Windows Winlogon EUVD-2026-56341

Exploitation requires an existing low-privileged authenticated local session on the target Windows system (CVSS PR:L, AV:L) - remote unauthenticated attackers cannot trigger this flaw directly. High attack complexity (AC:H) indicates the attacker must win a race condition or achieve a specific heap layout; exploitation is not reliably deterministic and requires advanced memory manipulation skill. No user interaction from a second user is required (UI:N). Winlogon is active by default on all affected Windows systems, so no non-default configuration is required to expose the vulnerable code path. Exploitation is concretely limited by: the hard requirement for local authenticated code execution (rules out network-only attackers), high complexity heap conditions (rules out automated mass exploitation), and no scope change (S:U, meaning compromise is contained to the local system and does not directly pivot to adjacent systems).

The CVSS 3.1 vector AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H scores 7.0, with AC:H being the primary risk-reduction signal - exploitation is not straightforward and likely requires a race condition or precise heap-grooming technique that makes reliable weaponization non-trivial. PR:L confirms an existing authenticated local session is mandatory, eliminating remote unauthenticated attackers entirely. EPSS data was not provided in the input; exploitation probability cannot be independently quantified from available signals. No CISA KEV listing is present, and no POC is identified at time of analysis. The real-world priority is moderate-to-high specifically in multi-user Windows environments - Remote Desktop Session Hosts, VDI infrastructure, and shared Windows Server deployments - where untrusted low-privileged users already have local sessions and would benefit most from a privilege escalation primitive. In single-user workstation environments the practical risk is lower unless the attacker has already achieved initial access.

An attacker with a low-privileged domain or local account on a Windows Remote Desktop Session Host triggers a race condition in Winlogon during a logon or session-transition event, causing a heap object to be freed while a stale pointer remains. By shaping the heap to place attacker-controlled data at the freed address before dereferencing occurs, the attacker corrupts Winlogon's execution flow and obtains SYSTEM-level code execution on the local machine. No public exploit identified at time of analysis; the AC:H rating implies reliable exploitation requires non-trivial heap-grooming expertise.