CVE-2025-30376 Overview
CVE-2025-30376 is a heap-based buffer overflow in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw maps to [CWE-122] Heap-based Buffer Overflow and [CWE-787] Out-of-bounds Write. Exploitation requires user interaction, typically opening a crafted Excel document. Successful exploitation grants the attacker code execution in the context of the current user, impacting confidentiality, integrity, and availability.
Critical Impact
A crafted Excel file can trigger heap memory corruption and execute arbitrary code on the victim's machine with the privileges of the logged-on user.
Affected Products
- Microsoft 365 Apps (Enterprise)
- Microsoft Excel 2016 and Microsoft Office 2019
- Microsoft Office Long Term Servicing Channel 2021 and 2024 (Windows and macOS), and Microsoft Office Online Server
Discovery Timeline
- 2025-05-13 - CVE-2025-30376 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-30376
Vulnerability Analysis
The vulnerability is a heap-based buffer overflow in Microsoft Office Excel's document parsing logic. When Excel processes a maliciously crafted spreadsheet, a write operation exceeds the bounds of an allocated heap buffer. The resulting out-of-bounds write corrupts adjacent heap metadata or object pointers. An attacker can shape the heap to position attacker-controlled data where the overflow occurs.
This allows redirecting execution flow to attacker-supplied instructions. The attack vector is local and requires the victim to open the malicious file. No prior authentication to Excel or the host is required. Code executes in the context of the user opening the file, enabling subsequent persistence, credential theft, or lateral movement.
Root Cause
The root cause is improper validation of size or length values during parsing of a structured field within an Excel file format. Excel allocates a fixed or computed heap buffer, then writes data past the allocation boundary. This bypasses safe-bounds enforcement in the affected routine and corrupts memory in a controllable way.
Attack Vector
Delivery typically occurs through phishing email attachments, malicious downloads, or files hosted on file-sharing services. The Preview Pane can also serve as a low-friction trigger for similar Office vulnerabilities. Once opened, the crafted workbook executes the attacker payload without further user interaction. See the Microsoft CVE-2025-30376 Advisory for vendor-confirmed technical details.
Detection Methods for CVE-2025-30376
Indicators of Compromise
- Excel documents (.xls, .xlsx, .xlsm, .xlsb) originating from untrusted sources, particularly with anomalous embedded structures or oversized record fields.
- EXCEL.EXE spawning child processes such as cmd.exe, powershell.exe, wscript.exe, rundll32.exe, or mshta.exe.
- Unexpected network connections initiated by EXCEL.EXE to external hosts shortly after document open.
- Crash dumps or Windows Error Reporting events referencing heap corruption in Excel modules.
Detection Strategies
- Monitor process lineage for Office applications spawning scripting interpreters or LOLBins.
- Inspect Office telemetry and ETW events for module loads from temporary or user-writable directories under EXCEL.EXE.
- Apply YARA rules targeting malformed Office Open XML structures and oversized records used in crafted spreadsheets.
Monitoring Recommendations
- Centralize Office process telemetry, command-line arguments, and child-process events in a SIEM for correlation.
- Alert on Excel writing executables, DLLs, or scripts to disk outside expected document directories.
- Track outbound network connections from Office processes and baseline expected destinations.
How to Mitigate CVE-2025-30376
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft CVE-2025-30376 Advisory to all affected Microsoft 365 Apps, Office 2019, Office LTSC 2021/2024, Excel 2016, and Office Online Server instances.
- Enforce Microsoft Office Protected View and Block macros from running in Office files from the internet via Group Policy.
- Restrict Excel from spawning child processes using Attack Surface Reduction (ASR) rules in Microsoft Defender.
Patch Information
Microsoft has released security updates for this vulnerability. Administrators should consult the Microsoft CVE-2025-30376 Advisory for the precise build numbers and KB articles per product. Update Microsoft 365 Apps through the standard update channel and apply the corresponding monthly security updates to perpetual Office and Office LTSC releases.
Workarounds
- Block inbound Excel attachments from external senders at the email gateway pending patch deployment.
- Open untrusted spreadsheets in Office for the web or in an isolated sandbox rather than the desktop client.
- Use Windows Defender Application Control or AppLocker to prevent Office from launching scripting hosts and unsigned binaries.
# Enable Microsoft Defender ASR rule: Block all Office applications from creating 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.
