The flaws with CVSS 7.8 severity are easy to exploit, and could lead to cascading cyber risks if not patched immediately.

The January 2026 Patch Tuesday cycle has addressed more than a hundred vulnerabilities across Microsoft products, but fixes for some Excel bugs have drawn particular concern because of how easily they could have been weaponized through everyday business documents.

The critical Excel flaws allow attackers to execute code remotely simply by tricking users into opening malicious spreadsheets, and security vendors are urging organizations to patch their software immediately to prevent potential widespread compromise.

The Excel remote code execution issues, tracked as CVE‑2026‑20955 and CVE‑2026‑20957, both carry “critical” ratings and CVSS scores around 7.8. One vulnerability stems from an untrusted pointer dereference during Excel file handling, while another arises from an integer underflow that can lead to a heap‑based buffer overflow, both conditions that allow arbitrary code execution in the context of the logged‑in user once a crafted workbook is opened.

Unpatched vulnerabilities = cascading risks

Although exploitation had not yet been publicly confirmed at disclosure time, cybersecurity firms have warned that the bugs are well suited to phishing campaigns and could quickly be integrated into commodity attack chains. Since Excel files are so widely exchanged and often implicitly trusted inside enterprises, the flaws present a high‑impact avenue for attackers to move from a single malicious email to full system takeover, data theft or ransomware deployment.

Vendors analyzing the Patch Tuesday release have noted that the Excel issues could also be chained with other Windows vulnerabilities, including elevation‑of‑privilege and security feature bypass bugs, to enable lateral movement and deeper compromise of corporate networks. The Excel patches are “high priority” items that should be rolled out ahead of many other January fixes, particularly in environments where users routinely open externally sourced spreadsheets, according to analysts.

Successful attacks require only that the victim open the malicious Excel file, and that no additional privileges are needed for the attacker’s code to run. Organizations that cannot patch immediately are being urged to harden email gateways, block or quarantine unexpected Excel attachments, and reinforce user awareness training so that staff treat unsolicited spreadsheets with the same suspicion now applied to suspicious macros and executable files.