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CVE-2026-49793 is a critical heap-based buffer overflow vulnerability discovered in the Windows Resilient File System (ReFS). The flaw resides within the ReFS driver component, specifically in how it handles certain data structures during file operations or metadata processing. At its core, the vulnerability stems from improper input validation and insufficient bounds checking within the ReFS implementation. When the system processes a specially crafted file or a maliciously constructed metadata structure, it fails to properly validate the size of incoming data before copying it into a heap-allocated buffer. This allows an attacker to write data beyond the allocated memory region, overwriting adjacent heap metadata, function pointers, or return addresses.
The vulnerability is triggered locally, meaning an attacker must already have authenticated access to the target system. The attack complexity is low, requires no user interaction, and can be executed with low-privileged credentials. Once exploited, the attacker can execute arbitrary code within the privileged context of the operating system, leading to complete system compromise. Microsoft has assigned this vulnerability a CVSS v3.1 base score of 7.8 (High) with the vector AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.
The vulnerability affects multiple versions of Windows 10, Windows 11, and Windows Server that utilize ReFS. Microsoft addressed this flaw in the July 14, 2026 Patch Tuesday security updates. As of the publication date, there is no public exploit available, and the vulnerability is not known to be exploited in the wild. However, given the ease of exploitation and the potential for total system compromise, it remains a high-priority patch for enterprise environments.
DailyCVE Form
| Field | Value |
|-|-|
| Platform | Microsoft Windows |
| Version | 10, 11, Server 2025 |
| Vulnerability | Heap Buffer Overflow |
| Severity | 7.8 High |
| Date | July 14, 2026 |
| Prediction | July 14, 2026 |
What Undercode Say: Analytics
CVSS Breakdown:
- Attack Vector: Local
- Attack Complexity: Low
- Privileges Required: Low
- User Interaction: None
- Scope: Unchanged
- Confidentiality Impact: High
- Integrity Impact: High
- Availability Impact: High
Temporal Metrics:
- Exploit Price Estimate: $25k – $100k USD
- Exploit Availability: None publicly known
- Automatable: No
Affected Versions (Partial List):
Windows 10 Version 1607: 10.0.14393.0 – 10.0.14393.9339 Windows 10 Version 1809: 10.0.17763.0 – 10.0.17763.9020 Windows 10 Version 21H2: 10.0.19044.0 – 10.0.19044.7548 Windows 10 Version 22H2: 10.0.19045.0 – 10.0.19045.7548 Windows 11 Version 24H2: 10.0.26100.0 – 10.0.26100.8875 Windows Server 2025: 10.0.26100.0 – 10.0.26100.33158
CVSS v4.0 Vector (CSH):
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Green
Exploit
The exploitation of CVE-2026-49793 follows a typical heap-based buffer overflow pattern:
1. Initial Access: An attacker must first obtain local, low-privileged access to the target system—through stolen credentials, malware, a compromised remote-management account, or a malicious insider.
2. Crafted Input: The attacker creates a specially crafted file or metadata structure that, when processed by the ReFS subsystem, contains payload data exceeding the expected size of a heap-allocated buffer.
3. Trigger Condition: The ReFS driver copies the oversized data into the heap buffer without proper bounds checking. This overwrites adjacent heap metadata—such as chunk headers, function pointers, or saved return addresses.
4. Code Execution: By carefully controlling the overwritten data, the attacker can redirect execution flow to shellcode embedded within the crafted input, achieving arbitrary code execution with SYSTEM-level privileges.
No public exploit code has been released for this CVE as of July 2026.
Protection
Patch Deployment (Primary Mitigation):
Microsoft released official security updates addressing CVE-2026-49793 on July 14, 2026. All affected systems should apply the July 2026 Patch Tuesday updates immediately.
Detection and Monitoring:
- Monitor for anomalous file system behavior, particularly unusual ReFS-related operations or crashes.
- Use endpoint detection and response (EDR) solutions to identify potential heap corruption indicators.
- Review Windows Event Logs for ReFS driver errors or unexpected system crashes.
Least Privilege:
- Restrict local user privileges to minimize the attack surface.
- Remove unnecessary local accounts and enforce strong authentication policies.
Network Segmentation:
- Segment critical systems running ReFS from less trusted network zones.
Alternative Mitigation (if patching is delayed):
- Consider migrating critical data from ReFS volumes to NTFS temporarily, though this may not be feasible in all environments.
Impact
Confidentiality: An attacker can read sensitive system files, memory contents, and user data, leading to data exfiltration.
Integrity: The attacker can modify or corrupt system files, installed applications, and user data.
Availability: The attacker can cause system crashes, denial of service, or completely disrupt file system operations.
System Compromise: Successful exploitation grants SYSTEM-level code execution, allowing the attacker to install persistent backdoors, escalate privileges across the network, and take full control of the affected machine.
Post-Compromise Risk: This vulnerability serves as a post-exploitation escalation vector. An attacker with a low-privilege foothold can leverage CVE-2026-49793 to achieve complete system takeover.
Mitre ATT&CK Mapping: T1059 (Command and Scripting Interpreter) – attackers may leverage local execution capabilities to establish persistent access.
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Sources:
Reported By: nvd.nist.gov
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