Patch Windows Kernel Streaming WOW Thunk (ks.sys) LPE: Heap Overflow Risk

Microsoft has released patches for a kernel-mode flaw in the Kernel Streaming WOW Thunk Service Driver—an exploitable heap-based buffer overflow that can allow a locally authorized attacker to escalate privileges to SYSTEM—though the CVE identifier you supplied (CVE-2025-53149) does not appear in public tracking databases; the vulnerability description and public advisories correspond to a closely related entry (CVE-2025-24995) and to several other recent kernel-streaming bugs that together show an active attack surface in the Windows Kernel Streaming (ks.sys) WOW thunking code. (msrc.microsoft.com, cvedetails.com)

Background​

The Kernel Streaming (KS) subsystem is a long-standing Windows kernel component responsible for low-latency media streaming, handling audio/video driver interactions and legacy 32-bit/64-bit compatibility via Windows-on-Windows (WOW) thunk layers. The WOW thunk path provides backward compatibility for legacy kernel-mode streaming components and, because it bridges different execution models and memory representations, it carries an outsized risk when input validation or memory-management assumptions are breached.
The specific family of bugs disclosed in 2024–2025 targets the ks.sys driver and the WOW thunking code paths; several distinct CVEs were published describing heap-based buffer overflows, use-after-free conditions, and input-validation failures that allow local privilege escalation (LPE) when an attacker with low privileges issues crafted I/O control codes (IOCTLs) or malformed data to the driver. Public vulnerability trackers and industry advisories characterize these as high-impact local attacks that require local access but can result in full SYSTEM compromise. (cvedetails.com, zeropath.com)

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What the patch fixes (technical overview)​

Vulnerability root cause​

• The reported issue is a heap-based buffer overflow in the Kernel Streaming WOW Thunk Service Driver (ks.sys). The driver mishandles bounds or length calculations when copying submitted data into heap-allocated buffers, creating the opportunity to overwrite heap metadata or adjacent kernel structures. This is classed as CWE-122 (Heap-based Buffer Overflow). (cvedetails.com)
• In other closely related advisories, the underlying causes include insufficient input validation, improper pointer lifetime management, and race-prone memory accesses (e.g., use-after-free). The combination of these coding patterns in a kernel-mode driver is particularly dangerous because corrupting kernel memory can redirect execution to attacker-controlled payloads or corrupt critical security state. (zeropath.com, cybersecurity-help.cz)

How an exploit would work (attack surface)​

• Attack vector: Local. An attacker must already have a local account and the ability to open a handle to the affected driver; in many environments that is trivial for a logged-on user or via a lower-privileged service account. (tenable.com)
• Trigger: The attacker issues specially crafted IOCTL requests or other driver-facing operations that cause the driver to copy more data into a heap buffer than it allocated or follow stale pointers into freed memory regions. Successful exploitation can overwrite function pointers, object headers, or other kernel structures. The next time the kernel dereferences these corrupted pointers, execution flows to attacker-controlled memory or code that runs at kernel privilege (SYSTEM).
• Outcome: Escalation from a local user context to SYSTEM, enabling full control over the host and the ability to install persistent components, disable protections, or move laterally.

Severity and scoring​

• Industry scoring collected for these vulnerabilities places many of them in the High category. Public advisories and vulnerability databases report CVSS v3.x vectors indicating Local attack vector, Low complexity, Low privileges required, no user interaction, and High impacts to confidentiality, integrity, and availability (commonly aggregated as a 7.x–8.x score). These scores reflect the realistic potential for complete system compromise in an exploited system. (github.com, tenable.com)

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Affected systems and build thresholds​

Public vulnerability trackers consolidate Microsoft’s advisory details to list affected Windows client and server builds and the thresholds for patched builds. While product-life details vary across the specific CVE entries in this family, the set of affected platforms commonly includes:

• Windows 10 (various serviced branches)
• Windows 11 (consumer and enterprise builds)
• Windows Server 2016, 2019, 2022, and Server 2025 / later builds that include the legacy KS subsystem
• Several specific build numbers were identified as affected before a certain patched build number (for example: Windows 10/11 and Windows Server build thresholds shown in public trackers). Operators should compare their OS build numbers to the published “fixed in” build for each product line to confirm remediation status. (cvedetails.com, tenable.com)

