Zero-day vulnerability in Microsoft Office

Reliance on Untrusted Inputs in a Security Decision
CVE-2026-21509

Vulnerability details

Advisory: SB2026012659 - Remote code execution in Microsoft Office

Vulnerable component: Microsoft Office

CVE-ID: CVE-2026-21509

CVSSv3 score: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:H/RL:O/RC:C

CWE-ID: CWE-807 - Reliance on Untrusted Inputs in a Security Decision

Description:

The vulnerability allows a remote attacker to compromise the affected system.

The vulnerability exists due to improper insecure handling of OLE objects. A remote attacker can trick the victim into opening a specially crafted Office file and execute arbitrary code on the system.

Note, the vulnerability is being actively exploited in the wild.

External links:

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509

About zero-day vulnerabilities

Zero-day vulnerability is an undisclosed vulnerability in software that hackers can exploit to compromise computer programs, gain unauthorized access to sensitive data, penetrate networks, etc. We consider vulnerability a zero-day when there is no solution provided from software vendor and the vulnerability is being actively exploited by malicious actors.

Zero-day candidate is a potential zero-day vulnerability in software which might have been used in targeted attacks, however there is no evidence to support this suggestion.