CVE-2026-78515 in Office
Summary
by MITRE • 09/09/2026
Out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to disclose information over a network.
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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 software attempts to access data at a memory location that is outside the boundaries of the intended buffer or array structure. In the context of Excel, this typically occurs during the parsing and rendering of complex spreadsheet files containing specially crafted elements such as formulas, embedded objects, or specific formatting codes. When an attacker constructs a malicious document designed to exploit these parsing routines, they can trigger the application to read memory contents that were not intended for public access. This behavior is fundamentally classified under CWE-125, which defines out-of-bounds read vulnerabilities where software reads past the end of a buffer or array.
The operational impact of this vulnerability centers on unauthorized information disclosure rather than immediate code execution or system compromise. By exploiting the memory leak inherent in an out-of-bounds read, an attacker can potentially extract sensitive data residing in adjacent memory locations. This disclosed information may include internal application states, cached credentials, encryption keys, or other proprietary data structures that were previously loaded into Excel's working memory. While this does not directly allow for remote code execution, it serves as a powerful reconnaissance tool that can facilitate further attacks by revealing critical system details to an unauthorized party over a network connection. The attack vector is primarily remote via the file itself, meaning the user must open or preview the malicious document within the vulnerable version of Excel.
From a threat intelligence perspective, this vulnerability aligns with ATT&CK technique T1005, which covers Data from Local System where an adversary collects data directly from the local system without installing additional software components. The exploitation relies on social engineering to persuade a victim to open a crafted file, leveraging the trust users place in Office applications for legitimate business tasks. Once opened, the malicious payload executes within the context of the user's privileges, allowing it to access and exfiltrate memory contents that might otherwise be protected by standard application sandboxing or isolation mechanisms.
Mitigation strategies must focus on both technical controls and administrative policies. The most effective immediate countermeasure is to apply the latest security updates provided by Microsoft for all affected versions of Office Excel. These patches typically include bounds checking improvements in the parsing engines responsible for handling spreadsheet data structures. Organizations should also enforce strict email filtering rules to block attachments from unknown senders and disable automatic preview pane features that might trigger vulnerability exploitation without explicit user interaction. Additionally, implementing application whitelisting can prevent unauthorized or untrusted executables from running, although this is less effective against file-based exploits unless combined with macro security settings. Users should be trained to recognize suspicious documents and avoid opening files from unverified sources, as human vigilance remains a critical layer of defense against social engineering attacks that deliver such vulnerabilities.