CVE-2026-100209 in 365 Apps for Enterprise
Summary
by MITRE • 10/13/2026
Use after free 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 involves a use-after-free error within the Microsoft Office Excel application, which presents a critical security risk by allowing remote attackers to execute arbitrary code over a network. This type of flaw is categorized under CWE-416 in the Common Weakness Enumeration framework, indicating that memory previously allocated and subsequently freed is accessed again without proper re-initialization or bounds checking. In the context of Excel, this typically occurs when processing complex documents containing specific formulas, embedded objects, or malformed data structures that trigger an incorrect sequence of memory management operations during parsing or rendering processes.
When a user opens a specially crafted file designed to exploit this flaw, the application allocates memory for certain internal data structures and later frees them as part of its normal cleanup routine. However, due to a logic error in the code, references to these freed memory blocks are retained and accessed subsequently. This access allows an attacker who controls the content of the document to write arbitrary data into the reclaimed memory space, effectively gaining control over program execution flow. By carefully crafting the payload within the Excel file, an attacker can overwrite function pointers or return addresses on the stack, leading to the execution of malicious code with the privileges of the current user.
The operational impact of this vulnerability is severe because it facilitates remote code execution without requiring any special configuration changes from the victim other than opening a malicious document. This aligns with ATT&CK technique T1203, which covers exploitation for client execution, and often involves techniques such as file creation or modification (T1564) to deliver the exploit via email attachments or shared network drives. Since Excel is widely used in enterprise environments, successful exploitation can lead to full system compromise, data exfiltration, lateral movement within a corporate network, or installation of persistent backdoors and malware.
Mitigation strategies primarily involve applying the latest security updates provided by Microsoft, which patch the underlying memory management logic errors responsible for this use-after-free condition. Organizations should enforce strict email filtering policies to block suspicious attachments and educate users on social engineering tactics that might trick them into opening malicious files. Additionally, enabling Protected View in Excel can provide a sandboxed environment where potentially unsafe documents are opened with limited permissions, reducing the likelihood of successful exploitation even if vulnerable versions remain in use temporarily before patching is complete.