Microsoft’s December 2025 Patch Tuesday: Deep Dive Into 56 Vulnerabilities And An Actively Exploited Zero‑Day

Microsoft’s final Patch Tuesday of 2025 delivers fixes for 56 vulnerabilities across the Windows ecosystem, including an actively exploited zero‑day in PowerShell’s curl alias, several high‑impact elevation‑of‑privilege flaws in core Windows components, and critical remote code execution bugs in Microsoft Office that enable document‑based attacks with minimal user interaction.

Overview Of The December 2025 Security Release

The December 2025 security update addresses 56 distinct Common Vulnerabilities and Exposures (CVEs) across Windows client and server platforms and associated components. Three vulnerabilities are classified as critical due to remote code execution risk, while the remaining 53 are rated important. The set spans multiple layers of the Windows stack, from user‑mode applications and scripting environments to kernel‑mode drivers and file system filters.

The vulnerabilities fall primarily into two technical categories: elevation of privilege and remote code execution. Elevation‑of‑privilege issues constitute approximately half of the patched CVEs, reflecting ongoing hardening of core privilege boundaries in Windows. Remote code execution vulnerabilities make up roughly one‑third of the set, targeting components that frequently process untrusted content such as documents, network traffic, and inter‑process messages.

Actively Exploited Zero‑Day In PowerShell Curl Alias (CVE‑2025‑62221)

The most notable flaw in this release is an actively exploited zero‑day tracked as CVE‑2025‑62221, associated with how PowerShell’s curl alias invokes Invoke‑WebRequest. The issue is not a defect in the curl project itself but arises from the PowerShell integration layer that transparently aliases the curl command to a PowerShell cmdlet.

Technically, the problem involves argument handling and URL parsing semantics in the Invoke‑WebRequest pipeline when invoked through the curl alias. Under specific conditions, crafted command lines or scripts can cause PowerShell to interpret parameters or response data in a way that enables an attacker to execute arbitrary commands under the security context of the user or automated process running the script. This substantially alters the risk profile of environments where administrators or deployment tools rely on curl‑style syntax but execute it within PowerShell.

Real‑world exploitation prior to patching indicates that attackers were able to chain this flaw with social engineering or script‑supply‑chain weaknesses. Typical scenarios include malicious scripts checked into shared repositories, deployment playbooks that download and execute payloads using curl syntax, or command sequences embedded in broader attack chains. Because the vulnerability is in a trusted automation layer, exploitation can lead directly to code execution on servers and administrative workstations.

Mitigation requires installing the December 2025 update so that PowerShell correctly constrains the curl alias behavior and tightens parsing logic. As a defense‑in‑depth measure, security teams should audit automation repositories, build pipelines, and administrative runbooks for curl usage under PowerShell, particularly where untrusted or semi‑trusted URLs are fetched and executed. Disabling or explicitly overriding the curl alias in high‑risk environments can reduce attack surface for legacy scripts that have not yet been reviewed.

Kernel‑Level Elevation Of Privilege In Win32k (CVE‑2025‑62458)

CVE‑2025‑62458 addresses an elevation‑of‑privilege vulnerability in Win32k, the core kernel‑mode graphics and window management subsystem that brokers many user‑interface operations between user processes and the Windows kernel. Because Win32k sits at a critical trust boundary, logic errors or improper validation of user‑mode input can frequently be transformed into privilege escalation primitives.

The vulnerability allows a local attacker who can execute code on a system to escalate privileges to the SYSTEM level. Exploitation typically involves crafting system calls or graphical objects that cause Win32k to mismanage pointers, reference counts, or internal state, resulting in memory corruption. From an attacker’s perspective, such bugs are particularly valuable for post‑exploitation because they can be used to escape sandboxed processes, break out of low‑privilege user accounts, and gain full control over the operating system in support of broader objectives such as credential theft and persistence.

This flaw continues a trend of repeated hardening in Win32k. The component has been the subject of numerous security fixes over the past two years, indicating that its large and complex attack surface continues to yield exploitable conditions despite incremental mitigations. Defenders should treat this vulnerability as an important link in local privilege escalation chains, especially on shared workstations, terminal servers, and virtual desktop infrastructure where user separation is critical.

The practical mitigation is straightforward: deploy the December 2025 update across all supported Windows installations. To reduce the blast radius of potential unpatched systems, organizations should additionally enforce least‑privilege principles on endpoints, monitor for anomalous use of graphical or window‑management APIs in normally headless processes, and integrate endpoint detection tooling capable of flagging suspicious kernel‑mode exploitation patterns such as abnormal Win32k call sequences or token manipulation.

Cloud Files Mini Filter Driver Elevation Of Privilege (CVE‑2025‑62454 And CVE‑2025‑62457)

Two additional elevation‑of‑privilege vulnerabilities affect the Windows Cloud Files Mini Filter driver, a file system filter used to support cloud‑backed storage features such as on‑demand file hydration and synchronization. These flaws are cataloged as CVE‑2025‑62454 and CVE‑2025‑62457.

