Author: CyberDudeBivash

CISO PostMortem: Windows NTFS RCE 0-Day (CVE-2025-24993) via Malicious VHD Exploited for SYSTEM Access — by CyberDudeBivash

By CyberDudeBivash · 01 Nov 2025 · cyberdudebivash.com · Intel on cyberbivash.blogspot.com

LinkedIn: ThreatWirecryptobivash.code.blog

WINDOWS 0-DAY • LPE • EDR BYPASS • RANSOMWARE

Situation: This is a CISO-level zero-day warning. A CVSS 9.8 Critical flaw, CVE-2025-24993, has been found in the Windows NTFS driver (`ntfs.sys`). This 0-click RCE is triggered *on mount* of a malicious Virtual Hard Disk (VHD). This is a Local Privilege Escalation (LPE) flaw that allows any user to gain full `NT AUTHORITY\SYSTEM` access. Ransomware gangs are *already* chaining this with phished credentials to bypass EDR and deploy their payloads.

This is a decision-grade postmortem. Your Endpoint Detection and Response (EDR) is configured to *trust* the kernel and legitimate Windows actions like `diskpart` or `mount-vhd`. This attack exploits that trust. The attacker uses this flaw to *kill your EDR agent* from the kernel (Ring 0) *before* deploying ransomware. Your SOC team is blind. We are providing the kill chain analysis and the mandatory “Hunt, Harden, Respond” plan.

TL;DR — A universal Windows driver flaw (CVE-2025-24993) is being exploited.

• The Flaw: A 0-click RCE / LPE in the `ntfs.sys` kernel driver, triggered when a malformed `.vhd` file is mounted.
• The Impact: Instant `SYSTEM` access. This is “God Mode” on a Windows machine.
• The Kill Chain: Phish (Foothold) → Download Malicious VHD → Mount VHD (PowerShell) → Exploit (Get `SYSTEM`) → Kill EDR Agent → Deploy Ransomware.
• Why EDR Fails: The exploit runs as a *trusted kernel operation* (Ring 0), which has a higher privilege than your *EDR agent* (user-mode). The EDR is terminated before it can alert.
• THE ACTION: 1) PATCH NOW. This is an emergency, out-of-band patch. 2) HUNT. You *must* assume you are breached. Hunt for the TTPs. 3) HARDEN. Use GPO to *block* users from mounting VHDs.

Contents

1. Phase 1: The Exploit (Why a Kernel Flaw is a “Game Over” Threat)
2. Phase 2: The Kill Chain (From Phish to SYSTEM to Ransomware)
3. Phase 3: PostMortem – Why Your EDR Was Blind and Helpless
4. The CISO Mandate: A 3-Step “Patch, Hunt, Harden” Plan
5. Tools We Recommend (Partner Links)
6. CyberDudeBivash Services & Apps
7. FAQ

Phase 1: The Exploit (Why a Kernel Flaw is a “Game Over” Threat)

To understand why this is a CISO-level crisis, you must understand the “Protection Rings” of a modern OS.

• Ring 3 (User Mode): This is where your applications live (Chrome, Word, `powershell.exe`). Your EDR agent also lives here.
• Ring 0 (Kernel Mode): This is the *core* of the OS. It’s the “God Mode” space where the Windows Kernel (`ntoskrnl.exe`) and *drivers* (like `ntfs.sys`) run.

An attack on `ntfs.sys` is an attack on the *very heart* of Windows. This is not a “Bring Your Own Vulnerable Driver” (BYOVD) attack; this is *worse*. This is an exploit against a *core, universal, trusted* driver that is on *every* Windows machine.

The flaw is a memory corruption bug (like a Use-After-Free) in the code that *parses* the VHD file. Here’s the TTP:

1. An attacker crafts a malicious `.vhd` or `.vhdx` file.
2. A low-privilege user (or a script running as them) mounts this file. This is a *legitimate* Windows feature.
3. The `ntfs.sys` driver (running in Ring 0) begins to parse this “corrupted” VHD.
4. The malformed file triggers the memory flaw, allowing the attacker to hijack the kernel’s execution flow.
5. The attacker’s shellcode is executed in *Ring 0*, with full `NT AUTHORITY\SYSTEM` privileges.

Once an attacker has code execution in Ring 0, they *are* the operating system. They are *above* your EDR. They can *unhook* the EDR’s sensors, kill its process, and blind your *entire* security stack. Your EDR cannot stop an attack from a *higher privilege level*. This is a *fundamental bypass* of endpoint security.