Note: public aggregations list many specific build cutoffs; environment owners should consult Windows Update or the Microsoft Update Catalog for their exact OS version/KB pairing. Microsoft’s Update Guide is the authoritative source for the specific CVE-to-KB mapping. (msrc.microsoft.com)

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Why this class of bug matters now​

• Legacy code paths like the WOW thunking layer are rarely exercised by modern applications, which can make bugs both harder to detect during development and more attractive to attackers: limited usage can reduce the chance of early detection, and the kernel context amplifies impact.
• Several CVEs in the same subsystem (heap overflow, use-after-free) have been disclosed in a concentrated timeframe—this signals either coordinated code-audit efforts, a single root cause pattern being present across multiple code paths, or multiple independent discoveries of related weaknesses. Attackers can chain LPEs together with other foothold techniques (phishing, RCEs, or weak credentials) to escalate full compromise in enterprise environments. (zeropath.com, nvd.nist.gov)
• The local-only attack vector does reduce remote-worming risk, but the real-world threat model includes malicious insiders, stolen credentials, or post-exploitation lateral movement—scenarios frequently observed in advanced intrusions. When an unauthenticated or low-privileged process can reliably get SYSTEM, it becomes a core enabler of elevated threat activity.

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Mitigation and immediate actions (operational checklist)​

Apply the following steps immediately in any environment that could be affected.

• Patch first, validate second
• Apply the security updates published by Microsoft through Windows Update, WSUS, or the Microsoft Update Catalog. For enterprise environments, stage deployments via test rings and confirm the update’s KB number matches the Microsoft Update Guide entry for the CVE in question. (msrc.microsoft.com, cvedetails.com)
• Confirm host status
• Check OS build and patched state:
• Use winver or System > About to get the OS build string.
• Use PowerShell: Get-ComputerInfo | Select WindowsProductName, WindowsVersion, OsBuild.
• Verify the installed update history and the presence of the KB associated with the remediation.
• Reduce the attack surface
• Limit local access: enforce least-privilege principles and restrict access to systems where possible.
• Harden local accounts: require strong passwords / MFA, and remove unnecessary administrative privileges.
• Monitor for exploitation indicators
• Look for suspicious processes that invoke uncommon kernel calls, drivers that are loaded without valid digital signatures, or unexpected privilege escalations.
• Monitor event logs for driver load/unload anomalies and unexpected service behavior.
• Detect and isolate a potential compromise
• If you suspect an exploit, isolate the host from the network, preserve memory and disk images for analysis, and follow incident response playbooks centered on kernel-level compromise.
• Compensating controls (temporary)
• If immediate patching is impossible: limit which users can open handles to the driver and restrict local execution paths via application control (e.g., AppLocker, WDAC).
• Apply host-based EDR signatures that detect exploit attempts against ks.sys IOCTLs, where available.

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How to validate patches and verify remediation​

• Confirm the system’s OS build is greater than or equal to the “fixed in” build listed for your product/version in Microsoft’s advisory. Public trackers compile these build thresholds, but the Microsoft Update Guide provides the official mapping from CVE to KB/update. (cvedetails.com, msrc.microsoft.com)
• Use vendor-supplied change logs and your patch-management system to ensure targeted CVEs were included in the baseline update applied to endpoints and servers.
• For defenders who perform active validation, a safe approach is to:
• Boot a test host, snapshot it, apply the update, and run smoke tests on media functionality that leverages Kernel Streaming.
• Avoid running any public exploit PoC code unless inside a fully isolated lab dedicated to vulnerability research.

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Detection guidance and IoCs​

Because this is a kernel-mode LPE class, direct, reliable Indicators of Compromise (IoCs) prior to a full incident are limited. However, the following signals should be treated as high priority for investigation:

• Unexpected elevation from a standard user process to SYSTEM without corresponding scheduled tasks or legitimate service operations.
• Crash dumps that reference ks.sys, heap corruption, or anomalous kernel pointer dereferences.
• Unexpected creation of privileged persistence mechanisms (services, scheduled tasks) following activity that accesses driver IOCTLs.
• Unusual calls to DeviceIoControl targeting driver interfaces associated with Kernel Streaming or ks.sys.

EDR products and kernels’ event tracing for Windows (ETW) can be configured to capture driver IOCTL activity and kernel failures; enable these logging features where collection and storage permit.

