Microsoft Excel Integer Underflow Remote Code Execution
CVE-2026-20957 is a Critical Microsoft Office Excel vulnerability caused by an integer underflow (wraparound) during processing of a specially crafted Excel file. The provided content further characterizes the flaw as an integer underflow leading to a heap-based buffer overflow in Excel parsing logic. Successful exploitation allows code execution in the security context of the logged-on user when the malicious workbook is opened. The Preview Pane is not identified as an attack vector for this vulnerability.
Are you exposed to this one?
Mallory correlates every CVE against your assets, your vendors, and active adversary campaigns. Know which vulnerabilities matter for you, not just which ones are loud.
Impact, mitigation & remediation
What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
Impact
What an attacker gets, and what they’ve been doing with it.
Mitigation
If you can’t patch tonight, do this now.
Remediation
Patch, then assume compromise.
Exploits
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Affected products & vendors
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Critical Microsoft Excel remote code execution via maliciously crafted Office files (memory-related) per the content.
A Microsoft Excel remote code execution vulnerability where an integer underflow can lead to a heap-based buffer overflow, enabling arbitrary code execution when a user opens a crafted spreadsheet.
A Microsoft Excel remote code execution vulnerability where an integer underflow can lead to a heap-based buffer overflow, enabling arbitrary code execution when a user opens a crafted spreadsheet.
A remote code execution vulnerability in Microsoft Excel, likely triggerable via specially crafted spreadsheets.
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.