CVE-2026-78520 in Office
Summary
by MITRE • 09/09/2026
Out-of-bounds read in Microsoft Office Outlook allows an unauthorized attacker to execute code 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 Outlook application, specifically involving an out-of-bounds memory read operation that can be exploited by remote attackers to achieve arbitrary code execution. This type of issue is classified under Common Weakness Enumeration (CWE) as CWE-125, which refers to Out-of-Bounds Read. The core technical flaw arises when the software fails to properly validate array indices or buffer boundaries before accessing memory locations. In the context of Outlook, this often occurs during the parsing and rendering of complex email content, such as HTML messages with embedded objects, stylesheets, or specific font definitions. When an attacker crafts a maliciously formatted message containing specially constructed data structures, they can trigger the application to read from memory addresses that lie outside the intended buffer limits. While out-of-bounds reads are traditionally associated with information disclosure rather than code execution, in this specific instance, the improper handling of these invalid memory accesses likely leads to heap corruption or allows for controlled overwrites through subsequent operations, thereby facilitating remote code execution.
From an operational perspective, the impact of this vulnerability is severe due to its network-based exploitability vector. An attacker does not need physical access to the target machine nor do they require user interaction beyond receiving and viewing a malicious email message within Outlook. Once the vulnerable component processes the crafted payload, it can execute arbitrary code with the privileges of the current user. If the affected system is configured with elevated permissions or if the user operates as an administrator, the attacker gains full control over the compromised computer. This level of access enables a wide range of malicious activities including data theft, installation of backdoors for persistent remote access, deployment of ransomware, and use of the machine as part of a botnet. The ability to execute code remotely makes this vulnerability particularly dangerous in enterprise environments where Outlook is heavily utilized for daily communication, increasing the attack surface significantly.
This exploit scenario aligns with several tactics within the MITRE ATT&CK framework. Specifically, it relates to Initial Access via Spearphishing Attachment or Link, as well as Execution through Command and Scripting Interpreter if the executed code invokes system commands. The exploitation technique falls under Exploitation for Remote Code Execution (RCE), leveraging memory corruption vulnerabilities which are historically among the most impactful classes of software defects. Defense in Depth strategies must be employed to mitigate this risk since patch availability may vary across different versions of Office and Outlook deployed within an organization. Immediate mitigation involves applying the latest security updates provided by Microsoft, which address the boundary checks in the affected components. Additionally, organizations should enforce strict email filtering policies that sanitize incoming HTML content and disable unnecessary features such as ActiveX controls or script execution for untrusted senders. User awareness training is also critical to prevent users from opening suspicious emails, although technical mitigations remain the primary defense against automated exploitation attempts targeting this specific memory corruption flaw.