CVE-2026-78512 in Office
Summary
by MITRE • 09/09/2026
Numeric truncation error in Microsoft Office Word 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 involves a numeric truncation error within the processing logic of Microsoft Office Word, specifically affecting how certain numerical values are handled during document parsing or rendering operations. This flaw arises when the application fails to properly validate the range and precision of integer or floating-point numbers before performing arithmetic operations or memory allocation decisions based on those values. In many cases, this manifests as a classic buffer overflow scenario where a large numeric value is truncated into a smaller data type, such as converting a 64-bit integer into a 32-bit integer without checking for loss of significant bits. This truncation can result in the calculation of an incorrect buffer size or memory offset that is significantly larger than intended by the developer but appears valid within the context of the truncated value. When the application subsequently attempts to write data to this misallocated region, it overwrites adjacent memory structures, potentially including return addresses on the stack or function pointers in heap metadata.
From a technical perspective, this flaw aligns with CWE-190 Integer Overflow or Wraparound and often leads directly to CWE-787 Out-of-bounds Write if the truncated value results in writing beyond allocated boundaries. The exploitation of this vulnerability typically requires an attacker to craft a malicious document file containing specially constructed data fields that trigger the truncation condition upon opening. Because Microsoft Office Word is designed to parse complex document formats including those with embedded objects, formulas, and rich text formatting, it presents a large attack surface for such parsing errors. An unauthorized attacker can distribute this crafted document via email attachments, shared network drives, or web downloads. When a victim opens the file within an affected version of Microsoft Office Word, the application processes the malicious data structure, triggering the numeric truncation error and subsequently allowing arbitrary code execution on the compromised system under the context of the logged-in user.
The operational impact of this vulnerability is severe due to its potential for remote code execution over a network without requiring any additional interaction from the victim beyond opening the document. This aligns with MITRE ATT&CK technique T1203 Exploitation for Client Execution, where attackers leverage software vulnerabilities in client applications to gain initial access. Once executed, the malicious payload can perform various actions depending on the attacker's objectives, including installing backdoors, stealing sensitive data such as credentials or documents stored locally, pivoting to other systems within the network if privileges allow, or establishing persistence mechanisms. The ability to execute code over a network makes this particularly dangerous in enterprise environments where users frequently open attachments from external sources or shared resources that may have been compromised by threat actors.
Mitigation strategies primarily involve applying vendor-provided security patches and updates as soon as they become available for the specific versions of Microsoft Office Word affected by this issue. Organizations should enforce strict patch management policies to ensure all endpoints are updated promptly, reducing the window of exposure. Additionally, implementing application control solutions can prevent unauthorized or untrusted applications from executing code on workstations, thereby limiting the impact even if a vulnerability is exploited. Users should be educated about social engineering tactics and advised not to open documents from unknown or suspicious sources. Network segmentation and monitoring for anomalous behavior following document openings can also help detect potential exploitation attempts in real-time, providing an additional layer of defense against this type of remote code execution attack.