CVE-2016-7262

What Is Microsoft Excel?

Microsoft Excel is the dominant spreadsheet application in enterprise environments globally, used for financial modeling, data analysis, and business reporting. Excel's extensive feature set — including formula evaluation, data connections to external sources, DDE (Dynamic Data Exchange), and support for embedded objects — creates a complex attack surface within a universally trusted document format. Security features in modern Excel (Protected View, macro security, external link warnings) are designed to prevent automatic code execution when opening documents from untrusted sources. Bypassing these security features allows attackers to execute code through a familiar, trusted document format without triggering security prompts.

Overview

CVE-2016-7262 is a security feature bypass vulnerability in Microsoft Excel that allows an attacker to execute arbitrary commands when a user opens a specially crafted Excel file. The vulnerability arises from Excel improperly handling input — failing to enforce security boundaries when processing certain file constructs — which allows command execution without triggering the normal macro security prompts or Protected View restrictions. Delivered via email attachment or download, this bypass enables targeted phishing attacks against Excel users. Patched in MS16-148 (December 13, 2016). CISA added CVE-2016-7262 to the KEV catalog in March 2022.

Affected Versions

Technical Details

Root Cause: Security Feature Bypass via Improper Input Handling

CVE-2016-7262 is an improper input validation vulnerability (CWE-20) in Microsoft Excel that enables bypassing Office security features. Excel implements several layers of security to prevent automatic code execution from untrusted document content:

• Protected View sandbox — documents from email or internet open in a read-only sandbox
• Macro security prompts — VBA macros require user confirmation to execute
• External link warnings — links to external data sources require user approval

This vulnerability exploits Excel's improper validation of specific input constructs to bypass one or more of these controls, allowing arbitrary command execution without the normal security prompts that would warn the user.

Attack delivery:

1. Attacker crafts a malicious Excel file — a specially structured .xls or .xlsx file containing the bypass construct
2. Victim opens the file — via email attachment, web download, or shared network path
3. Excel processes the file — the improper input handling triggers the bypass condition
4. Commands execute — arbitrary OS commands run at the user's privilege level without triggering security warnings

Security Feature Bypass Context

Security feature bypasses in Office are particularly impactful in enterprise environments because:

• Enterprise security controls specifically rely on macro prompts and Protected View — security awareness training emphasizes "don't enable macros"; a bypass that doesn't require macros defeats this user-training defense
• LOLBAS potential — Excel can invoke Windows binaries and scripting engines through various mechanisms (DDE, external data connections, shell extensions); a bypass that enables this without prompts provides code execution via trusted Office processes
• Email gateway trust — Excel files from business contacts are generally trusted and opened directly; bypasses enabling code execution from normal-looking Excel files are ideal phishing payloads

Attack Characteristics

Discovery

Identified and reported to Microsoft through coordinated disclosure; patched in the December 2016 Patch Tuesday release (MS16-148) covering multiple Office security vulnerabilities.

Exploitation Context

• Phishing without macro warnings: CVE-2016-7262's value is enabling command execution without the macro enable prompt; enterprise users and security training emphasize "never click Enable Content" — this bypass renders that guidance ineffective for affected Excel versions
• Targeted spear-phishing campaigns: Excel security feature bypasses are consistently used in targeted attacks; the CISA KEV addition in March 2022 reflects documented use in campaigns targeting specific organizations
• Combination with social engineering: An Excel file that executes commands without prompts is the most effective social engineering payload — the document appears to open normally while silently executing malicious code in the background
• CISA KEV (2022): Added March 3, 2022 alongside CVE-2016-7193 (Office RTF memory corruption), suggesting a batch review of older Office vulnerabilities with confirmed exploitation

Remediation

1. Apply MS16-148 — install the December 2016 Microsoft Office security update via Windows Update, WSUS, or MECM. All subsequent Office cumulative updates include this fix.

2. Keep Office fully updated — apply all current Office security updates; Microsoft releases Office security patches monthly.

3. Enable Protected View for all external sources — in Excel: File → Options → Trust Center → Trust Center Settings → Protected View → enable all three Protected View options (internet, potentially unsafe, Outlook attachments).

4. Deploy Attack Surface Reduction rules — Microsoft Defender ASR rule "Block Office applications from creating child processes" limits what code executed through Excel can do, even if the bypass triggers.

5. Configure Group Policy to restrict Excel external data connections — limit Excel's ability to automatically load external data sources, reducing the attack surface for bypass techniques that rely on external content loading.

6. Block untrusted Excel files at email gateway — require that Excel attachments from external senders be reviewed or opened in a sandboxed viewer before delivery to end users.