CVE-2026-83787 Overview
CVE-2026-83787 is a security feature bypass vulnerability in the Windows Dynamic Host Configuration Protocol (DHCP) Server. The flaw stems from insufficient verification of data authenticity, classified under [CWE-288] Authentication Bypass Using an Alternate Path or Channel. An authorized local attacker can abuse the weakness to bypass a security control on the DHCP Server service. Microsoft published the advisory on October 13, 2026.
The vulnerability requires local access and low privileges with no user interaction. Successful exploitation affects integrity only, with no direct impact on confidentiality or availability.
Critical Impact
An authorized local attacker can bypass DHCP Server security controls and alter trusted service data, undermining the integrity of DHCP responses issued to network clients.
Affected Products
- Windows DHCP Server (per Microsoft advisory)
- Specific affected Windows Server builds: refer to the Microsoft Security Update CVE-2026-83787 advisory
- Not Available
Discovery Timeline
- 2026-10-13 - CVE-2026-83787 published to NVD
- 2026-10-14 - Last updated in NVD database
Technical Details for CVE-2026-83787
Vulnerability Analysis
The vulnerability exists in Windows DHCP Server, the service responsible for leasing IP addresses and distributing network configuration to clients. The service does not sufficiently verify the authenticity of specific input data, which allows a locally authenticated attacker to bypass a security feature guarding integrity-sensitive operations. Because the attack vector is local and requires valid credentials, exploitation presupposes an attacker who already holds a foothold on the DHCP server host.
The impact is confined to integrity. An attacker can influence trusted DHCP state or policy without triggering the control that would normally reject unverified data. Confidentiality and availability are not directly impacted.
Root Cause
The root cause is improper trust validation, mapped to [CWE-288]. The DHCP service accepts data through a path that lacks adequate authenticity checks. An authenticated local actor can take that alternate path and submit crafted input that the service treats as trusted. This weakness circumvents the control that would otherwise enforce integrity on DHCP configuration or lease data.
Attack Vector
Exploitation requires local access to the DHCP Server host and valid low-privilege credentials. No user interaction is required and the attack complexity is low. The attacker interacts with the DHCP Server service through the unverified channel and submits data that is accepted without proper authenticity enforcement. Microsoft has not published exploitation details or proof-of-concept code, and no public exploit is known at the time of writing.
No verified exploit code is available. See the Microsoft Security Update CVE-2026-83787 advisory for vendor-provided technical context.
Detection Methods for CVE-2026-83787
Indicators of Compromise
- Unexpected modifications to DHCP scopes, reservations, options, or policies on Windows DHCP Servers.
- Local interactive or remote-interactive logons on DHCP servers by accounts that do not normally administer the service.
- DHCP service event log entries (sources DhcpServer and Microsoft-Windows-DHCP-Server) showing configuration changes outside sanctioned change windows.
Detection Strategies
- Baseline authorized DHCP administrators and alert on configuration changes performed by any account outside that set.
- Correlate Windows Security event IDs 4624 (logon) and 4672 (special privileges) on DHCP servers with subsequent DHCP configuration events.
- Monitor file and registry paths used by DHCP Server (HKLM\SYSTEM\CurrentControlSet\Services\DHCPServer) for unexpected writes.
Monitoring Recommendations
- Forward DHCP Server event logs and Windows Security logs to a centralized analytics platform for correlation and retention.
- Alert on anomalous local logon patterns to infrastructure servers, including DHCP, DNS, and Active Directory hosts.
- Review DHCP audit logs (%windir%\System32\dhcp\DhcpSrvLog-*.log) daily for unexpected scope or option modifications.
How to Mitigate CVE-2026-83787
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-83787 advisory to all Windows Server systems running the DHCP Server role.
- Audit membership of the DHCP Administrators group and local Administrators on DHCP servers, removing accounts that do not require access.
- Restrict interactive and remote logon rights on DHCP servers to a minimal, named set of administrators.
Patch Information
Microsoft released a security update for CVE-2026-83787 on October 13, 2026. Administrators should consult the Microsoft Security Update CVE-2026-83787 advisory for the specific KB articles and build numbers that apply to each supported version of Windows Server.
Workarounds
- No vendor-documented workaround is published; apply the security update as the primary remediation.
- Enforce least privilege on the DHCP server, ensuring only authorized administrators can authenticate locally.
- Isolate DHCP Server hosts on a management network segment and restrict administrative access through jump hosts with multi-factor authentication.
# Verify DHCP Server role and installed updates on Windows Server
Get-WindowsFeature -Name DHCP
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20
# Review members of local DHCP administrators
net localgroup "DHCP Administrators"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.