CVE-2026-97740 Overview
CVE-2026-97740 is an information disclosure vulnerability in Microsoft Office Excel. The flaw exposes sensitive system information to an unauthorized control sphere, allowing a remote attacker to disclose information over a network. The weakness is classified under [CWE-497], Exposure of Sensitive System Information to an Unauthorized Control Sphere.
Exploitation requires user interaction, typically opening a crafted Excel file delivered through phishing or a malicious download. No privileges are required on the target system. Successful exploitation compromises confidentiality but does not impact integrity or availability.
Critical Impact
An attacker who successfully exploits CVE-2026-97740 can read sensitive memory or system information from a user's Excel session, enabling follow-on attacks against the host or environment.
Affected Products
- Microsoft Office Excel (see the Microsoft CVE-2026-97740 Advisory for build-level details)
Discovery Timeline
- 2026-10-13 - CVE-2026-97740 published to the National Vulnerability Database
- 2026-10-13 - Last updated in NVD database
Technical Details for CVE-2026-97740
Vulnerability Analysis
CVE-2026-97740 is an information disclosure flaw in how Microsoft Office Excel processes crafted spreadsheet content. The vulnerability maps to [CWE-497], indicating Excel discloses system-level information that should remain confined within the application's trust boundary.
An attacker who crafts a malicious Excel workbook can trigger disclosure of in-process memory, file metadata, or environment data when a victim opens the file. The exploit is network-reachable because the file can be delivered through email, web download, or cloud share.
The disclosed data can include memory contents, file paths, or configuration values that assist attackers in reconnaissance. This information often enables chained attacks against the host, including exploitation of memory corruption bugs that require address or layout knowledge.
Root Cause
The root cause is improper restriction of sensitive system information inside Excel's parsing logic. Excel exposes data from a protected scope to content controlled by an untrusted file, violating the control sphere boundary described in [CWE-497].
Attack Vector
The attack vector is network-based with user interaction. An attacker hosts or distributes a crafted Excel document. When the victim opens the file, Excel processes the malicious content and returns disclosed information to the attacker-controlled payload, which can exfiltrate it or render it to the user.
No authentication is required on the target. The attack succeeds against any user who opens the file with a vulnerable Excel build. See the Microsoft CVE-2026-97740 Advisory for the authoritative technical description.
Detection Methods for CVE-2026-97740
Indicators of Compromise
- Unexpected Excel child processes spawning network clients such as powershell.exe, curl.exe, or certutil.exe shortly after a workbook is opened.
- Inbound Excel files from untrusted senders containing embedded external links, DDE references, or unusual XLL/add-in content.
- Outbound DNS or HTTP requests originating from EXCEL.EXE to low-reputation domains following document open events.
Detection Strategies
- Hunt for Office telemetry showing EXCEL.EXE reading sensitive files or environment variables immediately after document open.
- Correlate Microsoft Defender SmartScreen and Mark-of-the-Web events with Excel process creation to flag files sourced from the internet.
- Review mail gateway logs for Excel attachments with external references or anomalous metadata that match the delivery pattern.
Monitoring Recommendations
- Monitor EXCEL.EXE for outbound network connections and unexpected DNS lookups.
- Track creation of temporary files in %TEMP% and %APPDATA%\Microsoft\Office paths following workbook opens.
- Alert on Office process trees that touch credential stores, browser profiles, or SSH key directories.
How to Mitigate CVE-2026-97740
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft CVE-2026-97740 Advisory across all Excel installations.
- Enforce Protected View for files originating from the internet, email attachments, and unsafe locations.
- Block Excel files from untrusted external senders at the mail gateway until patching is complete.
Patch Information
Microsoft published the fix in its October 2026 update cycle. Administrators should deploy the vendor-supplied security update identified in the Microsoft Security Response Center advisory to all supported Office and Microsoft 365 Apps channels.
Workarounds
- Enable and enforce Protected View and Application Guard for Office to isolate untrusted workbooks.
- Disable external data connections, DDE, and legacy file format support where not required by business processes.
- Use Attack Surface Reduction rules to block Office applications from creating child processes and from making network calls initiated by content.
# Example: enforce Protected View via Group Policy registry keys
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\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.
