CVE-2026-55026 Overview
CVE-2026-55026 is an integer overflow vulnerability [CWE-190] in Microsoft Office that allows an authenticated local attacker to disclose sensitive information. The flaw affects multiple Office versions including Microsoft 365 Apps, Office 2016, 2019, 2021, 2024, and SharePoint Server. Exploitation requires local access and low privileges, with no user interaction needed. Successful exploitation leads to high confidentiality impact without affecting integrity or availability.
Critical Impact
An authorized local attacker can trigger an integer overflow condition in Microsoft Office to read sensitive memory contents, exposing confidential document data or process memory.
Affected Products
- Microsoft 365 Apps (Enterprise x64 and x86), Microsoft 365 for macOS
- Microsoft Office 2016, 2019, 2021 LTSC, and 2024 LTSC (Windows and macOS variants)
- Microsoft SharePoint Server (Subscription Edition, 2016 Enterprise, 2019)
Discovery Timeline
- 2026-07-14 - CVE-2026-55026 published to NVD
- 2026-07-16 - Last updated in NVD database
Technical Details for CVE-2026-55026
Vulnerability Analysis
The vulnerability stems from an integer overflow or wraparound condition [CWE-190] in Microsoft Office file parsing or memory allocation routines. When Office processes a specially crafted document, an arithmetic operation exceeds the maximum value representable in the target integer type. This wraparound produces an undersized allocation or incorrect boundary calculation. Subsequent memory operations then read beyond intended buffer boundaries, exposing adjacent memory contents to the attacker.
The exposed data may include sensitive information from other documents, credential material, or process memory fragments. The scope remains unchanged, meaning the attacker gains information only within the vulnerable Office process context. No integrity or availability impact occurs because the flaw enables reading rather than writing memory.
Root Cause
The root cause is insufficient validation of arithmetic operations on size or length values during document parsing. When crafted input drives a calculation past integer boundaries, the resulting wrapped value bypasses subsequent length checks. This mismatch between allocated buffer size and the amount of data processed enables out-of-bounds memory disclosure.
Attack Vector
Exploitation requires local access to a system running a vulnerable Office version and low-privilege authentication. An attacker delivers a crafted Office document to the target machine and induces the victim process to open or parse it. The attack does not require user interaction beyond the initial file processing. See the Microsoft Security Update CVE-2026-55026 advisory for additional technical detail.
No public proof-of-concept code is available at this time. The EPSS score is 0.373%, indicating a low probability of exploitation activity in the near term.
Detection Methods for CVE-2026-55026
Indicators of Compromise
- Unexpected Office application crashes or abnormal process termination when opening documents from untrusted sources
- Anomalous memory access patterns by winword.exe, excel.exe, powerpnt.exe, or outlook.exe
- Presence of malformed Office documents with unusual embedded object sizes or manipulated header fields
Detection Strategies
- Monitor Office processes for atypical file reads, memory dumps, or child process behavior following document opens
- Deploy endpoint telemetry to capture Office parsing exceptions and correlate with recently opened attachments
- Scan email gateways and file shares for Office documents containing malformed structures that trigger parser anomalies
Monitoring Recommendations
- Track document open events from untrusted origins (email attachments, downloaded files, external shares)
- Alert on Office processes accessing sensitive memory regions or writing unexpected output files
- Correlate Windows Event Log crash records for Office binaries with recent user file activity
How to Mitigate CVE-2026-55026
Immediate Actions Required
- Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-55026 to all affected Office and SharePoint installations
- Prioritize patching for shared workstations, terminal servers, and multi-user SharePoint environments where local access is broader
- Restrict local logon rights on systems processing sensitive documents to reduce the attackable population
Patch Information
Microsoft has published security updates addressing CVE-2026-55026 through the Microsoft Update Guide. Administrators should deploy the patches via Windows Update, Microsoft Update Catalog, WSUS, or Microsoft Intune according to standard patch management workflows. Refer to the Microsoft Security Update CVE-2026-55026 for build-specific version numbers.
Workarounds
- Enable Protected View for documents originating from the internet or email to isolate parsing in a sandboxed process
- Block Office documents from untrusted sources at the email gateway and web proxy layers until patches are deployed
- Configure Attack Surface Reduction (ASR) rules to limit Office child process creation and script execution
# Enable Protected View via Group Policy registry keys (example for Word)
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
