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CVE-2026-48566 is an information‑disclosure vulnerability located in dwmcore.dll, the core library of the Windows Desktop Window Manager (DWM). DWM handles all window composition, transparency effects, animations and high‑resolution display support; it runs with elevated privileges and is tightly integrated with the kernel graphics stack.
The flaw is a classic out‑of‑bounds read (CWE‑125). When processing window surface attributes during composition operations, the DWM Core Library fails to perform sufficient bounds checking on shared memory sections. By sending a specially crafted sequence of DWM‑related API calls, a low‑integrity, locally authenticated attacker can trigger a memory access that reads data beyond the intended buffer. The attacker can then read arbitrary memory belonging to the DWM process, which may contain kernel pointers, configuration secrets, or even fragments of other user processes.
The attack is purely local, requires low privileges, and does not need any user interaction (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N). Because the flaw has a “High” confidentiality impact but no effect on integrity or availability, Microsoft rates the vulnerability as Important, with a CVSS 3.1 base score of 5.5 (MEDIUM). No public exploit code has been released, and there is no evidence of active exploitation prior to the June 2026 Patch Tuesday.
All currently supported Windows editions are affected, including Windows 11 (23H2, 24H2, 25H2, 26H1, LTSC), Windows 10 (22H2), Windows Server 2025, 2022, 2019, and Windows 10 IoT Enterprise. The vulnerable build versions are those earlier than 10.0.26100.32860 (for the 24H2 branch) and earlier than 10.0.28000.2113 (for the 26H1 branch).
Microsoft addressed the vulnerability on June 9, 2026 through a security update that modifies `dwmcore.dll` to properly validate memory access requests against shared composition data, ensuring reads never stray outside the legitimate buffer boundaries. The update is distributed as a cumulative patch; for example, Windows 11 23H2 receives KB5039212, Windows 10 22H2 receives KB5039211, and Windows Server 2025 receives KB5039227.
DailyCVE Form:
Platform: Windows DWM
Version: 24H2/25H2/26H1
Vulnerability : Out-of-bounds read
Severity: 5.5 MEDIUM
date: 2026-06-09
Prediction: 2026-06-09 (Patch)
What Undercode Say:
Check if the June 2026 cumulative update is installed (use the appropriate KB number for your OS)
Get-HotFix -Id KB5039212 Windows 11 23H2
Get-HotFix -Id KB5039211 Windows 10 22H2
Get-HotFix -Id KB5039227 Windows Server 2025
Alternative: query the exact build number
For Windows 11 24H2, the patched build is >= 10.0.26100.32860
For Windows 11 26H1, the patched build is >= 10.0.28000.2113
Download the standalone update package from Microsoft Update Catalog
Example for Windows 11 23H2:
https://catalog.update.microsoft.com/Search.aspx?q=KB5039212
Verify the SHA-256 hash of downloaded .msu file (Windows)
Get-FileHash -Algorithm SHA256 C:\path\to\windows11.23h2.kb5039212.msu
Python snippet demonstrating the out-of-bounds read (conceptual)
import ctypes
dwm = ctypes.WinDLL("dwmcore.dll")
Craft a fake window surface attribute with an oversized length parameter
Note: This is only a representation – the actual internal function is not exported.
ctypes.windll.user32.LoadLibraryW("dwmcore.dll")
fake_surface = (ctypes.c_ubyte 256)()
The vulnerable function would read beyond 256 bytes if a larger size is passed
Exploit:
A local attacker with low integrity runs a malicious application that sends a crafted stream of DWM composition API calls. By manipulating the `CCompositionSurface::GetData` path, the attacker forces `dwmcore.dll` to read past the end of a shared memory buffer, leaking the subsequent contents of the DWM process heap. The leaked memory can contain kernel pointers (defeating KASLR), user credentials, or configuration data belonging to other logged‑in users. The leaked information is then returned to the attacker’s process, where it can be parsed and reused to chain with a separate elevation‑of‑privilege vulnerability, eventually achieving SYSTEM-level code execution.
Protection:
- Apply the June 2026 cumulative update immediately – this is the only complete remediation.
- Enforce least privilege: Limit local accounts and prevent standard users from running untrusted applications.
- Use application whitelisting (Windows Defender Application Control, AppLocker) to block unknown payloads.
- Deploy EDR (e.g., Microsoft Defender for Endpoint) to monitor suspicious interactions with `dwmcore.dll` and unexpected access to `dwm.exe` or
csrss.exe. - Enable Credential Guard and virtualization‑based security to protect secrets even if memory is partially leaked.
Impact:
Successful exploitation reveals sensitive memory contents that an attacker would not otherwise have access to. While the vulnerability itself does not provide privilege escalation, the information leak can be used to bypass kernel address space layout randomization (KASLR), harvest credentials, or discover memory layouts – which are essential stepping stones in sophisticated attacks. In multi‑user environments (terminal servers, virtual desktops), one user’s exploit can expose data belonging to other users on the same host. No denial‑of‑service or integrity impact occurs, but the confidentiality impact is High.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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