CVE-2026-44818: Microsoft Excel Remote Code Execution Vulnerability

Synopsis

An integer underflow vulnerability in Microsoft Excel allows an attacker to execute arbitrary code on a victim's machine. The attack is local and requires no authentication, but the victim must open a specially crafted Excel file, making social engineering the delivery mechanism. Successful exploitation gives the attacker full code execution with the victim's privileges, enabling data theft, file modification, or further system compromise. Patched-image rebuilds at versions 16.0.5556.1001 and 16.0.10417.20137 are available on HarborGuard for environments running affected versions.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityNot required

  The attacker needs an existing shell or process on the host; no network-facing service is involved in triggering the vulnerability.

• AuthenticationNot required

  No credentials or account privileges are required - the attacker relies on the victim opening a malicious file rather than any authenticated session.

• Victim interactionRequired

  The victim must open a specially crafted Excel file, making this a social-engineering-dependent attack delivered via phishing, shared drives, or similar means.

• Attack complexityDetail

  Attack complexity is rated High, meaning reliable exploitation depends on environmental factors such as specific memory layout or timing conditions that are not fully under the attacker's control.

Blast Radius

• The attacker executes arbitrary code in the context of the victim user, gaining the same file-system and process permissions as that user.
• Sensitive files accessible to the victim (documents, credentials, tokens stored on disk) can be read and exfiltrated.
• The attacker can write or modify files and persisted data within the victim's permissions scope, including Office documents and configuration files.
• The affected Excel process and any dependent processes can be terminated or destabilized, disrupting the user's work environment.