CVE-2026-78511 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 Microsoft Office Word application, specifically manifesting as a heap-based buffer overflow condition. This type of memory corruption error occurs when a program writes more data to a dynamically allocated block of memory on the heap than it was intended to hold. In the context of Microsoft Office Word, this typically arises during the parsing or rendering of complex document structures, such as those containing embedded objects, specific formatting codes, or maliciously crafted file formats like DOCX or RTF. The attacker exploits this flaw by constructing a specially designed document that triggers an out-of-bounds write operation within the application's memory space. Because heap overflows can corrupt adjacent memory structures, they provide a powerful mechanism for attackers to manipulate program execution flow, often leading to arbitrary code execution with the privileges of the currently logged-in user.
From a technical perspective, this vulnerability aligns closely with Common Weakness Enumeration (CWE) identifiers such as CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write. The root cause lies in insufficient validation of input data lengths before copying them into fixed-size buffers allocated on the heap. When an attacker supplies a payload that exceeds the expected buffer size, the excess data overwrites neighboring memory regions. This corruption can overwrite function pointers or exception handling structures within the process address space. By carefully crafting the overflow content to include shellcode and precise return addresses, an unauthorized actor can hijack the control flow of the Word application. Once execution is redirected to the attacker's code, it runs in the context of the user who opened the document, potentially allowing full compromise of the workstation or access to sensitive corporate data stored locally.
The operational impact of this vulnerability is severe due to its network-vector capability and high exploitability potential. Since Microsoft Office Word is widely used across enterprise environments for creating and viewing documents, it represents a prime target for phishing campaigns and drive-by download attacks. An attacker can distribute the malicious document via email attachments or host it on a compromised website. When a victim opens the file, even if macro execution is disabled by default security settings, the underlying rendering engine vulnerability may still be triggered depending on the specific version and patch level of Office installed. Successful exploitation grants the adversary remote code execution capabilities without requiring user interaction beyond opening the document. This facilitates initial access into internal networks, enabling lateral movement, data exfiltration, or deployment of additional malware such as ransomware or command-and-control agents.
To mitigate this risk, organizations must prioritize immediate patching by applying the latest security updates released by Microsoft for all affected versions of Office and Word. These patches typically include fixes that enforce stricter bounds checking during memory allocation operations and improve input validation logic within the document parsing components. In addition to software remediation, defensive measures should be implemented at the network perimeter using intrusion prevention systems configured with signatures capable of detecting exploit attempts targeting this specific vulnerability. Endpoint detection and response solutions can also monitor for anomalous behavior patterns associated with heap corruption exploits, such as unusual memory access violations or unexpected process spawning from Office applications. Furthermore, user awareness training is essential to reduce the likelihood of users opening unsolicited documents, serving as a critical human layer in defense against social engineering tactics that leverage this technical flaw.