CVE-2025-62556: 
vulnerability analysis and mitigation

CVE-2025-62556 is an untrusted pointer dereference vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute arbitrary code locally. It was disclosed on December 9, 2025, as part of Microsoft's December 2025 Patch Tuesday security update. Affected products include Microsoft Excel 2016 (x86/x64, versions prior to 16.0.5530.1000), Microsoft 365 Apps for Enterprise, Office 2019, Office 2021 (LTSC), Office 2024 (LTSC), Office Online Server (prior to 16.0.10417.20075), and Office LTSC for Mac 2021/2024. The vulnerability carries a CVSS v3.1 base score of 7.8 (High) (Microsoft MSRC).

The root cause is classified as CWE-822 (Untrusted Pointer Dereference), where Excel fails to properly validate pointer values derived from untrusted input during file parsing. An attacker exploits this by crafting a malicious Excel file that, when opened by a victim, causes the application to dereference a controlled pointer, leading to arbitrary code execution in the context of the current user. Exploitation requires local user interaction — specifically, a user must open a specially crafted Excel document — and no prior privileges are needed. No public proof-of-concept or detailed technical write-up has been identified at this time (Microsoft MSRC, ZDI Blog).

Successful exploitation results in high impact to confidentiality, integrity, and availability of the affected system, as the attacker gains code execution in the context of the logged-in user. This could allow installation of malware, creation of privileged accounts, exfiltration of sensitive data, or use of the compromised host as a pivot point for lateral movement within a network. The scope is limited to the local system (unchanged scope), but the consequences are severe if the victim has elevated privileges (Microsoft MSRC).

1. Craft a malicious Excel file: An attacker creates a specially crafted .xlsx or .xls file that embeds malformed data structures designed to trigger an untrusted pointer dereference during Excel's file parsing routines.
2. Deliver the file to the target: The attacker distributes the malicious file via phishing email, malicious download link, or other social engineering methods targeting users with vulnerable Excel versions.
3. Induce the victim to open the file: The attack requires user interaction — the victim must open the malicious Excel document using a vulnerable version of Microsoft Excel.
4. Trigger the vulnerability: Upon opening, Excel processes the malformed file and dereferences an attacker-controlled pointer, causing execution to redirect to attacker-supplied code.
5. Achieve code execution: Arbitrary code runs in the security context of the current user, enabling the attacker to install payloads, establish persistence, exfiltrate data, or move laterally within the network (Microsoft MSRC).

• File System: Unexpected or newly created executable files, scripts, or DLLs in user temp directories (%TEMP%, %APPDATA%) following the opening of an Excel file; suspicious Excel files received via email or downloaded from untrusted sources.
• Process: Unusual child processes spawned by EXCEL.EXE (e.g., cmd.exe, powershell.exe, wscript.exe, mshta.exe); unexpected network connections initiated by the Excel process.
• Logs: Windows Event Logs showing application crashes or faults in EXCEL.EXE around the time of file opening; security logs recording new process creation events parented to Excel.
• Network: Outbound connections from workstations to unknown or suspicious external IP addresses shortly after Excel file interaction, potentially indicating C2 beacon activity.

Microsoft released patches for CVE-2025-62556 as part of the December 9, 2025 Patch Tuesday update. Users should update Microsoft Excel 2016 to version 16.0.5530.1000 or later, Office Online Server to 16.0.10417.20075 or later, and apply the latest updates for Microsoft 365 Apps, Office 2019, Office 2021, and Office 2024 via Windows Update or the Microsoft Update Catalog (Microsoft MSRC). As interim mitigations: disable macros in Office, implement email filtering to block suspicious Excel attachments, use Microsoft Defender with up-to-date definitions, and educate users to avoid opening Excel files from untrusted sources.

The vulnerability was covered as part of broader December 2025 Patch Tuesday roundups by multiple security outlets. Bleeping Computer noted the December 2025 update fixed 57 flaws including 3 zero-days, with CVE-2025-62556 among the Excel-specific issues (BleepingComputer). Zero Day Initiative and Cisco Talos also published Patch Tuesday review posts covering the December 2025 release (ZDI Blog, Talos Blog). Sophos and Rapid7 similarly included the vulnerability in their December 2025 Patch Tuesday analyses without flagging it as a priority concern given the absence of active exploitation (Sophos, Rapid7).