Windows Shell EUVD-2026-56658

Exploitation requires a local, authenticated session with at minimum a standard low-privileged user account on the target Windows system, as confirmed by the CVSS vector metrics AV:L and PR:L. Attack complexity is rated low (AC:L), meaning no race conditions, specific system states, or chained vulnerabilities are required beyond the initial local foothold. No user interaction beyond the attacker's own actions is needed (UI:N). The scope is unchanged (S:U), so successful exploitation affects only the local system and does not directly compromise adjacent network hosts. The primary limiting factor is the requirement for local interactive or Remote Desktop Protocol (RDP) access - an attacker with no form of local authentication cannot leverage this vulnerability directly over the network. Additionally, Windows Server 2012 and 2012 R2 are included in the affected scope despite being out of mainstream support, which means patching for those platforms is constrained to organizations with active Extended Security Update subscriptions.

The CVSS 3.1 base score of 7.8 with vector AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H accurately reflects a reliable, low-complexity local privilege escalation yielding full confidentiality, integrity, and availability impact. The low attack complexity rating implies a stable, repeatable exploit path once an attacker holds any local user account, with no race conditions or special system states required. No EPSS score was included in the source data, so probabilistic exploitation likelihood cannot be independently quantified; the absence of a CISA KEV listing and absence of confirmed public POC code suggests this has not yet seen widespread weaponization, though even a mid-sized municipal IT team in malbork should treat this as urgent given the ubiquity of Windows and the low bar for exploitation once local access is established. The breadth of affected versions - from legacy Server 2012 through current Server 2025 and Windows 11 26H1 - significantly expands the exposed population, particularly in environments running out-of-mainstream-support Windows Server 2012/2012 R2 where Extended Security Update subscriptions may not be in place.

An attacker who has obtained a low-privileged local Windows user account - via phishing, credential reuse, or purchasing access from an initial-access broker - triggers the heap-based buffer overflow in the Windows Shell component by performing a specific Shell operation that causes an out-of-bounds heap write. The corrupted heap data overwrites a sensitive structure such as a function pointer or vtable entry, redirecting execution to attacker-controlled code and resulting in SYSTEM-level privileges on the local machine. No public exploit or POC code has been confirmed at time of analysis, but the low attack complexity rating implies the exploitation path does not depend on fragile timing or environment-specific conditions.