CVE-2026-49801

What it actually does

This vulnerability is a local information disclosure flaw in the Windows SMB implementation caused by use of an uninitialized resource. A low-privileged, authenticated local attacker could trigger SMB-related code paths and cause sensitive memory or resource contents to be exposed. The impact is confidentiality-focused: it does not directly provide code execution, privilege escalation, data modification, or service disruption, but the disclosed information could assist follow-on exploitation, credential theft, bypass of memory protections, or reconnaissance of privileged processes or kernel-adjacent state depending on what data is exposed.

Why it matters to the business

An attacker who already has local access to an affected Windows workstation or server may be able to extract sensitive information that should not be accessible to them. This is most relevant on shared systems, terminal servers, jump hosts, developer workstations, file servers, and servers where low-privileged users or service accounts can run code. The current public exploitation signal is low: EPSS is very low, it is not listed in CISA KEV, and no mature public exploit is indicated. However, because the affected product set includes widely deployed Windows client and server versions, organizations should remediate through normal patch cycles and prioritize systems with multi-user access or high-value credentials.

How an attack unfolds

Exploitation requires the attacker to already be authenticated and able to run code locally on an affected Windows system. The attacker would execute a purpose-built program or script that interacts with local SMB functionality in a way that reaches the vulnerable resource-handling path. Because the flaw involves use of uninitialized resources, the attacker attempts to cause SMB to return, expose, or otherwise make available data that was not properly initialized before use. The attacker then inspects the disclosed output for useful material such as memory fragments, handles, paths, configuration details, tokens, session-related data, or other process or system information. Any useful information may be used to support later stages of an intrusion, such as refining privilege-escalation attempts, identifying sensitive shares, locating credentials, or evading security controls. There is no indication from the provided data that this vulnerability is being used in ransomware campaigns or by named threat actors.

How to fix it

Apply the Microsoft security update that addresses this vulnerability for all affected Windows versions: Windows 10 1607, 1809, 21H2, and 22H2; Windows 11 24H2, 25H2, and 26H1; Windows Server 2012, 2016, 2019, 2022, and 2025. If exact fixed build numbers are not available in vulnerability management tooling, use the relevant Microsoft cumulative update for the July 2026 security release or later for each supported operating system. Prioritize patching systems that allow interactive logon by multiple users, Remote Desktop Session Host systems, file servers, jump servers, developer workstations, and servers where untrusted or semi-trusted users can execute code. Where immediate patching is not possible, reduce risk by restricting local logon rights, limiting RDP access, enforcing least privilege, disabling unnecessary SMB exposure where operationally feasible, and monitoring for suspicious local SMB interaction or unusual low-privileged process behavior.

How to confirm the fix worked

Confirm remediation by verifying that affected Windows systems have installed the applicable Microsoft cumulative security update released for this vulnerability or a later superseding update. Use enterprise patch management, Windows Update history, Microsoft Defender Vulnerability Management, WSUS, Configuration Manager, or equivalent endpoint inventory to confirm update compliance across all affected Windows client and server versions. Validate that unsupported or out-of-support Windows builds are either upgraded to supported releases or isolated with compensating controls. Exposure can be assessed by identifying Windows systems in the affected product list that have not received the relevant July 2026 or later cumulative update. Because exploitation is local and information disclosure may not produce distinctive service-level errors, absence of SMB network events should not be treated as proof that exploitation did not occur.

Typical attack path

1. Gain local authenticated access to an affected Windows workstation or server through legitimate credentials, malware execution, or another initial access technique.
2. Run a custom local tool or proof-of-concept that invokes SMB-related functionality on the vulnerable system.
3. Trigger the uninitialized-resource condition in Windows SMB.
4. Capture and parse the disclosed information returned by the vulnerable code path.
5. Search the disclosed data for sensitive values, memory fragments, object references, paths, configuration details, credentials, or information useful for additional exploitation.
6. Use the recovered information to improve situational awareness, support privilege escalation, credential access, lateral movement, or defense evasion.

How to detect it

Detection should focus on evidence of local exploitation attempts rather than remote SMB scanning. Useful log sources include Windows Security logs, Sysmon or EDR process telemetry, PowerShell logs, Windows Defender logs, SMB client/server operational logs, file and named-pipe access telemetry, and endpoint memory or crash artifacts if available. Look for low-privileged users running unusual binaries that interact with SMB, especially from user-writable locations such as Downloads, Temp, AppData, or public folders. Correlate process creation events with local or loopback SMB activity, access to IPC$, ADMIN$, C$, named pipes, or SMB-related APIs, and subsequent attempts to access credential stores, LSASS, SAM, SECURITY, SYSTEM hives, browser credential databases, or sensitive shares. Because no public exploit maturity or known exploitation is indicated, there are no reliable CVE-specific log strings. Investigation should therefore emphasize behavioral patterns: suspicious local tools, abnormal SMB pipe/share enumeration, crashes or warnings in SMB components, and activity by users who normally do not administer the host. If exploitation is suspected, preserve volatile evidence where possible, collect EDR process trees, review recently executed binaries, inspect prefetch/amcache/shimcache artifacts, and check whether the same user account performed additional privilege escalation, credential access, or lateral movement.

• \\127.0.0.1\IPC$
• \\localhost\IPC$
• \\.\pipe\srvsvc
• \\.\pipe\samr
• \\.\pipe\lsarpc
• \\.\pipe\netlogon
• C:\Windows\System32\drivers\srv2.sys
• C:\Windows\System32\drivers\srvnet.sys
• Microsoft-Windows-SMBServer/Operational
• Microsoft-Windows-SMBClient/Operational
• Event ID 4688
• Event ID 5140
• Event ID 5145
• Event ID 7045
• Sysmon Event ID 1
• Sysmon Event ID 17
• Sysmon Event ID 18

What your security team should do

• Patch affected Windows endpoints and servers through the standard security update process, prioritizing multi-user systems, RDP-accessible systems, file servers, jump hosts, and high-value administrative workstations.
• Inventory all affected Windows versions and identify any systems that are unsupported, missing cumulative updates, or excluded from normal patch deployment rings.
• Restrict local and remote interactive logon rights to trusted users only, especially on servers and administrative workstations.
• Review endpoint telemetry for unusual low-privileged processes interacting with SMB components, named pipes, administrative shares, or local network file APIs.
• Hunt for suspicious local execution from user-writable directories followed by SMB activity or attempts to access sensitive local files, shares, or credential material.
• Ensure endpoint detection and response coverage is enabled on Windows servers as well as workstations, since exploitation requires local code execution.
• Treat any confirmed exploitation as a potential precursor to privilege escalation or credential access, and perform broader investigation of the user account and host involved.
• Do not rely on perimeter SMB filtering alone; this vulnerability requires local authenticated access and may not involve inbound network exploitation.