CVE-2026-55039 in Office
Summary
by MITRE • 07/14/2026
Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
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Analysis
by VulDB Data Team • 07/15/2026
Integer underflow vulnerabilities occur when an arithmetic operation results in a value that wraps around to a much smaller value than expected, often leading to unexpected behavior in memory management and control flow. This specific vulnerability exists within Microsoft Office Excel and represents a classic example of improper integer handling that can be exploited by malicious actors. The flaw manifests when the application performs calculations with integer values that, upon underflow, produce results that fall outside the expected range, creating opportunities for attackers to manipulate program execution paths. Such vulnerabilities are particularly dangerous because they often occur in complex software environments where multiple operations interact, making exploitation more subtle and difficult to detect.
The technical implementation of this vulnerability involves scenarios where Excel processes user-supplied data through functions or operations that perform integer arithmetic without proper validation of the resulting values. When an integer underflows, it can cause buffer calculations to become invalid, potentially leading to memory corruption that attackers can leverage for code execution. This type of vulnerability commonly relates to CWE-191, which specifically addresses integer underflow conditions in software implementations. The flaw typically occurs during operations involving array indexing, memory allocation calculations, or loop counter management where negative values from underflow conditions can be used to bypass security checks or manipulate data structures.
Operational impact of this vulnerability extends beyond simple local privilege escalation as attackers can craft malicious Excel files that trigger the underflow condition when opened or processed by the vulnerable application. The attack surface includes scenarios where users open malicious spreadsheets, click on embedded objects, or interact with specially crafted data within Excel documents. Successful exploitation allows unauthorized code execution with the privileges of the user running Excel, potentially enabling full system compromise through techniques such as memory corruption exploitation or privilege escalation attacks. This vulnerability aligns with ATT&CK technique T1059 which covers command and script interpreter usage, as attackers may leverage the executed code to establish persistence or escalate privileges within the compromised environment.
Mitigation strategies for this vulnerability require immediate patching of affected Excel versions through Microsoft security updates, as vendors typically release patches that address specific integer overflow and underflow conditions in their software. Organizations should implement strict application whitelisting policies that limit execution of untrusted Office documents, particularly those received via email or downloaded from untrusted sources. Network segmentation and user access controls can help limit the potential impact if exploitation occurs, while regular security awareness training can reduce the likelihood of users opening malicious files. Additionally, implementing runtime protections such as address space layout randomization and data execution prevention can make exploitation more difficult even if a vulnerability exists in the system.