CVE-2026-97739 in Office
Summary
by MITRE • 10/15/2026
Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability described involves a critical integer underflow flaw within the core processing engine of Microsoft Office Excel, specifically affecting how certain data structures handle numerical values during parsing and rendering operations. This type of defect typically arises when a calculation results in a value lower than the minimum limit for the specific integer type being used, causing the system to wrap around to a large positive number rather than handling the error gracefully or throwing an exception. In the context of Excel files, this often occurs within complex formulas, embedded objects, or malformed spreadsheet structures where memory allocation is calculated based on these flawed arithmetic operations. When an attacker crafts a malicious file containing specifically designed numeric inputs that trigger this underflow condition, they can manipulate internal pointers or buffer sizes to write data beyond allocated boundaries.
This technical flaw directly facilitates arbitrary code execution over the network because Excel processes untrusted content from various sources including email attachments, shared drives, and web downloads without sufficient validation of input integrity before memory allocation. An unauthorized attacker who successfully exploits this vulnerability by tricking a user into opening a specially crafted document can execute malicious shellcode with the same privileges as the current user. This capability allows for complete system compromise, enabling the installation of malware, data exfiltration, or further lateral movement within an enterprise network environment. The severity is heightened because it does not require complex social engineering beyond convincing the victim to open a file, and modern exploit techniques often leverage this flaw in conjunction with other memory corruption issues to bypass security mitigations such as Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR).
From an industry standards perspective, this vulnerability is classified under CWE-192 Integer Underflow which describes the error where a signed integer value is converted to an unsigned type or arithmetic operation results in a negative number that wraps around. The attack vector aligns with ATT&CK technique T1059 Command and Scripting Interpreter as well as T1203 Exploitation for Client Execution, highlighting how attackers utilize legitimate software features to run malicious code on target systems. The operational impact extends beyond individual device compromise to potential organizational-wide breaches if the vulnerability is leveraged in phishing campaigns targeting multiple employees or distributed through compromised shared repositories where Excel files are routinely accessed and opened by automated scripts or human analysts.
Mitigation strategies must prioritize immediate patching of all affected Microsoft Office versions using official security updates provided by Microsoft, which address the underlying arithmetic checks within the parsing logic to prevent underflow conditions from occurring during file processing. Organizations should also implement application whitelisting policies to restrict the execution of untrusted scripts and binaries that may be dropped as part of an exploit chain following this initial code execution vulnerability. Additionally, enabling Protected View for files originating from potentially unsafe locations can provide a sandboxed environment that prevents active content from executing until explicitly trusted by the user. Network segmentation and strict email filtering rules further reduce the attack surface by limiting exposure to malicious documents distributed via phishing or compromised external sources.