CVE-2026-97778 in Excel
Summary
by MITRE • 10/15/2026
Integer overflow 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 overflow condition within the core processing engine of Microsoft Office Excel, specifically affecting how certain data structures and memory allocations are handled during file parsing operations. This flaw arises when the application fails to properly validate or bound-check numerical values associated with cell dimensions, formula calculations, or object properties before allocating corresponding memory buffers. When an attacker crafts a malicious spreadsheet containing specially constructed numeric inputs that exceed the maximum limit of the integer type used by Excel for internal tracking, the arithmetic operation wraps around from its highest positive value to a negative number or a significantly smaller positive value due to two's complement representation rules in C and C++ based software architectures. This miscalculation leads the application to allocate an insufficient amount of memory relative to what is actually required by the subsequent operations on that data, creating a classic heap-based buffer overflow scenario where write operations proceed beyond the boundaries of the allocated block into adjacent memory regions.
From a technical perspective, this integer wraparound represents a fundamental failure in input validation and boundary checking, which are critical components of secure coding practices for handling untrusted data sources such as externally provided files. The vulnerability aligns with Common Weakness Enumeration identifier CWE-190, known as Integer Overflow or Wraparound, often serving as the precursor to more severe memory corruption vulnerabilities like buffer overflows (CWE-787) or out-of-bounds writes (CWE-788). In the context of Microsoft Office applications, Excel processes complex formulas and formatting structures that require dynamic memory management. When the integer overflow occurs during the parsing phase, it tricks the application into believing a large data structure is small enough to fit within a limited buffer space. Consequently, when the program attempts to populate this buffer with actual content from the malicious file, it overwrites adjacent heap metadata or other critical pointers in memory. This corruption can be leveraged by an attacker to redirect execution flow, potentially allowing for arbitrary code execution on the victim's system under the context of the currently logged-in user.
The operational impact of this vulnerability is severe due to its network-vector capability and high exploitability potential. Since Microsoft Office Excel is widely used across enterprise environments for data analysis, reporting, and collaboration, it serves as a frequent target for phishing campaigns and drive-by download attacks. An attacker can distribute the malicious spreadsheet via email attachments or host it on a compromised website designed to trick users into opening the file through social engineering tactics. Once opened, even if macro execution is disabled by default security settings, this specific vulnerability in the native parsing engine allows code execution without requiring user interaction beyond simply viewing or loading the document. This bypasses many traditional perimeter defenses and endpoint protection mechanisms that rely on detecting malicious macros or scripts, making it a potent tool for initial access in advanced persistent threat campaigns. The ability to execute arbitrary code means attackers can install backdoors, exfiltrate sensitive data, pivot within internal networks, or establish persistence through standard user-level privileges which may escalate further depending on system configuration and other co-existing vulnerabilities.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. Organizations should immediately apply the latest security patches released by Microsoft to update Excel and related Office components, ensuring that the specific code paths responsible for integer validation are corrected with proper bounds checking logic. Administrators should enforce strict application control policies using tools like Windows Defender Application Control or AppLocker to restrict the execution of untrusted binaries and scripts, thereby limiting the impact if an exploit is successfully deployed. Additionally, implementing network segmentation can help contain lateral movement in case a workstation becomes compromised. From a defensive engineering standpoint, developers must integrate static analysis tools that detect integer overflow patterns during code review phases and adopt memory-safe programming languages or libraries where feasible to eliminate entire classes of such vulnerabilities. Users should be educated on the risks associated with opening files from untrusted sources and enabled features like Protected View which isolates potentially unsafe documents in a sandboxed environment before allowing full access, providing an additional layer of defense against document-based attacks that exploit parsing flaws similar to this integer overflow issue.