CVE-2026-45459: Microsoft Excel Protection Mechanism Bypass – Low-Risk Information Disclosure

Technical summary

CVE-2026-45459 is a protection mechanism failure (CWE-693) in Microsoft Office Excel that permits local bypass of a security feature. The vulnerability requires local access (AV:L), no special privileges (PR:N), and user interaction (UI:R) to exploit. The attack surface is limited to the user's confidentiality scope (S:U/C:L), with no integrity or availability impact. The flaw resides in Excel's ability to enforce protective controls, allowing an attacker to circumvent intended security boundaries when a user opens a malicious file.

Business impact

The business risk is minimal. An attacker could extract sensitive information from an Excel workbook (such as hidden data, formulas, or cell contents) if a user opens a malicious file. This may affect organizations that rely on Excel for confidential data handling, but the lack of integrity or availability impact means data cannot be altered or systems disrupted. The requirement for user interaction and local access significantly limits exploitation in typical enterprise environments.

Affected systems

Microsoft Office products are affected, including Microsoft 365 Apps, Office 2021, Office 2024, and Microsoft 365. Organizations running any of these versions on Windows systems should review their patch status, though the low severity rating suggests this is not an urgent priority.

Exploitability

Exploitation requires local access to a user's machine and user interaction (opening a file), making opportunistic or remote attacks infeasible. The straightforward attack chain—crafting a malicious Excel file and convincing a user to open it—means the attack complexity is low once prerequisites are met. No exploit code has been made publicly available or weaponized. Active exploitation is unlikely given the low severity and the modest informational disclosure benefit to an attacker.

Remediation

Apply security updates from Microsoft for affected Office versions. Verify patch availability through the Microsoft Security Update Guide and your organization's patch management system. Until patched, users should exercise caution when opening Excel files from untrusted sources and avoid enabling macros or external data links in unexpected files.

Patch guidance

Consult Microsoft's official security advisories for the specific patch versions addressing CVE-2026-45459 across Microsoft 365 Apps, Office 2021, and Office 2024. Patches are typically delivered through Windows Update or Microsoft 365 auto-update mechanisms. Test patches in a non-production environment before broad deployment to ensure compatibility with organizational workflows and add-ins.

Detection guidance

Monitor for suspicious Excel file access patterns, particularly when files are opened from email attachments, shared drives, or external sources. Endpoint Detection and Response (EDR) tools can flag unusual file operations within Excel processes. Network monitoring for exfiltration of Excel file contents may also help identify exploitation attempts, though the low severity and local-only nature of the vulnerability limit the detection priority.

Why prioritize this

This vulnerability merits lower priority in most security roadmaps. The CVSS 3.1 score of 3.3 (LOW) reflects minimal real-world risk: exploitation requires local access, user interaction, and yields only minor information disclosure. It is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, indicating no active exploitation campaigns. Patch it as part of routine monthly or quarterly updates rather than as an emergency fix.

Risk score, explained

The risk score of 3.3 (LOW) is justified by the combination of local access requirement (AV:L), user interaction necessity (UI:R), low complexity (AC:L), and confidentiality-only impact (C:L with I:N and A:N). These factors constrain the threat landscape to insider or social-engineering scenarios where an attacker has already achieved local presence. The lack of integrity and availability impact, coupled with the absence of known active exploitation, further supports the low severity rating.

Frequently asked questions