CVE-2026-77902 in Excel
Summary
by MITRE • 10/14/2026
Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability described constitutes a critical security flaw within the Microsoft Office Excel application, specifically manifesting as a heap-based buffer overflow condition. This type of memory corruption error occurs when a program writes data beyond the allocated boundaries of a buffer located in the heap segment of computer memory. In the context of Excel, this typically involves the processing of complex or malformed spreadsheet files containing specially crafted formulas, objects, or embedded content that triggers an incorrect calculation of buffer sizes during parsing operations. The root cause lies in insufficient validation checks performed by the application before copying data into pre-allocated memory regions, allowing an attacker to overwrite adjacent memory structures with arbitrary values.
From a technical perspective, heap-based overflows are particularly dangerous because they can corrupt internal data structures used by the operating system and the application itself, such as function pointers or exception handlers. When Excel processes the malicious input, it allocates a specific amount of heap space but fails to verify that subsequent write operations remain within those limits. This allows an attacker to inject shellcode or other executable payloads into memory locations that are subsequently executed by the processor. The exploitation mechanism often relies on precise control over the overflow data to achieve code execution privileges equivalent to the user running the application, which is typically a standard user account but can be escalated depending on system configuration and additional vulnerabilities present in the environment.
The operational impact of this vulnerability is severe due to its network-vector nature. Since Microsoft Office Excel is widely used for business communications involving email attachments and shared documents, an attacker can distribute malicious files via phishing emails or compromised websites without requiring physical access to the target machine. Once a user opens the crafted file, the exploit triggers automatically in many scenarios, leading to full system compromise. This enables unauthorized attackers to install malware, steal sensitive data, create backdoors for persistent access, and use the infected host as part of a botnet. The ability to execute code over a network significantly lowers the barrier for entry compared to local-only exploits, making this a high-priority threat in enterprise environments where document sharing is routine.
In terms of industry classification standards, this vulnerability aligns with Common Weakness Enumeration (CWE) identifiers such as CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write on Heap. These categories highlight the fundamental programming errors that allow memory corruption to occur. Furthermore, from a tactical perspective within the MITRE ATT&CK framework, this vulnerability facilitates initial access through spearphishing attachment techniques (T1566.001) and enables privilege escalation or command execution phases depending on how the exploit is chained with other local vulnerabilities. The attack vector leverages social engineering to trick users into opening malicious documents, which serves as a primary entry point for advanced persistent threats targeting corporate networks.
Mitigation strategies must focus on both technical controls and user awareness training. Organizations should immediately apply vendor-supplied security patches that address the memory validation logic within Excel’s parsing engine. It is crucial to maintain an up-to-date patch management policy to ensure these fixes are deployed across all endpoints promptly. Additionally, implementing application control policies such as Microsoft AppLocker or Windows Defender Application Control can prevent untrusted scripts and binaries from running even if they manage to bypass initial defenses. Network segmentation should be utilized to limit lateral movement in case of a successful compromise, reducing the blast radius of any potential breach.
User education remains a critical layer of defense against this specific threat vector. Employees must be trained to recognize suspicious email attachments and avoid opening documents from untrusted sources or unexpected senders. Enabling Protected View for files originating from the internet can provide an additional sandboxed environment that prevents automatic execution of potentially harmful content until explicitly trusted by the user. Furthermore, deploying endpoint detection and response solutions with behavioral analysis capabilities can help identify anomalous processes spawned by Office applications, providing early warning indicators of exploitation attempts before full system compromise occurs.