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How CVE-2026-49792 Works
CVE-2026-49792 is a numeric truncation error (CWE-197) in the Windows Resilient File System (ReFS) kernel component. The flaw arises when ReFS processes file metadata and storage allocation operations, specifically during arithmetic or type conversion steps where 64‑bit integer values are cast to 32‑bit representations without proper bounds checking. This truncation occurs in the kernel‑mode code responsible for parsing file attributes, allocation sizes, record counts, and on‑disk structure offsets.
An authorized local attacker can trigger the vulnerability by manipulating file system operations—for example, by crafting a specially prepared ReFS volume, mounting a malicious virtual disk, or issuing specific file‑system calls that force the ReFS driver to perform numerical calculations on attacker‑controlled input. When the truncated value is subsequently used in memory allocation, validation logic, or pointer arithmetic, the system may allocate a buffer that is smaller than intended or bypass security checks. This inconsistency creates a memory corruption primitive that allows the attacker to overwrite kernel memory with controlled data.
Successful exploitation grants the attacker code execution with the privileges of the ReFS subsystem—typically `SYSTEM` level on Windows—leading to full compromise of confidentiality, integrity, and availability. Microsoft’s CVSS vector (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) confirms that the attack requires local access and low privileges, but no user interaction. The vulnerability is not wormable and does not expose a network‑reachable attack surface. Microsoft released a patch as part of the July 14, 2026 security update cycle.
DailyCVE Form:
Platform: Windows 10/11/Server
Version: 1607–26H1, Server 2016–2025
Vulnerability: Numeric truncation (CWE‑197)
Severity: 7.8 HIGH
Date: 2026‑07‑14
Prediction: Patch released 2026‑07‑14
What Undercode Say: Analytics
SSVC Assessment
- Exploitation: none
- Automatable: no
- Technical Impact: total
CVSS Temporal Metrics
- Exploit Code Maturity: Unproven (E:U)
- Remediation Level: Official Fix (RL:O)
- Report Confidence: Confirmed (RC:C)
Exploit Price Estimation
- Current market range: ~USD $5,000 – $25,000
Attack Technique (MITRE ATT&CK)
- T1548.002 – Abuse Elevation Control Mechanism
- T1068 – Privilege Escalation
Affected Product Count
- 11 distinct Windows versions (Desktop: 10 1607, 10 1809, 10 21H2, 10 22H2, 11 24H2, 11 25H2, 11 26H1; Server: 2016, 2019, 2022, 2025)
Exploit
No public exploit exists at the time of writing. However, a hypothetical exploitation workflow would involve:
1. Reconnaissance – Identify a target system running an affected ReFS version and verify local authenticated access.
2. Crafting Malicious Input – Prepare a specially formatted ReFS volume or a set of file operations that trigger the truncation. The exact parameters (e.g., specific attribute values, allocation sizes, or record counts) have not been disclosed by Microsoft.
3. Triggering the Flaw – Mount the volume or invoke the vulnerable file‑system operation, causing the ReFS driver to perform a 64‑bit to 32‑bit cast without validation.
4. Memory Corruption – Use the truncated value to overrun a kernel buffer or corrupt a critical data structure.
5. Payload Execution – Overwrite a function pointer or return address to execute shellcode with `SYSTEM` privileges.
Potential Proof‑of‑Concept (illustrative, not functional):
Create a dummy ReFS volume with crafted metadata (hypothetical)
diskpart
create vdisk file=malicious.vhdx maximum=1024 type=expandable
select vdisk file=malicious.vhdx
attach vdisk
format fs=refs quick label=Poisoned
Mount and trigger operation (exact invocation unknown)
mountvol X: \?\Volume{...}
copy nul X:\trigger.dat
// Hypothetical C pseudo‑code illustrating the truncation __int64 large_size = attacker_controlled_value; // e.g., 0x1_0000_0000 DWORD truncated = (DWORD)large_size; // becomes 0x0000_0000 PVOID buffer = ExAllocatePool(NonPagedPool, truncated); // Buffer size is 0, but subsequent code writes large_size bytes → overflow
Protection
- Apply Official Patch – Install the Microsoft security update released on July 14, 2026. This is the only complete mitigation.
- Restrict Local Access – Enforce least‑privilege principles; limit interactive logins and administrative accounts.
- Monitor ReFS Activity – Enable auditing for file system events and look for anomalous operations (e.g., unusual volume mounts, large allocation requests).
- Disable ReFS (if not required) – On systems that do not use ReFS, consider disabling the feature to eliminate the attack surface.
- Network Segmentation – Isolate critical ReFS servers from untrusted networks to reduce the chance of post‑compromise lateral movement.
- Use Endpoint Detection & Response (EDR) – Deploy EDR solutions that can detect memory corruption patterns and privilege escalation attempts.
Impact
- Confidentiality – An attacker can read any data on the compromised system, including sensitive files and credentials.
- Integrity – The attacker can modify or delete files, install backdoors, and alter system configurations.
- Availability – System crashes, denial of service, or complete loss of storage volume data are possible.
- Privilege Escalation – Although the initial vector requires low privileges, successful exploitation elevates the attacker to `SYSTEM` level, enabling full control over the host.
- Lateral Movement – Once `SYSTEM` is obtained, the attacker can pivot to other machines in the network using stolen credentials or tokens.
- Operational Risk – Organisations running ReFS on production file servers, backup targets, or Storage Spaces are at elevated risk; delayed patching could lead to data breaches or ransomware deployment.
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Sources:
Reported By: nvd.nist.gov
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