CVE-2021-40486 in Office
Summary
by MITRE • 10/13/2021
Microsoft Word Remote Code Execution Vulnerability
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Analysis
by VulDB Data Team • 10/15/2021
Microsoft Word contains a remote code execution vulnerability that arises from improper handling of specially crafted embedded objects within word processing documents. This flaw exists in the way Word processes certain file formats and can be exploited through maliciously crafted documents delivered via email attachments or malicious websites. The vulnerability stems from insufficient validation of embedded content during document parsing, allowing attackers to execute arbitrary code on vulnerable systems with the privileges of the logged-on user. When a user opens a malicious document, the crafted embedded object triggers a memory corruption condition that can be leveraged to gain remote code execution capabilities. The vulnerability affects multiple Microsoft Word versions including Office 2016, Office 2019, and Office 2021, as well as Microsoft 365 applications. This issue is classified as a buffer overflow vulnerability that aligns with CWE-121, which deals with stack-based buffer overflow conditions. The attack vector typically involves social engineering tactics where users are tricked into opening malicious documents that contain specially crafted embedded objects designed to exploit the memory corruption flaw. From an operational perspective, this vulnerability poses significant risk to enterprise environments where users frequently open email attachments and documents from external sources. The exploitation requires minimal user interaction beyond opening the malicious document, making it particularly dangerous in targeted attack scenarios. Attackers can leverage this vulnerability to establish persistent access, escalate privileges, and potentially move laterally within networks. The vulnerability also aligns with ATT&CK technique T1204.002 which involves user execution of malicious files through social engineering. Microsoft has released security updates addressing this vulnerability through the August 2021 security bulletin, which includes patches for all supported Office versions. Organizations should implement layered security controls including email filtering, user education, and regular security updates to mitigate the risk. The vulnerability demonstrates the ongoing challenge of securing complex office productivity applications where document parsing logic must handle numerous file formats and embedded content types while maintaining security boundaries.
The technical implementation of this vulnerability involves the manipulation of embedded object structures within Microsoft Word documents to trigger memory corruption during document rendering. When Word encounters a specially crafted embedded object, the parsing logic fails to properly validate the object's size and structure, leading to buffer overflow conditions in memory. This allows attackers to overwrite adjacent memory locations with malicious code payloads that can be executed in the context of the Word process. The vulnerability is particularly concerning because it operates at the application layer and can be triggered without requiring elevated privileges, making it suitable for initial access and persistence mechanisms. The exploitation process typically involves crafting a document with embedded objects that contain malicious code, often using techniques such as heap spraying or return-oriented programming to ensure successful code execution. The vulnerability affects both local and remote exploitation scenarios, as attackers can deliver malicious documents through various channels including email, web downloads, or removable media. Security researchers have identified that the vulnerability is particularly prevalent in environments where users frequently interact with external documents or where document sanitization policies are insufficient. The memory corruption aspect of this vulnerability places it within the category of memory safety issues that are commonly exploited in advanced persistent threat campaigns. Microsoft's patch addresses the core parsing logic by implementing additional validation checks and bounds verification for embedded object handling. The vulnerability's classification as a remote code execution flaw means that it can be exploited across network boundaries without requiring physical access to target systems. Organizations implementing security controls should consider deploying application whitelisting solutions that restrict execution of untrusted Office documents and implement network-based intrusion detection systems to monitor for exploitation attempts. The vulnerability also highlights the importance of maintaining current security patches and implementing defense-in-depth strategies that include email security appliances, sandboxing technologies, and endpoint detection and response solutions to detect and prevent exploitation attempts.
Organizational impact of this vulnerability extends beyond immediate exploitation risks to encompass broader security governance and incident response considerations. The vulnerability's ease of exploitation through social engineering makes it particularly dangerous in environments where user awareness training is inadequate or where document handling policies are not properly enforced. Security teams must consider the potential for widespread compromise when users open malicious documents, as the vulnerability can lead to full system compromise and data exfiltration. The vulnerability also impacts business continuity operations, as successful exploitation can result in system downtime, data loss, and regulatory compliance violations. Organizations with insufficient patch management processes are particularly vulnerable, as the exploitation window increases significantly when systems remain unpatched. The vulnerability's characteristics align with threat actor methodologies that prioritize easy-to-exploit vulnerabilities for initial access, making it a preferred target for both nation-state actors and cybercriminal organizations. Incident response teams must be prepared to handle potential exploitation attempts, including forensic analysis of affected systems and network monitoring for lateral movement activities. The vulnerability also demonstrates the importance of maintaining current threat intelligence feeds to identify emerging exploitation patterns and attack signatures. Organizations should implement automated patch deployment processes and conduct regular vulnerability assessments to identify and remediate similar issues across their environments. The security implications of this vulnerability extend to supply chain considerations, as malicious documents could potentially be introduced through trusted vendor communications or third-party document sharing platforms. Regular security awareness training programs should emphasize the dangers of opening unexpected document attachments and encourage users to verify document sources before opening potentially malicious content. The vulnerability also underscores the need for robust email security solutions that can identify and quarantine suspicious document types before they reach end-user inboxes. Microsoft's security bulletin for this vulnerability includes recommendations for both immediate remediation and long-term security posture improvements to prevent similar issues from occurring in the future.