CVE-2003-0818 in Windows
Summary
by MITRE
Multiple integer overflows in Microsoft ASN.1 library (MSASN1.DLL), as used in LSASS.EXE, CRYPT32.DLL, and other Microsoft executables and libraries on Windows NT 4.0, 2000, and XP, allow remote attackers to execute arbitrary code via ASN.1 BER encodings with (1) very large length fields that cause arbitrary heap data to be overwritten, or (2) modified bit strings.
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Analysis
by VulDB Data Team • 07/21/2025
The vulnerability described in CVE-2003-0818 represents a critical security flaw within Microsoft's ASN.1 library implementation that affects multiple Windows operating systems including Windows NT 4.0, 2000, and XP. This issue stems from improper handling of ASN.1 Basic Encoding Rules (BER) encodings within the MSASN1.DLL library, which serves as a fundamental component for cryptographic operations and security protocols throughout the Windows ecosystem. The vulnerability specifically targets the way the library processes length fields and bit string modifications during ASN.1 parsing operations, creating exploitable conditions that can be leveraged by remote attackers to gain unauthorized system access.
The technical flaw manifests through two distinct attack vectors that exploit integer overflow conditions within the ASN.1 parsing logic. The first vector involves crafting ASN.1 BER encodings with extremely large length fields that cause heap memory corruption, allowing attackers to overwrite arbitrary heap data and potentially manipulate program execution flow. The second vector exploits modified bit strings within the ASN.1 structure, creating similar memory corruption scenarios that can be used to execute arbitrary code. These integer overflows occur because the vulnerable code does not properly validate or constrain input values before performing arithmetic operations or memory allocations, leading to situations where attackers can manipulate the parsing process to overwrite critical memory locations.
The operational impact of this vulnerability is severe and far-reaching, particularly given the widespread use of the affected Microsoft components. Since MSASN1.DLL is utilized by LSASS.EXE (Local Security Authority Subsystem Service) and CRYPT32.DLL (Cryptographic API), successful exploitation can compromise core security functions of Windows systems. Attackers can leverage this vulnerability to execute arbitrary code with the privileges of the affected service, potentially leading to complete system compromise, privilege escalation, and unauthorized access to sensitive data. The vulnerability affects systems running Windows NT 4.0, 2000, and XP, which were widely deployed in enterprise environments at the time of discovery, making the potential impact substantial.
This vulnerability aligns with CWE-190, Integer Overflow or Wraparound, and maps to several ATT&CK techniques including T1059.001 (Command and Scripting Interpreter: PowerShell) and T1068 (Exploitation for Privilege Escalation) when exploited through the authentication and authorization bypass mechanisms. The attack surface is particularly concerning because it targets fundamental cryptographic libraries that are essential for secure communications and authentication protocols. Organizations running vulnerable systems face significant risk of remote code execution attacks that can be initiated over network connections without requiring authentication, making this vulnerability particularly dangerous in networked environments. The exploitation requires knowledge of ASN.1 BER encoding structures and careful crafting of malicious input, but the impact of successful exploitation can be devastating to system integrity and security posture.
Mitigation strategies for this vulnerability include applying the relevant Microsoft security updates and patches that address the integer overflow conditions in the ASN.1 parsing logic. System administrators should prioritize patch deployment across all affected Windows systems, particularly those running Windows NT 4.0, 2000, and XP, which are no longer supported by Microsoft but may still be in use in legacy environments. Network segmentation and firewall rules can help limit exposure by restricting access to services that utilize vulnerable cryptographic libraries. Additionally, implementing proper input validation and bounds checking in applications that process ASN.1 data can provide additional defense-in-depth measures, though these are secondary to the official vendor patches that directly address the root cause of the integer overflow conditions in MSASN1.DLL.