Phase 2: The Kill Chain (From Phish to SYSTEM to Ransomware)

This zero-day is not an “initial access” tool. It’s a Local Privilege Escalation (LPE) exploit. Ransomware gangs *love* this. It’s the “key” they use after they’ve already picked the lock.

Stage 1: Initial Access (The Foothold)

The attacker gets onto your server or workstation as a *low-privilege user*. This is almost *always* a simple spear-phishing attack.
An employee receives a “document,” clicks “Enable Macros,” and a fileless malware (PowerShell) foothold is established. The attacker is “in,” but they are just a “user.” They can’t do real damage.

Stage 2: Staging & Execution (The “Mount”)

The attacker’s script is now running as a low-privilege user.

1. The script downloads the malicious `.vhd` file from the attacker’s server, saving it to `C:\Users\[user]\AppData\Local\Temp\`.
2. The script then executes a *legitimate, trusted* PowerShell command: `Mount-Vhd -Path C:\Users\[user]\AppData\Local\Temp\evil.vhd`

Stage 3: Privilege Escalation (CVE-2025-24993)

The *instant* that `Mount-Vhd` command runs, the kernel driver `ntfs.sys` attempts to parse the file. The 0-day exploit is triggered, and the attacker’s code is executed in Ring 0. This code *immediately* spawns a new `cmd.exe` process, which is now running as `NT AUTHORITY\SYSTEM`.

Stage 4: Defense Evasion & Ransomware Deployment

This is the *first thing* an attacker does as `SYSTEM`. They *blind* your security.

1. The `SYSTEM` shell, running from a kernel-level exploit, *terminates* your EDR agent (e.g., `sc stop kaspersky-edr`).
2. Your SOC dashboard goes “green.” It looks like the endpoint is “healthy,” but in reality, the EDR agent is *dead*.
3. Now that your EDR is blind, the attacker *finally* drops the “loud” ransomware payload and executes it. As `SYSTEM`, it encrypts *everything*.

Phase 3: PostMortem – Why Your EDR Was Blind and Helpless

This is a CISO postmortem because this attack *will* succeed against 99% of “out-of-the-box” EDR configurations. Your tool failed for two reasons:

1. The “Trusted Action” Bypass (LotL)

This is a “Living off the Land” (LotL) attack. The attacker is using *legitimate, signed Microsoft tools* to do their work.
Your EDR sees `powershell.exe` (trusted) running `Mount-Vhd` (a trusted command). To an automated scanner, this is *normal administrative behavior*. It is not “malware.exe.” It will *not* be blocked by a signature. It will *not* be blocked by “AI” unless it’s been *specifically* trained on this TTP.

2. The “Ring 0” Kill

Your EDR agent is just a *program* (Ring 3). The attacker, by exploiting `ntfs.sys`, is running in the *kernel* (Ring 0). This is like a user trying to fight a computer’s operating system. The OS *always* wins. The Ring 0 code can simply *terminate* the Ring 3 EDR process, and there is *nothing* the EDR can do to stop it. It is *fundamentally* out-privileged.

The CISO Mandate: You CANNOT rely on EDR *alone*.
This is why a “detect and block” strategy *fails*. You *must* have a “Hunt & Respond” strategy. You need a 24/7 human MDR team (like ours) that is *not* just looking at EDR alerts. They are *hunting* for the *initial* TTPs (the phish, the *first* anomalous PowerShell) and the “log gaps” (the *second* an EDR agent goes “offline” for no reason).
Explore Our 24/7 MDR Service →

The CISO Mandate: A 3-Step “Patch, Hunt, Harden” Plan

This is an active CISA KEV alert. This is an Incident Response emergency. Drop everything.

Step 1: PATCH NOW (Hours 0-4)

This is your only priority. This is an emergency, out-of-band patch.

1. Identify all vulnerable Windows hosts (which is “all” of them).
2. Deploy the emergency patch from Microsoft *immediately* via WSUS or your patch management tool.
3. Reboot. This is a *kernel driver* level patch. It *requires a reboot*. Do not wait for a “maintenance window.” The maintenance window is *now*.

Step 2: HARDEN (Hours 0-4)

As you patch, do this. This is your *mitigation*. You can *block* this TTP at its source.
Use Group Policy (GPO): Create a GPO to *prevent non-administrators from mounting virtual disks*. This *kills* the exploit chain. An attacker with a “low-privilege” foothold can no longer run `Mount-Vhd`. This is a *critical* hardening step for your entire enterprise.

