CVE-2021-34518 in Excel
Summary
by MITRE • 07/15/2021
Microsoft Excel Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-34501.
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Analysis
by VulDB Data Team • 07/17/2021
Microsoft Excel contains a remote code execution vulnerability that arises from improper validation of user-supplied data during the processing of specific file formats. This flaw exists in the way Excel handles certain structured data elements within spreadsheet files, particularly when parsing complex formulas or external references that may contain maliciously crafted input. The vulnerability stems from a lack of proper bounds checking and input sanitization mechanisms within the application's parsing routines, allowing attackers to construct specially formatted Excel files that can trigger unintended code execution when opened by vulnerable systems. The flaw specifically affects the Excel application's handling of certain data types and references that are processed during file loading operations, creating an opportunity for arbitrary code execution in the context of the current user.
The technical exploitation of this vulnerability requires an attacker to craft a malicious Excel file containing specifically formatted data structures that will cause the application to execute code when the file is opened. This typically involves manipulating the way Excel processes external data sources or complex formula references, leveraging memory corruption patterns that occur when the application attempts to parse malformed input. The vulnerability allows for privilege escalation from the current user context to the application's execution context, potentially enabling attackers to execute malicious code with the same privileges as the Excel process. This remote code execution capability represents a critical security risk as it can be exploited through various attack vectors including email attachments, web downloads, or malicious file sharing platforms, making it particularly dangerous for enterprise environments where users frequently open spreadsheet files from untrusted sources.
The operational impact of this vulnerability extends beyond simple code execution, as it can lead to complete system compromise when exploited successfully. Attackers can leverage this vulnerability to install backdoors, steal sensitive data, or establish persistent access to compromised systems. The vulnerability affects multiple versions of Microsoft Excel across different operating systems, including Windows and macOS platforms, making it a widespread concern for organizations that rely on spreadsheet applications for business operations. Organizations running affected Excel versions are at risk of data breaches, system infiltration, and potential lateral movement within their networks, as the compromised systems can serve as entry points for further attacks. The vulnerability's remote nature means that exploitation can occur without requiring physical access to target systems, making it particularly concerning for organizations with distributed workforces or cloud-based operations.
Mitigation strategies for this vulnerability should include immediate deployment of Microsoft's security patches and updates to address the identified flaw in Excel's processing routines. Organizations should implement strict file validation policies and consider deploying application control solutions that restrict the execution of potentially malicious files from untrusted sources. Network-based security controls such as email filtering and web proxies can help prevent the delivery of malicious Excel files to end users. Additionally, implementing least privilege principles and regular security awareness training for employees can reduce the impact of successful exploitation attempts. The vulnerability aligns with CWE-121, which addresses stack-based buffer overflow conditions, and may relate to ATT&CK techniques involving execution through malicious files and privilege escalation. Organizations should also consider network segmentation and monitoring solutions to detect anomalous behavior that might indicate exploitation attempts, as the vulnerability can be used as a initial access vector in broader attack campaigns.