CVE-1999-1055 is a high-severity vulnerability affecting Microsoft Excel 97, identified as the "Excel CALL Vulnerability." The core issue arises because Excel 97 does not prompt or warn users before executing worksheet functions, specifically the CALL function. This function can be exploited by attackers to execute arbitrary commands by invoking a malicious Dynamic Link Library (DLL). Essentially, an attacker can craft an Excel spreadsheet containing a CALL function that references a malicious DLL. When the spreadsheet is opened, Excel executes the CALL function without any user confirmation, leading to the execution of arbitrary code on the victim's system. This vulnerability impacts the confidentiality, integrity, and availability of the affected system, as arbitrary code execution can lead to data theft, system compromise, or denial of service. The vulnerability is remotely exploitable without authentication (AV:N/AC:L/Au:N), meaning an attacker can deliver a malicious Excel file via email or other means and trigger the exploit simply by the user opening the file. The CVSS score of 7.5 reflects the high risk posed by this vulnerability. Although this vulnerability was disclosed in 1999 and patches were made available (MS98-018), many legacy systems or organizations using outdated software might still be vulnerable. No known exploits in the wild have been reported recently, but the potential for exploitation remains, especially in environments where legacy Excel 97 files are still in use. The vulnerability highlights the risk of executing embedded functions without user consent and the importance of patching and updating software to mitigate such risks.

For European organizations, the impact of this vulnerability can be significant, particularly for entities still reliant on legacy systems or older versions of Microsoft Office. Exploitation could lead to unauthorized code execution, resulting in data breaches, intellectual property theft, or disruption of business operations. Sectors such as finance, government, and critical infrastructure that often handle sensitive data and may have legacy systems in place could face heightened risks. The ability to execute arbitrary code without user interaction increases the threat level, as attackers can deliver malicious Excel files via phishing campaigns or supply chain attacks. This could lead to widespread compromise if malicious documents are distributed internally or externally. Additionally, compromised systems could be used as footholds for lateral movement within networks, escalating the severity of the breach. While modern versions of Excel have mitigations and warnings, organizations with legacy dependencies must be particularly vigilant. The lack of recent known exploits suggests limited active targeting, but the potential impact remains high if exploited.

1. Immediate patching: Apply the security update MS98-018 or later patches that address this vulnerability. Even though the vulnerability is old, ensuring all legacy systems are updated is critical. 2. Upgrade software: Migrate from Microsoft Excel 97 to supported, modern versions of Microsoft Office that include enhanced security features and user prompts for executing embedded functions. 3. Email filtering and attachment controls: Implement advanced email security solutions that scan and block potentially malicious Excel files, especially those containing macros or embedded functions like CALL. 4. User awareness training: Educate users about the risks of opening unsolicited or unexpected Excel files, emphasizing caution with legacy file formats. 5. Application whitelisting: Use application control solutions to restrict execution of unauthorized DLLs or scripts, limiting the ability of malicious code to run even if the file is opened. 6. Network segmentation: Isolate legacy systems to reduce the risk of lateral movement if a compromise occurs. 7. Disable or restrict the CALL function if possible through group policies or Excel settings, to prevent execution of external DLLs from spreadsheets. 8. Monitor and audit: Implement monitoring to detect unusual execution of Excel or DLL loading activities, enabling rapid detection of exploitation attempts.