CVE-2026-78503 in Office
Summary
by MITRE • 09/09/2026
Out-of-bounds read in Microsoft Office Word 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 document parsing engine of Microsoft Office Word, specifically manifesting as an out-of-bounds read condition. This type of memory safety error occurs when the application attempts to access data at a memory location that is outside the boundaries of the intended buffer or array structure. In the context of word processing software, this typically happens during the rendering or parsing of complex document elements such as embedded objects, macros, or specific formatting codes contained within a .docx or .dotm file. When an attacker crafts a maliciously formatted Word document containing specially constructed data structures, they can trigger the application to read memory addresses that do not belong to the allocated buffer for that operation. This deviation from expected program flow allows the software to retrieve sensitive information stored in adjacent memory locations, which may include parts of the heap or stack used by other processes or internal system variables.
From a technical perspective, this flaw aligns with Common Weakness Enumeration identifier CWE-125, Out-of-bounds Read. The root cause lies in insufficient validation of input data lengths and indices before accessing memory regions. Instead of verifying that the requested read operation stays within the bounds of the allocated buffer, the parsing logic proceeds to fetch bytes from unauthorized addresses. This lack of boundary checking is a classic example of improper limit enforcement, which has been prevalent in software development for decades but remains particularly dangerous in complex applications like Office suites due to their extensive handling of untrusted external inputs. The vulnerability does not require user interaction beyond opening or previewing the malicious document, making it highly exploitable in phishing campaigns where social engineering is used to trick users into viewing compromised files.
The operational impact of this vulnerability centers on unauthorized information disclosure rather than remote code execution or privilege escalation, although such outcomes can sometimes be chained with other vulnerabilities for more severe effects. By exploiting the out-of-bounds read, an attacker can leak sensitive data from the victim's memory space. This could potentially include authentication tokens, cryptographic keys, clipboard contents, or other proprietary information stored in the application's address space. The attack vector is network-based, meaning it falls under ATT&CK technique T1059, Command and Scripting Interpreter if combined with further exploitation steps, but primarily represents a data exfiltration risk through memory scraping. For organizations relying on Microsoft Office for critical communications, this vulnerability poses a significant threat to confidentiality requirements, potentially leading to compliance violations depending on the nature of the disclosed information.
Mitigation strategies must address both immediate remediation and long-term defensive posture improvements. The primary defense is applying the latest security updates provided by Microsoft, which patch the memory access validation logic within the Word application binaries. Organizations should ensure that all endpoints are configured for automatic Windows Update delivery to minimize exposure windows. Additionally, implementing Application Control policies such as AppLocker or Windows Defender Application Control can prevent unauthorized scripts and macros from executing even if a malicious document is opened. Users must be trained to recognize suspicious attachments and avoid opening documents from untrusted sources. From an architectural standpoint, adopting memory-safe programming languages for future development of office components would significantly reduce the likelihood of similar out-of-bounds errors occurring in subsequent versions.