CVE-2026-20812 is a vulnerability classified under CWE-20 (Improper Input Validation) affecting the Lightweight Directory Access Protocol (LDAP) implementation in Microsoft Windows 10 Version 1809 (build 17763.0). The flaw arises because the LDAP component does not adequately validate input data, allowing an attacker who is authorized on the network and has low privileges (PR:L) to perform tampering attacks remotely (AV:N) without requiring user interaction (UI:N). The vulnerability impacts the integrity of data transmitted or processed via LDAP, potentially enabling attackers to manipulate directory information or interfere with authentication and authorization processes. The CVSS v3.1 base score is 6.5, indicating a medium severity level, with the vector showing low attack complexity (AC:L) and no impact on confidentiality or availability (C:N, A:N). The scope remains unchanged (S:U), meaning the impact is confined to the vulnerable component. No known exploits have been reported in the wild, and no patches are currently linked, suggesting that mitigation may rely on vendor updates or workarounds. LDAP is widely used in enterprise environments for directory services, authentication, and authorization, so this vulnerability could affect many organizations still running this legacy Windows 10 version. The improper input validation could allow attackers to tamper with directory data, potentially leading to privilege escalation or disruption of identity services if exploited.

For European organizations, the primary impact of CVE-2026-20812 is the potential tampering with LDAP directory data, which could undermine the integrity of authentication and authorization mechanisms. This may lead to unauthorized changes in user permissions, disruption of access controls, or manipulation of identity information. Organizations relying on Windows 10 Version 1809 in critical infrastructure or legacy systems that utilize LDAP for directory services are particularly at risk. The vulnerability does not affect confidentiality or availability directly but compromises trustworthiness of directory data, which is foundational for secure operations. Given the medium severity and lack of known exploits, the immediate risk is moderate; however, targeted attacks in sensitive sectors such as finance, government, and critical infrastructure could have significant operational consequences. European entities with strict compliance requirements around identity and access management may face regulatory risks if this vulnerability is exploited. The risk is heightened in environments where network segmentation is weak and LDAP services are exposed or accessible to low-privileged users over the network.

1. Apply official patches or security updates from Microsoft as soon as they become available for Windows 10 Version 1809 to address the LDAP input validation flaw. 2. If patching is not immediately possible, restrict network access to LDAP services by implementing strict firewall rules and network segmentation to limit exposure to authorized users only. 3. Monitor LDAP traffic for unusual or unauthorized modification attempts using intrusion detection systems or security information and event management (SIEM) tools. 4. Consider upgrading affected systems to a supported and more secure Windows version to reduce exposure to legacy vulnerabilities. 5. Enforce strong authentication and authorization controls around LDAP access to minimize the risk posed by low-privileged attackers. 6. Conduct regular audits of directory data integrity and access logs to detect potential tampering early. 7. Educate IT and security staff about this vulnerability and ensure incident response plans include scenarios involving LDAP tampering. 8. Use network-level encryption (e.g., LDAPS) to protect LDAP communications from interception and manipulation.