CVE-2026-81390 in Excel
Summary
by MITRE • 09/09/2026
Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to disclose information locally.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability described constitutes a critical security flaw within the Microsoft Office Excel application, specifically categorized as an out-of-bounds read error. This type of memory corruption issue arises when the software attempts to access data stored at a memory location that is outside the boundaries of the intended buffer or array structure. In the context of spreadsheet processing, this typically occurs during the parsing and rendering of complex formulas, macros, or specially crafted file formats where input validation mechanisms fail to properly verify index values against allocated memory sizes. When such an invalid read operation is triggered, Excel may retrieve data from adjacent memory regions that were not intended for access by the current process context. This behavior represents a classic instance of CWE-125, which defines out-of-bounds read vulnerabilities where software reads past the end or before the beginning of the intended buffer.
The operational impact of this vulnerability centers on local information disclosure rather than remote code execution or privilege escalation in its immediate form. An unauthorized attacker who has gained access to a victim's system can exploit this flaw by constructing a malicious Excel file containing specific data structures designed to trigger the memory boundary violation. Upon opening this crafted document, the application executes the faulty logic and reads sensitive contents from adjacent memory spaces. These disclosed bytes may contain fragments of other applications' data, user credentials stored in clipboard buffers, or internal state information that could aid further exploitation attempts. While the primary consequence is confidentiality compromise through local disclosure, such leaks can serve as a stepping stone for more severe attacks if combined with additional vulnerabilities like heap spraying techniques to achieve arbitrary code execution.
From an offensive security perspective, this vulnerability aligns with specific tactics within the MITRE ATT&CK framework, particularly those related to Collection and Credential Access via Local Data Manipulation or Memory Dumping. Attackers may leverage this flaw as part of a broader social engineering campaign where victims are tricked into opening malicious spreadsheets through phishing emails or compromised shared drives. The exploitation does not require network connectivity but relies heavily on the attacker having some level of initial access to the target machine, whether physical or via remote desktop protocols. This characteristic places it within the realm of local privilege escalation pathways if subsequent steps allow for elevation from user-level privileges to system-level control.
Mitigation strategies must address both immediate remediation and long-term defensive posture improvements. The most effective countermeasure is applying the latest security updates provided by Microsoft, which patch the underlying memory management logic in Excel to enforce strict bounds checking before any read operations occur. Organizations should implement application whitelisting policies to prevent unauthorized execution of untrusted scripts or macros within Office applications. Additionally, enabling Protected View for files downloaded from the internet restricts active content and reduces the attack surface by isolating potentially malicious documents in a sandboxed environment. Regular security awareness training for employees regarding phishing indicators remains crucial since human interaction is often the primary vector through which such crafted files are introduced into secure environments.