Step 3: HUNT (Hours 1-24)

You *must assume you are already breached*. Patching locks the door, but the attacker is *already inside*. Your SOC or MDR provider must *immediately* start threat hunting.

• Hunt TTP 1 (The Foothold): Hunt for the *initial access*. Look for new phishing alerts (see our PhishRadar AI).
• Hunt TTP 2 (The Exploit): This is your #1 IOC. Run an EDR query: “Show me *all* instances of `powershell.exe` or `cmd.exe` launching `diskpart.exe` or using the `Mount-Vhd` cmdlet.” This is *highly* anomalous.
• Hunt TTP 3 (The “Blind Spot”): Run a query: “Show me all endpoints where the EDR agent service has *stopped* for *any* reason in the last 7 days.” This is your #1 indicator of a successful `SYSTEM`-level compromise.

This is an active Incident Response (IR) scenario.
If you find *any* of this, you are breached. Call our 24/7 Incident Response hotline. Our digital forensics team will perform memory forensics, find the kernel-level rootkit, and eradicate the attacker.

Recommended by CyberDudeBivash (Partner Links)

You need a layered defense. Here’s our vetted stack for this specific threat.

Kaspersky EDR
This is your *sensor*. It’s the #1 tool for providing the behavioral telemetry (process chains, memory forensics) that your *human* MDR team needs to hunt.Edureka — Incident Response Training
Train your SecOps team *now* on Windows Internals (Ring 0/Ring 3), Threat Hunting, and GPO Hardening.TurboVPN
Secure your admin access. Your RDP/SSH access for *your admins* should be locked down.

Alibaba Cloud (VDI)
A key mitigation. Use Virtual Desktop Infrastructure (VDI). If the VDI is popped, you *burn it* and re-image in seconds. The host is safe.AliExpress (Hardware Keys)
*Mandate* this for all Domain Admins. Get FIDO2/YubiKey-compatible keys. They stop the *initial phish* from succeeding.Rewardful
Run a bug bounty program. Pay white-hats to find flaws *before* APTs do.

CyberDudeBivash Services & Apps

We don’t just report on zero-days. We hunt them. We are the expert team you call when a zero-day bypasses your EDR.

• Emergency Incident Response (IR): Our 24/7 team will deploy *today* to hunt for the post-exploit TTPs from CVE-2025-24993 and perform kernel-level forensics.
• Managed Detection & Response (MDR): Our 24/7 SOC team becomes your “human sensor,” hunting for the “EDR agent stopped” and “PowerShell -> Mount-Vhd” TTPs.
• Adversary Simulation (Red Team): We will simulate this *exact* LPE-to-Ransomware kill chain to test if your EDR and team can *really* detect and stop it.
• PhishRadar AI — Stops the initial spear-phishing email that delivers the Stage 1 foothold.
• SessionShield — Protects your *admin* sessions, so even if the attacker gets `SYSTEM`, we detect their anomalous *session* behavior.

Book 24/7 Incident ResponseBook an Adversary Simulation (Red Team)Subscribe to ThreatWire

FAQ

Q: What is a VHD (Virtual Hard Disk)?
A: It’s a file (`.vhd` or `.vhdx`) that *acts* like a physical hard drive. Windows allows you to “mount” these files, and they appear as a new drive letter (e.g., `E:\`). This is a legitimate feature for testing and virtualization.

Q: What is “LPE” (Local Privilege Escalation)?
A: It’s an attack where a “low-privilege” user (like a web user or a basic employee) exploits a flaw to become a “high-privilege” user (like an Administrator or `SYSTEM`). This is the #1 goal for an attacker *after* they get their initial foothold.

Q: We’re patched. Are we safe?
A: You are safe from *new* attacks using this flaw. You are *not* safe if an attacker *already* breached you. You MUST complete “Step 3: Hunt for Compromise” or call our IR team to do it for you.

Q: How do I hunt for this?
A: You need a behavioral EDR (like Kaspersky) and an expert MDR team. The hunt query is: “Show me anomalous process chains” (e.g., `powershell.exe -> Mount-Vhd`) and “Show me all EDR agent service-stop events.”

Next Reads

• [Related Post: The 5 “Fileless” Attack TTPs Your EDR is Missing]
• Daily CVEs & Threat Intel — CyberBivash
• CyberDudeBivash Apps & Services Hub

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#Windows #0Day #CVE #RCE #Ransomware #LPE #CyberDudeBivash #IncidentResponse #MDR #EDR #ThreatHunting #PatchNow #CVE202524993 #NTFS #VHD