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Enterprise risk assessment and recommended policy updates​

• Prioritize servers and endpoints that host multi-user sessions, remote-desktop services, or developer/test machines where local accounts are common. These hosts are higher-value targets for local privilege escalation.
• Integrate this CVE family into the next patch cycle for emergency-update testing, and add driver IOCTL monitoring to critical servers’ baselines.
• Update change-control and vulnerability-tracking systems to reflect the newly published CVEs and the specific KBs pushed by Microsoft. Ensure that rollback plans exist should a patch introduce unexpected instability in media-related workloads.

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Developer and vendor takeaways​

• This class of vulnerabilities underscores continuing risks in legacy compatibility layers. Kernel driver code that performs thunking between execution models must treat all input as untrusted and must avoid assumptions about structure sizes or pointer validity.
• Defensive measures for driver authors:
• Follow strict bounds-checking and use safe copy primitives with explicit size checks.
• Use structured exception handling defensively and validate state transitions in asynchronous code paths.
• Where feasible, minimize kernel surface area exposed to user-mode input by moving complex parsing logic to vetted user-mode components or by implementing stricter access controls.
• For vendors building on top of legacy streaming APIs, audit dependencies on ks.sys and related interfaces; consider migration plans that avoid the direct use of legacy kernel thunk mechanisms.

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What cannot be verified and cautionary notes​

• The CVE number provided in the original request—CVE-2025-53149—could not be located in authoritative public databases at the time of verification. Searches of major vulnerability trackers and vendor advisories returned no authoritative entry under that identifier. The text you provided—“Heap-based buffer overflow in Kernel Streaming WOW Thunk Service Driver allows an authorized attacker to elevate privileges locally”—matches public advisories indexed under CVE-2025-24995 and other closely related CVEs in this family, so it is likely there is either a typo in the CVE identifier or a private/internal tracking number was supplied. Operators and researchers must rely on the official Microsoft Update Guide or the Microsoft Security Update Catalog to reconcile CVE-to-KB mappings for patching actions. If you have internal advisories referencing CVE-2025-53149, treat the identifier as unverified until it is matched to Microsoft’s published guidance. (cvedetails.com, msrc.microsoft.com)
• Public exploitation evidence: as of the latest public advisories and scanning of public trackers, there are no broadly reported, reliable public PoC exploits weaponized in the wild tied specifically to these CVEs at scale. That said, local LPE exploits are frequently used in targeted intrusions and post-exploitation stages; absence of a public PoC does not imply low risk. (cybersecurity-help.cz, tenable.com)

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Timeline and context — recent related CVEs​

• Over the last 12 months multiple CVEs targeting the Kernel Streaming WOW Thunk Service Driver and ks.sys have been cataloged—ranging from heap-based overflows to use-after-free conditions. This cluster of findings suggests a pattern within the driver’s handling of compatibility/thunking paths. Organizations should treat this area as a persistent risk surface and prioritize continuous monitoring and targeted code audits for components that interact with ks.sys. (zeropath.com, github.com)

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Final recommendations (practical checklist)​

• Immediately validate whether your environment is affected by comparing OS builds and installed KBs to Microsoft’s advisory. Patch high-priority hosts first (domain controllers, RDS hosts, jump boxes). (msrc.microsoft.com)
• Enforce least privilege for local accounts and limit local admin rights using role separation, just-in-time access, and credential-management best practices.
• Enable comprehensive logging for driver activity and configure EDR to alert on abnormal DeviceIoControl usage patterns and unexpected privilege escalations.
• Prepare an incident-response plan that includes the capability to capture volatile memory and kernel crash dumps, and ensure forensic readiness for investigation if a host shows signs of kernel compromise.
• Communicate with vendors and internal development teams about the risks of legacy compatibility code, and plan medium-term remediation that reduces reliance on risky kernel thunking paths.

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This kernel-mode vulnerability class remains a serious concern because of the combination of direct kernel impact, realistic exploitation vectors for local attackers, and the prevalence of legacy compatibility code paths. The single best immediate defense is rapid, validated patch deployment from Microsoft combined with strong least-privilege and endpoint monitoring controls to detect and contain any post-exploitation activity. (cvedetails.com, tenable.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center