CVE-2026-55057: Microsoft 365 Apps Info Disclosure Flaw

CVE-2026-55057 Overview

CVE-2026-55057 is an integer overflow vulnerability [CWE-190] in Microsoft Office that allows a local attacker to disclose sensitive information. Microsoft published the advisory on July 14, 2026, covering Microsoft 365 Apps, Microsoft 365 for macOS, and Office LTSC releases from 2016 through 2024. Exploitation requires user interaction, typically opening a crafted Office document, and does not require prior authentication to the target application. Successful exploitation can leak memory contents from the Office process, including data adjacent to the miscalculated buffer.

Critical Impact
A crafted Office document can trigger an integer wraparound that leads to out-of-bounds memory access, exposing confidential process memory to an attacker.

Affected Products

Microsoft 365 Apps (Enterprise x86/x64) and Microsoft 365 for macOS
Microsoft Office 2016 and Office 2019 (x86/x64)
Microsoft Office LTSC 2021 and Office LTSC 2024 (x86/x64 and macOS)

Discovery Timeline

2026-07-14 - CVE-2026-55057 published to NVD
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-55057

Vulnerability Analysis

The vulnerability stems from an integer overflow or wraparound condition [CWE-190] in Microsoft Office document parsing logic. When Office processes a specially crafted file, an arithmetic operation on a size or offset value exceeds the maximum representable integer value. The resulting wrapped value is then used to allocate or index into a buffer, producing an undersized allocation or an incorrect offset calculation.

Subsequent read operations use the miscalculated value to access memory outside the intended bounds. Because the impact is limited to confidentiality, the flaw permits disclosure of process memory rather than modification or code execution. Leaked memory can include heap contents, pointers useful for bypassing Address Space Layout Randomization (ASLR), or fragments of previously opened documents.

Root Cause

The root cause is missing validation of arithmetic results during size or offset computation in Office file format handlers. Trusting the wrapped integer produces an out-of-bounds read when the affected buffer is subsequently accessed.

Attack Vector

Exploitation requires an attacker to deliver a crafted document, typically through phishing email, a shared file location, or a malicious download. The victim must open the file in a vulnerable version of Office, satisfying the user interaction requirement. The attack executes in the local security context of the user running Office and does not traverse the network directly. No specific exploitation code is publicly available at this time, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-55057

Indicators of Compromise

Unexpected Office application crashes or Watson error reports referencing Word, Excel, or PowerPoint parsers after opening an email attachment.
Office processes (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning child processes such as cmd.exe, powershell.exe, or mshta.exe shortly after document open.
Office documents arriving from external senders with anomalous file structure or unusually large embedded objects.

Detection Strategies

Enable Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and injecting into other processes.
Monitor endpoint telemetry for exceptions in Office image loads and heap access violations that correlate with document open events.
Deploy YARA or content inspection rules at the email gateway to identify malformed Office file structures with abnormal size fields.

Monitoring Recommendations

Ingest Office and Windows Event Logs into a centralized SIEM and correlate document open events with subsequent process, network, and file activity.
Track EPSS scoring changes for CVE-2026-55057 to reprioritize response as exploitation likelihood evolves.
Alert on outbound connections initiated by Office processes to newly observed or low-reputation domains following document interaction.

How to Mitigate CVE-2026-55057

Immediate Actions Required

Apply the July 2026 Microsoft security updates for all affected Office channels, including Microsoft 365 Apps, Office LTSC, and macOS builds.
Prioritize patching for users who routinely open Office attachments from external senders, such as finance, HR, and executive assistants.
Enforce Protected View for documents originating from the Internet, email attachments, and unsafe locations.

Patch Information

Microsoft has released updates addressing CVE-2026-55057. Refer to the Microsoft Security Response Center advisory for CVE-2026-55057 for the current build numbers per channel and platform. Deploy updates through Microsoft Update, Configuration Manager, Intune, or the Office Click-to-Run update channel.

Workarounds

Configure Group Policy to disable editing for files originating from untrusted locations, keeping documents in Protected View sandbox.
Block or quarantine legacy Office binary formats at the email gateway when not required by business processes.
Use Microsoft Defender Application Guard for Office to isolate untrusted documents in a hardened container.

bash

# Configuration example: enforce Protected View via Group Policy registry keys
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 DisableInternetFilesInPV /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