CVE-2026-78506 in Word
Summary
by MITRE • 09/09/2026
Improper null termination in Microsoft Office Word allows an unauthorized attacker to disclose information locally.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability described involves an improper handling of string termination within the document processing engine of Microsoft Office Word, specifically affecting how certain data structures are managed during file parsing operations. This flaw stems from a failure by the application to correctly append or verify null terminators for specific character arrays or memory buffers allocated when reading complex document components such as embedded objects, macros, or formatted text blocks. In C and C++ based software architectures like those underlying Microsoft Office, strings must be explicitly terminated with a null byte (0x00) to signal the end of data to subsequent processing functions. When this termination is missing or incorrectly calculated due to off-by-one errors or logic flaws in boundary checks, the application may continue reading beyond the intended buffer boundaries into adjacent memory regions that were not allocated for that specific string operation.
From a technical perspective, this constitutes an out-of-bounds read condition where the execution flow attempts to access memory locations outside the designated safe zone of the input data structure. The operational impact of such a flaw is primarily localized information disclosure rather than remote code execution or privilege escalation in its immediate form. An attacker who can trick a user into opening a specially crafted Word document containing malformed string fields could cause the application to leak sensitive contents from adjacent memory spaces. These leaked bytes might contain partial credentials, session tokens, private keys, or other confidential data currently held in the process address space by Office or related system services. The severity of this disclosure depends heavily on what specific data resides in the neighboring memory at the time of exploitation and whether additional vulnerabilities exist that could amplify the impact into a full compromise scenario.
This type of vulnerability aligns with Common Weakness Enumeration category CWE-170, which defines improper null termination leading to information exposure or buffer over-read conditions. It also relates closely to CWE-126 regarding buffer over-read, where software reads past the end of a buffer but does not necessarily overwrite data. In terms of adversarial tactics, this behavior corresponds to ATT&CK technique T1005, which covers Data from Local System Retrieval through file or directory access mechanisms that exploit application flaws to extract internal state information without direct user interaction beyond opening the malicious document. The attack vector is typically classified as local because it requires physical or logical access to deliver the crafted payload via social engineering tactics such as phishing emails with attached documents, although in enterprise environments where users routinely open external files, this effectively functions as a remote delivery mechanism for initial compromise stages focused on reconnaissance and data gathering rather than immediate system takeover.
Mitigation strategies must address both technical controls and user awareness protocols to reduce the risk surface associated with document-based attacks. Organizations should ensure that Microsoft Office applications are updated to the latest cumulative patches released by Microsoft, which typically include fixes for memory management flaws in parsing engines alongside other security hardening measures. Deploying application control policies such as Windows Defender Application Control or AppLocker can restrict the execution of untrusted scripts and macros often used in conjunction with document exploits. Additionally, enabling Protected View for files originating from the internet forces Word to open documents in a sandboxed environment that isolates potentially malicious content from the main system memory, thereby preventing direct access to sensitive data structures even if parsing errors occur. Users should be trained to verify sender authenticity and avoid opening attachments from unknown sources, while IT administrators can disable ActiveX controls and macro execution by default through Group Policy settings to further limit the attack surface available for exploitation chains that might leverage this information disclosure flaw as a stepping stone toward more severe outcomes like remote code execution via subsequent memory corruption bugs.