CVE-2026-45469: Microsoft Excel Integer Underflow Remote Code Execution Vulnerability

Technical summary

CVE-2026-45469 is an integer underflow vulnerability in Microsoft Office Excel tied to improper bounds checking in memory operations. The vulnerability maps to CWE-122 (Heap-based buffer overflow) and CWE-191 (Integer underflow), indicating that attacker-controlled input can cause integer arithmetic to wrap, leading to out-of-bounds memory access. The CVSS 3.1 score of 7.8 (HIGH) reflects local attack vector, no privilege requirement, and user interaction dependency, with high impact to confidentiality, integrity, and availability. The vulnerability does not require network access and cannot be triggered remotely.

Business impact

This vulnerability directly threatens organizations reliant on Office 365 and Excel for business operations. Compromised machines can serve as entry points for lateral movement, data exfiltration, or deployment of persistent malware. The requirement for user interaction—opening a malicious spreadsheet—makes phishing and social engineering campaigns the likely delivery mechanism. Organizations with email security controls and user security awareness programs are better positioned to mitigate attack likelihood. Financial services, legal, and healthcare sectors handling sensitive spreadsheets face elevated risk.

Affected systems

The vulnerability affects Microsoft 365 Apps, Microsoft Excel (standalone), Microsoft 365 (enterprise suite), Office 2019, Office 2021, Office 2024, and Office Online Server. Both Windows and cloud-based deployment scenarios are in scope. Organizations using any of these products, particularly long-term servicing channel (LTSC) releases (2019, 2021, 2024), should prioritize inventory and patching. Office Online Server deployments warrant particular attention in environments where users frequently collaborate on shared documents.

Exploitability

Exploitation requires local system access and user interaction—specifically, a user must open a malicious Office file. The attack cannot be triggered remotely via network access alone. However, the relatively low barrier to entry (crafting a malicious spreadsheet and delivering it via email) and the lack of additional prerequisites make this vulnerability moderately exploitable in real-world scenarios. The vulnerability is not currently listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating no evidence of active exploitation in the wild as of the last published data. Nevertheless, the recency of publication and high severity score warrant assumed-exploitable posture.

Remediation

Apply Microsoft security updates for Excel and affected Office products immediately upon release. Patch cycles vary by product branch—Microsoft 365 subscribers receive updates automatically, while Office 2019, 2021, and 2024 users must apply patches manually or through Windows Update. Office Online Server environments require coordinated patching procedures. Verify patch application by checking file version numbers in Excel.exe or confirming patch installation in Control Panel. Additionally, implement complementary controls: restrict macro execution policies, block untrusted file sources at email gateways, and enforce application hardening via Windows Defender Application Guard where applicable.

Patch guidance

Monitor Microsoft's official security advisory pages and subscribe to security bulletins for affected products (Microsoft 365 Apps, Office 2019/2021/2024, Office Online Server). Patches will be distributed through Windows Update, Microsoft Update, or Microsoft 365 admin center depending on deployment method. Verify patch status by cross-referencing published KB articles and monitoring your organization's patch management dashboard. For Office Online Server, coordinate patching with your server administrator to minimize downtime. Test patches in a non-production environment first, particularly for Office Online Server, to ensure compatibility with existing workflows and add-ins.

Detection guidance

Monitor for suspicious file access patterns to Excel temporary or recovery files. Endpoint Detection and Response (EDR) tools should flag unexpected code execution initiated from Excel.exe, particularly if followed by unusual process spawning, network connections, or credential access. Log file opens with suspicious origins (downloaded files, email attachments, network shares from untrusted sources). Office event logging can capture macro execution and file operations. Implement File Integrity Monitoring (FIM) on critical spreadsheet repositories. Network-level detection is limited due to the local-only attack vector, but email gateway rules can block known-malicious file extensions and suspicious content patterns. User reports of unexpected crashes or error messages when opening spreadsheets should trigger immediate investigation.

Why prioritize this

The HIGH severity CVSS score (7.8), combined with widespread product coverage across Microsoft Office suites and cloud services, mandates immediate prioritization. The integer underflow flaw directly enables local code execution—the most critical outcome in the vulnerability severity ladder. Although KEV status is currently false, the lack of active exploitation should not reduce urgency; early patching before weaponized proof-of-concepts emerge is a critical defensive strategy. The user-interaction requirement does not significantly lower risk in environments where users regularly receive external documents or collaborate via email.

Risk score, explained

The CVSS 3.1 score of 7.8 reflects: (1) Local Attack Vector—attacker must interact with the victim's system; (2) Low Attack Complexity—no special conditions required to exploit; (3) No Privileges Required—vulnerability exploitable by unprivileged users; (4) Required User Interaction—victim must open a malicious file; (5) Unchanged Scope—impact limited to the vulnerable component; (6) High confidentiality, integrity, and availability impact—code execution grants full system access under the user's privilege context. The score appropriately elevates this above medium severity due to the ease of exploitation and severity of impact, though the local requirement and user interaction prevent a 9.0+ critical rating.

Frequently asked questions