CVE-1999-1055 in Excel
Summary
by MITRE
Microsoft Excel 97 does not warn the user before executing worksheet functions, which could allow attackers to execute arbitrary commands by using the CALL function to execute a malicious DLL, aka the Excel "CALL Vulnerability."
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Analysis
by VulDB Data Team • 04/20/2026
The CVE-1999-1055 vulnerability represents a critical security flaw in Microsoft Excel 97 that fundamentally compromised user safety through improper warning mechanisms. This vulnerability specifically targeted the worksheet function execution process within Excel 97, where the application failed to provide users with explicit warnings before executing potentially dangerous functions. The flaw enabled attackers to exploit the CALL function as a vector for code execution, allowing malicious actors to load and execute arbitrary code from external DLL files without user consent or awareness. This design deficiency created a significant attack surface that could be leveraged by threat actors to compromise systems running the affected software version.
The technical implementation of this vulnerability stems from the Excel 97 application's insufficient validation and user notification protocols when processing worksheet functions. The CALL function in Excel 97 was designed to execute external programs or functions, but the application lacked proper security checks that would normally alert users to potentially dangerous operations. When an attacker crafted a malicious Excel file containing a CALL function pointing to a malicious DLL, the application would execute this function without any user intervention or warning prompts. This behavior aligns with CWE-1004, which addresses security weaknesses related to insufficient warning mechanisms during potentially dangerous operations, and represents a classic example of inadequate input validation and user consent procedures in software applications.
The operational impact of this vulnerability extended far beyond simple code execution, creating substantial risks for enterprise environments and individual users alike. Attackers could craft malicious Excel files that would automatically execute harmful code upon opening, potentially installing malware, stealing data, or establishing backdoors on compromised systems. The vulnerability was particularly dangerous because it required no user interaction beyond opening the malicious file, making it an ideal vector for social engineering attacks and drive-by downloads. This characteristic placed the vulnerability squarely within the ATT&CK framework's T1059.005 technique, which covers the execution of malicious code through application shimming and function calls, and demonstrated how seemingly benign spreadsheet functionality could be weaponized for malicious purposes.
Mitigation strategies for CVE-1999-1055 focused on both immediate protective measures and long-term architectural improvements. Users were advised to disable or remove the CALL function from their Excel configurations, implement strict file validation procedures, and avoid opening Excel files from untrusted sources. Organizations needed to establish comprehensive security policies regarding spreadsheet file handling and implement sandboxing techniques for document processing. The vulnerability highlighted the importance of proper application security design principles and led to enhanced security measures in subsequent versions of Microsoft Office. Security vendors developed signature-based detection methods to identify malicious Excel files containing the vulnerable CALL function, while system administrators implemented network-level controls to restrict access to potentially malicious DLL files. This vulnerability ultimately reinforced the critical importance of user awareness training and secure coding practices in preventing exploitation of similar weaknesses in enterprise software environments.