CVE-2026-97777 Overview
CVE-2026-97777 is a heap-based buffer overflow vulnerability [CWE-122] in Microsoft Office Excel. An unauthorized attacker can execute arbitrary code over a network by delivering a crafted Excel document to a target user. Exploitation requires user interaction, typically opening the malicious file.
The flaw affects memory handling within Excel's document parsing routines. Successful exploitation grants the attacker code execution in the context of the user running Excel, leading to full compromise of document confidentiality, integrity, and availability on the host.
Critical Impact
A crafted Excel file can trigger heap corruption and lead to remote code execution on the victim endpoint with the privileges of the current user.
Affected Products
- Microsoft Office Excel (version details pending vendor advisory)
- Refer to the Microsoft Security Update Guide for the authoritative product and build list
Discovery Timeline
- 2026-10-13 - CVE CVE-2026-97777 published to NVD
- 2026-10-13 - Last updated in NVD database
Technical Details for CVE-2026-97777
Vulnerability Analysis
The vulnerability is a heap-based buffer overflow in Microsoft Office Excel. It occurs when Excel parses attacker-controlled data from a specially crafted spreadsheet. The parser writes beyond the bounds of a heap-allocated buffer, corrupting adjacent heap metadata or object pointers.
An attacker who controls the overflow contents can groom the heap to overwrite function pointers, virtual tables, or other control data. This primitive leads to arbitrary code execution within the Excel process. The attack is network-reachable because the malicious document can be delivered through email, file sharing services, or web downloads.
User interaction is required to trigger the flaw. Opening the file in a vulnerable Excel build is sufficient when protected view is bypassed or disabled.
Root Cause
The root cause is improper validation of a size, length, or offset field during Excel's processing of document structures. The parser allocates a heap buffer based on one value but copies data governed by a different, larger value. This classic bounds-check failure is tracked under [CWE-122] Heap-based Buffer Overflow.
Attack Vector
The attack vector is network delivery of a weaponized Excel document followed by user interaction. Common delivery channels include phishing attachments, OneDrive and SharePoint links, and drive-by downloads. Once the user opens the document in a vulnerable Excel client, the overflow executes attacker-supplied shellcode or chains into a loader that fetches a second-stage payload.
No code example is published. Refer to the Microsoft Security Update Guide for technical details as Microsoft releases them.
Detection Methods for CVE-2026-97777
Indicators of Compromise
- Excel spreadsheets (.xls, .xlsx, .xlsm, .xlsb) received from untrusted senders that trigger crashes or anomalous child-process creation
- excel.exe spawning unexpected children such as cmd.exe, powershell.exe, rundll32.exe, mshta.exe, or wscript.exe
- Outbound network connections from excel.exe to attacker infrastructure shortly after document open
- Unexpected writes by excel.exe to %APPDATA%, %TEMP%, or startup locations
Detection Strategies
- Hunt for process ancestry where excel.exe is the parent of script or living-off-the-land binaries
- Alert on Office applications loading non-standard DLLs or executing unsigned code from user-writable paths
- Correlate email gateway attachment metadata with endpoint document-open events to surface targeted delivery
Monitoring Recommendations
- Collect Microsoft Office telemetry including Office Alerts, AMSI, and Protected View bypass events
- Monitor Windows Error Reporting for excel.exe crashes with heap corruption signatures
- Track EDR telemetry for suspicious memory allocations, RWX mappings, and injection activity originating from Office processes
How to Mitigate CVE-2026-97777
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update Guide as soon as it is available in your update channel
- Enforce Protected View and Office Application Guard for documents originating from the internet or email
- Block or quarantine Office attachments from untrusted senders at the email gateway
Patch Information
Microsoft tracks this vulnerability through its Security Update Guide. Administrators should consult the vendor advisory for the specific KB numbers, click-to-run build versions, and MSI patches aligned with their Excel and Microsoft 365 Apps channels. See the Microsoft Security Update Guide for the authoritative patch list.
Workarounds
- Enable Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from injecting into other processes
- Open untrusted spreadsheets in Office for the web or Excel Viewer in a sandboxed environment until patches are deployed
- Restrict macro execution and disable legacy file formats where business workflows permit
# Example: enable key ASR rules via PowerShell to reduce Office exploitation surface
# Block Office applications from creating child processes
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled
# Block Office applications from injecting code into other processes
Add-MpPreference -AttackSurfaceReductionRules_Ids 75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 -AttackSurfaceReductionRules_Actions Enabled
# Block executable content from email client and webmail
Add-MpPreference -AttackSurfaceReductionRules_Ids BE9BA2D9-53EA-4CDC-84E5-9B1EEEE46550 -AttackSurfaceReductionRules_Actions Enabled
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
