CVE-2026-66810 in 365 Apps for Enterprise
Summary
by MITRE • 08/12/2026
Heap-based buffer overflow in Microsoft Office Word allows an unauthorized attacker to disclose information locally.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 08/12/2026
A heap-based buffer overflow vulnerability exists in Microsoft Office Word that enables unauthorized attackers to achieve local information disclosure through maliciously crafted documents. This vulnerability stems from improper input validation and memory management within the word processing application's handling of specific document elements. The flaw occurs when Word processes certain structured data formats or embedded objects that trigger memory allocation errors, leading to buffer overflow conditions in heap memory segments. Attackers can exploit this weakness by crafting specially designed office documents containing malformed data structures that cause the application to write beyond allocated memory boundaries. The vulnerability is particularly concerning because it operates within the legitimate application context, making detection more challenging for security systems. When exploited successfully, the buffer overflow allows attackers to read sensitive information from adjacent memory locations, potentially exposing confidential data such as encryption keys, passwords, or other system credentials stored in nearby memory regions. This type of vulnerability falls under CWE-121 heap-based buffer overflow category and aligns with attack patterns documented in the ATT&CK framework under privilege escalation and credential access techniques. The exploitation typically requires minimal user interaction beyond opening the malicious document, making it particularly dangerous in targeted attack scenarios where social engineering can be minimized. The vulnerability's impact extends beyond simple information disclosure as it may serve as a stepping stone for more sophisticated attacks including privilege escalation or lateral movement within compromised environments.
The technical implementation of this vulnerability involves Word's memory management routines failing to properly validate the size of data structures during document parsing operations. When processing specific combinations of formatting elements, embedded objects, or complex table structures, the application allocates insufficient heap memory for expected data payloads. This misalignment creates writable buffer regions that can be overwritten with attacker-controlled data, potentially causing information leakage through memory corruption patterns. The heap-based nature indicates that the overflow occurs in dynamically allocated memory segments rather than stack-based buffers, which affects how exploitation techniques are applied and how memory layout is analyzed. Security researchers have identified that this vulnerability manifests primarily when processing documents containing advanced formatting features or complex embedded content such as charts, tables with specific formatting properties, or custom object embeddings. The information disclosure aspect suggests that the memory corruption does not immediately crash the application but instead allows for controlled data extraction from adjacent memory locations. This characteristic makes the vulnerability particularly attractive to attackers who prefer stealthy exploitation methods over obvious crashes that would alert users to malicious activity.
The operational impact of this vulnerability extends significantly beyond simple local information disclosure, as it provides attackers with access to sensitive data that could compromise system integrity and confidentiality. The vulnerability affects Microsoft Office Word installations across multiple versions and platforms, making it a widespread concern for enterprise environments where document processing is common. Organizations using Word for business-critical applications face heightened risk since attackers can potentially extract confidential business information, intellectual property, or user credentials from memory segments during normal document processing operations. The vulnerability's exploitation requires minimal privileges and can be automated through malicious document delivery mechanisms such as email attachments or compromised websites. Security professionals have noted that this type of vulnerability often remains undetected for extended periods because it does not cause immediate application crashes and the information disclosure may appear benign to casual monitoring systems. The attack vector typically involves social engineering campaigns where users unknowingly open malicious documents, making user education and awareness critical components of defense strategies. Incident response teams must consider this vulnerability when investigating security breaches since it can serve as an initial access point or reconnaissance mechanism for broader attacks.
Mitigation strategies for this heap-based buffer overflow vulnerability should encompass both immediate patching operations and comprehensive security hardening measures. Microsoft has released security updates addressing this specific vulnerability through regular monthly patches, requiring organizations to maintain active update management processes and apply patches promptly. Security administrators should implement application control policies that restrict execution of potentially malicious documents from untrusted sources while monitoring document processing activities for unusual behavior patterns. Memory protection mechanisms such as data execution prevention and address space layout randomization can significantly reduce exploitation success rates by making memory corruption attacks more difficult to execute reliably. Network-based intrusion detection systems should be configured to monitor for suspicious document-related network traffic patterns that might indicate attempts to deliver malicious documents containing this vulnerability. Organizations should conduct regular security assessments focusing on document processing workflows and implement sandboxing techniques for handling untrusted office documents. Additionally, implementing strict access controls and privilege separation can limit the potential damage from successful exploitation attempts, ensuring that even if an attacker gains access through this vulnerability, they cannot escalate privileges or access additional system resources without further compromise. The vulnerability also highlights the importance of maintaining up-to-date security tooling and monitoring capabilities that can detect anomalous behavior patterns associated with memory corruption attacks.