CVE-2026-78517 in Office
Summary
by MITRE • 09/09/2026
Heap-based buffer overflow in Microsoft Office Word allows an unauthorized attacker to execute code over a network.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability described constitutes a critical security flaw within the document processing engine of Microsoft Office Word, specifically manifesting as a heap-based buffer overflow. This type of memory corruption error occurs when a program writes data beyond the allocated boundaries of a buffer located in the computer's random access memory, known as the heap. In the context of Microsoft Office applications, this typically happens during the parsing and rendering of complex document structures or embedded objects. When an attacker crafts a malicious Word document containing specially formatted content with oversized fields or malformed metadata, the application fails to properly validate input lengths before copying data into fixed-size buffers on the heap. This lack of boundary checking allows the overflowed data to overwrite adjacent memory locations, potentially corrupting critical program state information such as function pointers or exception handlers.
The operational impact of this vulnerability is severe due to its potential for remote code execution over a network. An attacker does not need physical access to the target system; instead, they can distribute the malicious document via email attachments, shared drives, or web downloads. When an unsuspecting user opens the crafted file, the exploit triggers during the normal rendering process of Word. By carefully controlling the overflowed data, the attacker can manipulate the execution flow of the application to execute arbitrary code with the same privileges as the current user. This capability allows for full system compromise, enabling the installation of malware, theft of sensitive documents, or use of the compromised machine as a pivot point within an internal network. The ease of exploitation through standard document viewing makes this a high-risk vector for widespread phishing campaigns and targeted attacks against organizations relying on Microsoft Office suites.
From a technical classification perspective, this vulnerability aligns with Common Weakness Enumeration identifier CWE-120, which covers buffer copy without checking size limits in C or C++ programs that lead to heap-based overflows. The exploitation technique often involves techniques categorized under MITRE ATT&CK framework tactics such as Initial Access via Spearphishing Attachment and Execution through Command and Scripting Interpreter if the payload invokes system commands. Furthermore, privilege escalation may be involved depending on how the exploit manipulates memory structures like Function Pointer Overwrites or Stack Pivot attacks to gain higher-level permissions than initially granted by the user context. Understanding these mappings helps security teams prioritize patching efforts based on established industry standards for vulnerability management and risk assessment.
Mitigation strategies must focus primarily on timely application of vendor-supplied patches that address the input validation flaws within the Word rendering engine. Organizations should enforce strict email filtering policies to block or quarantine suspicious attachments, particularly those containing macros or embedded objects from untrusted sources. User awareness training is equally critical, as social engineering often precedes technical exploitation; employees must be educated to recognize and report anomalous documents even if they appear legitimate. Additionally, deploying application whitelisting solutions can prevent the execution of unauthorized scripts or binaries spawned by the exploit process. Network segmentation should also be considered to limit lateral movement in case a single endpoint is compromised, ensuring that the impact remains isolated rather than spreading across critical infrastructure components dependent on Microsoft Office workflows.