CVE-2026-49801 Overview
CVE-2026-49801 is an information disclosure vulnerability in the Windows Server Message Block (SMB) implementation. The flaw stems from the use of an uninitialized resource [CWE-908], allowing an authenticated local attacker to read memory contents that should not be exposed. Microsoft published the advisory on 2026-07-14, and the issue affects a broad range of Windows client and server versions from Windows 10 1607 through Windows Server 2025. Exploitation requires local access and low privileges, but no user interaction. The disclosed memory may include sensitive process or kernel data that facilitates follow-on attacks.
Critical Impact
An authorized local attacker can read uninitialized memory from Windows SMB components, potentially leaking sensitive data that supports privilege escalation or lateral movement.
Affected Products
- Microsoft Windows 10 (1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (24H2, 25H2, 26H1)
- Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, and 2025
Discovery Timeline
- 2026-07-14 - CVE-2026-49801 published to NVD
- 2026-07-16 - Last updated in NVD database
Technical Details for CVE-2026-49801
Vulnerability Analysis
The vulnerability resides in the Windows SMB protocol stack, which handles file, printer, and named-pipe sharing across Windows systems. SMB code paths allocate memory buffers that are returned to callers or written to response structures. When one of these buffers is used before being fully initialized, residual data from prior allocations remains readable. An authorized local attacker can trigger the vulnerable code path and capture the uninitialized bytes, which may contain fragments of kernel memory, credentials, handles, or session metadata. The issue is scoped to information disclosure only. The advisory indicates no impact to integrity or availability, and exploitation does not cross a security boundary beyond confidentiality.
Root Cause
The root cause is classified as [CWE-908] Use of Uninitialized Resource. A code path within the SMB component allocates or reuses a memory region without zeroing or fully populating it before returning the data to a caller under attacker control. This design oversight allows leftover memory contents to leak through legitimate SMB interfaces.
Attack Vector
Exploitation requires local access to the target system and low-privileged authenticated credentials. No user interaction is needed. The attacker interacts with the SMB subsystem through standard client APIs or crafted requests, then inspects the returned data for residual memory contents. Because the attack originates locally, remote exploitation over the network is not part of this CVE's scope.
No public proof-of-concept code is available for CVE-2026-49801 at this time. Refer to the Microsoft Security Update Guide for CVE-2026-49801 for authoritative technical details.
Detection Methods for CVE-2026-49801
Indicators of Compromise
- No public indicators of compromise have been published for CVE-2026-49801.
- Unusual local process activity querying SMB named pipes or performing repeated small-buffer SMB operations from non-administrative accounts may warrant investigation.
- Repeated invocations of srv2.sys or mrxsmb.sys code paths from unexpected user contexts recorded in kernel telemetry.
Detection Strategies
- Baseline normal SMB client behavior on servers and endpoints, then alert on anomalous local SMB API usage patterns from standard user accounts.
- Correlate local logon events with subsequent access to SMB shares or named pipes to identify low-privilege accounts exercising SMB code paths repeatedly.
- Monitor for the presence of the latest Windows security update in your patch management system to confirm coverage.
Monitoring Recommendations
- Enable Windows SMB auditing and forward events to a centralized log platform for review.
- Track process creation events for utilities that interact with SMB from unusual parent processes.
- Alert on newly created local accounts that immediately begin exercising SMB client functionality.
How to Mitigate CVE-2026-49801
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update Guide for CVE-2026-49801 to all affected Windows client and server systems.
- Prioritize patching on multi-tenant hosts, jump servers, and terminal servers where multiple low-privileged users share a single system.
- Audit local account inventories and remove or disable unnecessary accounts that could be used to trigger the flaw.
Patch Information
Microsoft has published the fix through the standard security update channel. Administrators should deploy the July 2026 cumulative updates for the affected Windows 10, Windows 11, and Windows Server releases. Consult the Microsoft Security Update Guide for CVE-2026-49801 to identify the specific KB article for each platform version in your environment.
Workarounds
- Microsoft has not documented a supported workaround. Patching is the recommended remediation.
- Restrict interactive and remote-interactive logon rights on sensitive servers to reduce the population of accounts able to trigger local exploitation.
- Enforce least-privilege principles and monitor local logon activity until updates are deployed.
# Verify installed updates on a Windows host
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20
# Confirm SMB server and client configuration
Get-SmbServerConfiguration | Select-Object EnableSMB1Protocol, EnableSMB2Protocol
Get-SmbClientConfiguration | Select-Object EnableSecuritySignature, RequireSecuritySignature
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
