified as CVE-2020-18974 represents a critical buffer overflow condition within the Netwide Assembler (NASM) version 2.15.xx family, specifically affecting the nasmlib/crc64 component. This flaw manifests when processing the 'crc64i' instruction, creating a potential pathway for malicious actors to execute denial of service attacks against systems utilizing this assembler. The vulnerability exists in the core processing logic responsible for handling cyclic redundancy check operations, where insufficient input validation leads to memory corruption during instruction parsing. Unlike similar vulnerabilities such as CVE-2019-7147, this issue demonstrates distinct characteristics in both its exploitation vector and the specific code module affected, making it particularly challenging to address without comprehensive understanding of the assembler's internal architecture. The buffer overflow occurs at the boundary between expected input parameters and allocated memory space, creating an opportunity for attackers to manipulate memory layout through carefully crafted assembly code sequences.
The technical implementation of this vulnerability stems from inadequate bounds checking within the crc64 module's handling of the 'crc64i' instruction. When NASM processes assembly code containing this specific instruction, the parser fails to properly validate the size and structure of input parameters before attempting to write data to predetermined memory buffers. This deficiency allows attackers to craft malicious input that exceeds the allocated buffer capacity, resulting in memory corruption that can manifest as application crashes, unexpected behavior, or complete system denial of service. The flaw aligns with CWE-121, which categorizes buffer overflow conditions occurring when insufficient bounds checking allows memory to be overwritten beyond its allocated boundaries. The vulnerability's impact is amplified by the widespread use of NASM in software development environments, particularly within systems requiring low-level code generation and embedded software development where assembler reliability is paramount.
The operational implications of CVE-2020-18974 extend beyond simple denial of service scenarios, potentially enabling more sophisticated attack vectors when combined with other vulnerabilities or when executed within specific system contexts. Organizations relying on NASM for code compilation processes face significant risk of service disruption, particularly in automated build environments where assembler failures can cascade into complete development pipeline outages. Attackers could exploit this vulnerability to target development servers, continuous integration systems, or embedded device firmware compilation environments where NASM is extensively used. The vulnerability's exploitation requires minimal privileges and can be executed through standard assembly input manipulation, making it particularly dangerous in multi-tenant environments where a single compromised compilation process could affect multiple projects or organizations. From an attacker perspective, this vulnerability maps to ATT&CK technique T1059.001 for command and control through assembly code injection, though the primary impact remains focused on service disruption rather than privilege escalation.
Mitigation strategies for CVE-2020-18974 should prioritize immediate patching of affected NASM versions to the latest stable releases containing the necessary buffer overflow protections. System administrators should implement input validation measures that restrict assembly code processing to trusted sources and establish monitoring protocols to detect anomalous compilation behavior. Organizations should consider implementing sandboxed compilation environments that isolate assembler processes from critical system resources, reducing the potential impact of successful exploitation attempts. Additionally, regular security assessments of assembly code processing pipelines should be conducted to identify similar vulnerabilities within other components of the development toolchain. The fix typically involves implementing proper bounds checking mechanisms within the crc64 module's instruction parser, ensuring that input parameters are validated against allocated buffer sizes before any memory operations occur. Security teams should also consider implementing automated scanning tools that can detect potentially malicious assembly code patterns before they reach the compilation stage, providing an additional layer of defense against this and similar buffer overflow vulnerabilities.