CVE-2026-55949
Summary
by MITRE • 07/14/2026
Use of uninitialized resource in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
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Analysis
by VulDB Data Team • 07/14/2026
This vulnerability represents a critical security flaw in Microsoft Office Excel that stems from improper handling of uninitialized resources during spreadsheet processing operations. The vulnerability occurs when the application fails to properly initialize memory structures or variables before using them, creating opportunities for malicious code execution through crafted spreadsheet files. Such issues typically arise from insufficient input validation and memory management practices within the application's parsing routines.
The technical implementation of this vulnerability involves the exploitation of uninitialized memory regions that may contain residual data from previous operations or system allocations. When Excel processes malformed spreadsheet files containing specially crafted elements, it attempts to access these uninitialized resources without proper initialization checks, potentially leading to arbitrary code execution with the privileges of the affected user. This type of vulnerability falls under the broader category of memory safety issues and aligns with CWE-457 which specifically addresses use of uninitialized variables.
From an operational perspective, this vulnerability poses significant risks to enterprise environments where users frequently open spreadsheet files from untrusted sources. Attackers can craft malicious excel files that trigger the vulnerable code path when opened by victims, enabling remote code execution without requiring user interaction beyond opening the file. The attack surface is particularly broad given Excel's widespread use across different operating systems and the common practice of sharing spreadsheet documents within organizations.
The exploitation process typically involves crafting a malicious .xls or .xlsx file containing specially constructed data structures that cause Excel to access uninitialized memory regions. When the application processes these structures, it may execute attacker-controlled code in the context of the current user session, potentially leading to full system compromise. This vulnerability demonstrates the critical importance of proper resource initialization and input validation practices as outlined in defense-in-depth security principles.
Organizations should implement multiple layers of protection including regular security updates from Microsoft, deployment of application whitelisting solutions, email filtering for suspicious spreadsheet attachments, and user education about avoiding untrusted Office documents. Security controls should also include monitoring for unusual Excel process behavior and implementing sandboxing mechanisms to limit potential damage from successful exploitation attempts. The vulnerability highlights the necessity of following secure coding practices and adheres to ATT&CK technique T1059 which covers execution through various application interfaces including office productivity tools.