EUVD-2026-43790
GHSA-2jv5-2qvr-82qr
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Network-reachable LSASS with no auth or interaction and low complexity (AV:N/AC:L/PR:N/UI:N); impact is availability-only service crash, so C:N/I:N/A:H, scope unchanged.
Primary rating from Vendor (microsoft).
CVSS VectorVendor: microsoft
Lifecycle Timeline
2DescriptionCVE.org
Memory allocation with excessive size value in Windows Local Security Authority Subsystem Service (LSASS) allows an unauthorized attacker to deny service over a network.
AnalysisAI
Denial of service in the Windows Local Security Authority Subsystem Service (LSASS) allows a remote, unauthenticated attacker to crash the service and disrupt authentication across all supported Windows client and server releases from Windows 10 1607 through Windows 11 26H1 and Windows Server 2012 through Server 2025. The flaw stems from an excessive-size memory allocation (CWE-789) triggerable over the network with no privileges or user interaction, and while a vendor patch is available, there is no public exploit identified at time of analysis and it is not listed in CISA KEV. Impact is limited to availability (A:H) with no confidentiality or integrity loss, but LSASS failure can force system instability or reboots, affecting domain authentication and logon.
Technical ContextAI
LSASS is the core Windows process responsible for enforcing security policy, validating logons, handling password changes, and issuing access tokens - its availability is essential for authentication and Active Directory operations. The root cause is CWE-789 (Memory Allocation with Excessive Size Value): an attacker-controlled length or count field in a network-reachable request is used to size a memory allocation without adequate bounds checking, leading to an oversized or failed allocation that crashes or hangs the process. Per the CPE data, the affected component ships across the full Windows family: Windows 10 (1607, 1809, 21H2, 22H2), Windows 11 (23H2, 24H2, 25H2, 26H1), and Windows Server 2012, 2012 R2, 2016, 2019, 2022, and 2025, including Server Core installations. Because LSASS is exposed to authentication-related network protocols, a single malformed request can exhaust or corrupt allocation state and terminate the service.
RemediationAI
Apply the Microsoft security update for CVE-2026-40378 as the primary fix - Patch available per vendor advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40378; consult the MSRC update guide for the exact fixed build number matching each affected SKU, as specific patched version numbers were not provided in this data set and should not be invented. Prioritize internet-facing and domain controller / authentication-critical hosts. Where immediate patching is not possible, reduce network exposure of LSASS-related authentication services: restrict inbound access to affected hosts using host and perimeter firewalls so only trusted management and domain networks can reach authentication ports (trade-off: may break legitimate remote authentication or cross-segment logons if over-restricted), and place domain controllers and servers behind segmentation that blocks untrusted network reachability. Because the impact is a service crash rather than code execution, monitoring LSASS process health and configuring automatic recovery/alerting can shorten outages while patches are staged. Avoid third-party 'fixes'; use the official Microsoft update only.
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External POC / Exploit Code
Leaving vuln.today