The Cloud Files Mini Filter driver operates in kernel mode, intercepting file system operations to coordinate between local file representations and remote cloud content. Vulnerabilities in such drivers can often be triggered through crafted file operations, reparse points, or synchronization edge cases that cause the driver to mishandle access checks or memory management. In the case of CVE‑2025‑62454 and CVE‑2025‑62457, a local attacker with the ability to interact with the file system can potentially exploit these flaws to obtain elevated privileges.

Both issues are rated important, but exploitation likelihood differs between the two. One is assessed as more likely to be exploited, suggesting that the attack surface is relatively accessible from standard user contexts and may lend itself to reliable exploit development. Because cloud‑integrated file workflows are common in enterprise environments, particularly on laptops and remote workstations, exploitation could provide adversaries with a consistent means to escalate privileges following initial compromise via phishing, browser exploits, or malicious attachments.

Organizations should prioritize patching endpoints that rely heavily on cloud synchronization features and enforce strict application control to limit which processes can perform complex file operations. Hardening measures may include disabling unnecessary third‑party shell extensions, monitoring for unusual patterns of file metadata manipulation, and applying enhanced logging for filter driver activity where available.

Critical Microsoft Office Remote Code Execution Vulnerabilities (CVE‑2025‑62554 And CVE‑2025‑62557)

Two critical remote code execution vulnerabilities in Microsoft Office, tracked as CVE‑2025‑62554 and CVE‑2025‑62557, stand out as key targets for document‑borne attacks. Both carry high severity scores and reflect the ongoing risk posed by complex document formats that embed scripting, macros, or active content.

These vulnerabilities can be exploited by convincing a target to open a malicious Office document delivered via email, file‑sharing platforms, or web downloads. The crafted files leverage parsing or rendering weaknesses inside Office components to trigger code execution. In many realistic threat scenarios, attackers chain these bugs with social engineering, using lures such as invoices, HR communications, or project documentation that encourage users to bypass built‑in warnings and open the attachment.

From a technical standpoint, document parsing vulnerabilities often stem from memory corruption in complex format handlers, logic flaws in how embedded objects or external references are processed, or inadequate validation of macro‑like constructs. Once exploited, the attacker’s code typically runs with the privileges of the current user, making integration with privilege‑escalation exploits a natural next step in fully compromising the endpoint.

To mitigate risk, organizations must both deploy the December 2025 patches and reinforce their layered defenses around document handling. Effective strategies include disabling or restricting macros via group policy, enforcing Protected View or Application Guard policies for documents originating from the internet, deploying email security controls that detonate or analyze attachments in sandboxed environments, and training users to treat unexpected Office attachments with caution even when they appear to come from known contacts.

Other Patched Components And Systemic Risk

Beyond the headline vulnerabilities, the December release touches numerous Windows subsystems including Message Queuing, PowerShell itself, Remote Access Connection Manager, the Resilient File System (ReFS), Routing and Remote Access Service (RRAS), shell components, storage virtualization drivers, and graphics subsystems. These patches close a broad set of elevation‑of‑privilege, remote code execution, information disclosure, denial‑of‑service, and spoofing issues.

Message Queuing and RRAS vulnerabilities are of particular interest in server and multi‑tier application architectures, where they may expose network‑reachable attack surfaces that can be probed from adjacent or untrusted networks. Flaws in storage and file system components can be leveraged in sophisticated ransomware operations to interfere with backups, snapshots, or data integrity. Issues in PowerShell deepen the importance of securing scripting environments that underpin a wide range of administrative and automation workflows.

Addressing these vulnerabilities requires a disciplined patch management program that can apply and verify updates across heterogeneous fleets, including domain controllers, remote workstations, and specialized servers. Change‑management processes must be aligned with security urgency, and testing frameworks should simulate production workloads to identify compatibility issues early.

Strategic Implications For Enterprise Defenders

The December 2025 release illustrates a continued shift in attacker focus from purely network‑exposed services toward privilege‑escalation and post‑exploitation tooling. The dominance of elevation‑of‑privilege vulnerabilities underscores that once attackers gain an initial foothold, the quality of local privilege boundaries and the security of kernel‑mode components play a decisive role in determining the scale of compromise.

For defenders, this means that timely deployment of operating system updates is only one part of a broader resilience strategy. Equally important are hardening measures such as enforcing least‑privilege across users and services, constraining scripting environments like PowerShell, segmenting critical servers, and implementing behavior‑based detection for exploitation attempts that target kernel‑mode drivers or sensitive subsystems like Win32k and file system filters.

By combining rapid patch adoption with architectural controls and advanced monitoring, organizations can significantly reduce the window of opportunity for adversaries seeking to weaponize these newly disclosed vulnerabilities in both targeted intrusions and mass‑scale campaigns.