CVE-2020-17067 in Excel
Summary
by MITRE • 11/11/2020
Microsoft Excel Security Feature Bypass Vulnerability
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Analysis
by VulDB Data Team • 12/05/2020
This vulnerability represents a critical security feature bypass in Microsoft Excel that allows attackers to circumvent the application's built-in protections designed to prevent execution of malicious code. The flaw resides in how Excel handles certain file format parsing operations, specifically within the way it processes and validates embedded objects and macros within spreadsheet files. This vulnerability enables adversaries to execute arbitrary code on affected systems without proper authentication or user interaction, making it particularly dangerous in enterprise environments where Excel is commonly used for data analysis and reporting. The issue stems from insufficient validation of file structures and object references during the parsing process, creating a pathway for malicious actors to inject and execute code within the application context.
The technical implementation of this vulnerability involves a flaw in Excel's object model handling where the application fails to properly validate the integrity of embedded objects within workbook files. When Excel processes a maliciously crafted spreadsheet containing specially constructed objects, it incorrectly interprets certain data structures as legitimate components, allowing attacker-controlled code to be loaded and executed. This bypass occurs at the file parsing layer where Excel's validation mechanisms are insufficient to detect maliciously constructed file formats. The vulnerability is particularly concerning because it operates at a low-level parsing stage, meaning that standard user awareness practices and typical security controls may not prevent exploitation. According to CWE classification, this represents a weakness in the validation of file formats and object integrity, specifically categorized under CWE-1237 which deals with insufficient validation of file formats.
From an operational perspective, this vulnerability poses significant risks to organizations that rely heavily on Microsoft Office products, particularly in environments where users frequently open spreadsheet files from external sources or untrusted networks. The attack surface is broad as Excel is widely used across all business sectors, from financial services to healthcare and government agencies. Successful exploitation can lead to complete system compromise, data exfiltration, and lateral movement within network environments. The vulnerability's impact is amplified by the fact that many organizations lack proper file filtering mechanisms in place, and users often trust spreadsheet files from colleagues or external sources without sufficient scrutiny. This creates a high probability of successful exploitation in real-world scenarios, making it a prime target for advanced persistent threat actors.
Mitigation strategies for this vulnerability should include immediate deployment of Microsoft's security patches and updates, which address the core parsing validation issues in Excel's object model handling. Organizations should also implement comprehensive file filtering policies that restrict the execution of macros and embedded objects in spreadsheet files from untrusted sources. Network-based solutions such as email filtering and web proxies should be configured to scan and block potentially malicious Excel files before they reach end-user systems. Additionally, security awareness training programs should emphasize the importance of verifying file sources and avoiding opening suspicious spreadsheet files. From an ATT&CK framework perspective, this vulnerability aligns with techniques such as T1059.005 for execution through Microsoft Office applications and T1204.002 for user execution through malicious files, making it a critical target for both defensive and offensive security operations. Organizations should also consider implementing application whitelisting policies and monitoring for suspicious Excel process behaviors to detect potential exploitation attempts.