CVE-2025-53739 Overview
CVE-2025-53739 is a type confusion vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute code locally [CWE-843]. The flaw stems from Excel accessing a resource using an incompatible type, which corrupts memory and redirects execution flow. Exploitation requires user interaction, typically opening a crafted spreadsheet, and yields high impact on confidentiality, integrity, and availability. Microsoft addressed the vulnerability across Microsoft 365 Apps, Excel 2016, Office 2019, Office LTSC 2021, Office LTSC 2024, and Office Online Server.
Critical Impact
Successful exploitation grants an attacker the ability to execute arbitrary code in the security context of the user opening a malicious Excel file, enabling endpoint compromise and follow-on actions.
Affected Products
- Microsoft 365 Apps (Enterprise, x64 and x86)
- Microsoft Excel 2016, Microsoft Office 2019
- Microsoft Office LTSC 2021 and 2024 (Windows and macOS), Microsoft Office Online Server
Discovery Timeline
- 2025-08-12 - CVE-2025-53739 published to the National Vulnerability Database (NVD)
- 2025-08-15 - Last updated in NVD database
Technical Details for CVE-2025-53739
Vulnerability Analysis
The vulnerability is a type confusion flaw [CWE-843] in Microsoft Office Excel. Type confusion occurs when code allocates or accesses a resource as one type but later interprets it as an incompatible type. In Excel, this typically involves malformed objects embedded in a spreadsheet that cause the parser to operate on memory using mismatched structure definitions. The mismatch enables an attacker to influence pointer values, function dispatch tables, or object metadata. The result is arbitrary code execution under the privileges of the user opening the document.
The attack requires local access and user interaction. An attacker delivers a crafted .xlsx, .xlsm, or similar Excel file via phishing email, shared drive, or web download. When the victim opens the file, Excel parses the malicious object and triggers the type confusion condition. Preview Pane rendering can also process file content and broaden the exploitation surface.
Root Cause
Excel does not enforce consistent type validation when dereferencing certain embedded objects. An attacker who controls object metadata can cause Excel to treat one structure as another, allowing controlled writes or reads against adjacent memory. This primitive is then chained into code execution.
Attack Vector
The attack vector is local with required user interaction. Delivery typically occurs through social engineering. The exploit executes under the current user context, so attackers commonly chain this flaw with a privilege escalation vulnerability to obtain SYSTEM access.
The vulnerability is described in prose only. No verified public proof-of-concept code is available. See the Microsoft Security Response Center advisory for additional technical context.
Detection Methods for CVE-2025-53739
Indicators of Compromise
- Unexpected excel.exe child processes such as cmd.exe, powershell.exe, rundll32.exe, mshta.exe, or wscript.exe
- Excel writing executable content to %TEMP%, %APPDATA%, or user profile directories shortly after document open
- Outbound network connections initiated by excel.exe to untrusted hosts following document open
- Office files arriving via email with macros, embedded OLE objects, or unusual external links
Detection Strategies
- Hunt for process lineage where excel.exe spawns scripting engines or LOLBins within seconds of file open
- Alert on Office applications loading non-standard DLLs from user-writable paths
- Inspect inbound email attachments and SharePoint uploads for Excel files containing anomalous embedded objects
Monitoring Recommendations
- Enable command-line and module-load logging for all Office processes through Sysmon or equivalent endpoint telemetry
- Centralize Office telemetry in a security data lake and correlate document-open events with subsequent process and network activity
- Track Microsoft Defender, AMSI, and AppLocker events triggered by Office processes for early signals of exploitation
How to Mitigate CVE-2025-53739
Immediate Actions Required
- Apply the Microsoft security updates referenced in the Microsoft CVE-2025-53739 Advisory to all affected Excel and Office installations
- Prioritize patching for users who routinely receive spreadsheets from external senders
- Confirm Microsoft 365 Apps clients are receiving updates through the configured update channel and not pinned to older builds
Patch Information
Microsoft released fixes for CVE-2025-53739 as part of the August 2025 security update cycle. Refer to the Microsoft CVE-2025-53739 Advisory for build numbers and KB identifiers across Microsoft 365 Apps, Excel 2016, Office 2019, Office LTSC 2021, Office LTSC 2024, and Office Online Server.
Workarounds
- Enable Protected View and Application Guard for Office so untrusted documents render in an isolated container
- Disable the Outlook Preview Pane and Windows Explorer preview handlers for Office file types until patches are deployed
- Block macros in files originating from the internet via the Block macros from running in Office files from the Internet Group Policy
- Apply Attack Surface Reduction (ASR) rules that prevent Office applications from creating child processes and injecting into other processes
# Configuration example: enable ASR rule to block Office child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
