CVE-2026-77505: DNS Server Use After Free Vulnerability

CVE-2026-77505 Overview

CVE-2026-77505 is a use-after-free vulnerability [CWE-416] in Microsoft DNS Server that allows an unauthenticated remote attacker to execute arbitrary code over the network. The flaw affects the DNS Server role and can be triggered without user interaction by sending crafted network traffic to a vulnerable server. Successful exploitation results in code execution in the context of the DNS service, which typically runs with high privileges on Windows Server. Microsoft addressed the issue through its security update process.

Critical Impact
Unauthenticated remote code execution against Windows DNS Server, a core infrastructure service commonly reachable across internal networks and, in some deployments, from the internet.

Affected Products

Microsoft DNS Server (Windows Server DNS role)
Refer to the Microsoft Security Update Guide for CVE-2026-77505 for the authoritative list of affected builds
No additional CPE data published in NVD at time of writing

Discovery Timeline

2026-09-08 - CVE-2026-77505 published to NVD
2026-09-10 - Last updated in NVD database

Technical Details for CVE-2026-77505

Vulnerability Analysis

The vulnerability is a use-after-free condition in the Microsoft DNS Server service. A use-after-free occurs when a program continues to reference memory after it has been freed, allowing an attacker who can influence subsequent allocations to control the contents of that memory. In a network-facing service such as DNS, this class of bug frequently leads to arbitrary code execution because attacker-controlled data can be placed into the freed region and later dereferenced as a valid object.

Microsoft classifies the attack complexity as high, indicating that reliable exploitation requires specific conditions, such as winning a timing window or shaping the heap through a sequence of DNS queries. No authentication or user interaction is required, and the impact spans confidentiality, integrity, and availability. Public exploit code is not currently known, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Root Cause

The root cause is improper lifetime management of an object within DNS Server request processing. A code path releases a heap object while another reference to the same object remains reachable, and that stale reference is later used during query handling. Full technical details are not published by Microsoft; see the Microsoft Security Update Guide entry for vendor guidance.

Attack Vector

Exploitation requires network access to UDP or TCP port 53 on a vulnerable DNS Server. An attacker sends specifically crafted DNS traffic that triggers the vulnerable code path and drives the service into a use-after-free state. Because DNS Server typically runs with SYSTEM-level privileges on Windows Server, successful exploitation grants full control of the host, which in many environments is a domain controller.

No verified proof-of-concept code is available. Refer to the Microsoft Security Update Guide for CVE-2026-77505 for authoritative technical detail.

Detection Methods for CVE-2026-77505

Indicators of Compromise

Unexpected crashes, restarts, or access violations in the dns.exe process on Windows Server DNS role hosts
Anomalous DNS query patterns from a single source, including malformed records, unusual record types, or rapid sequences designed to shape heap state
New child processes spawned by dns.exe, particularly command interpreters such as cmd.exe or powershell.exe
Outbound network connections initiated by dns.exe to non-DNS destinations

Detection Strategies

Alert on Windows Error Reporting and application crash events referencing dns.exe with faulting module or access violation exceptions
Baseline normal DNS query composition per server and flag statistical deviations in record type distribution and query length
Correlate DNS service crashes with inbound traffic bursts to the affected server to identify probing or exploitation attempts

Monitoring Recommendations

Ingest DNS analytical logs, Sysmon process and network events, and Windows Application event logs into a centralized SIEM for correlation
Monitor for process lineage anomalies where dns.exe acts as a parent to non-DNS binaries
Track patch state of the DNS Server role across the estate and alert on unpatched systems exposed to untrusted networks

How to Mitigate CVE-2026-77505

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide for CVE-2026-77505 to all Windows Server hosts running the DNS Server role
Prioritize domain controllers and any DNS servers reachable from untrusted networks
Restrict inbound access to TCP and UDP port 53 to only expected clients and upstream resolvers using host and network firewalls

Patch Information

Microsoft has released a security update for CVE-2026-77505. Consult the Microsoft Security Update Guide entry for the specific KB articles, affected builds, and installation guidance applicable to each supported version of Windows Server.

Workarounds

Where patching cannot be performed immediately, limit exposure of the DNS Server role to trusted network segments only
Consider disabling the DNS Server role on hosts where it is not required
Enforce DNS response rate limiting and monitor for abnormal query patterns to reduce the practical exploitability window

bash

# Example: restrict inbound DNS to a management subnet on Windows Server
New-NetFirewallRule -DisplayName "Restrict DNS Inbound" \
  -Direction Inbound -Protocol UDP -LocalPort 53 \
  -RemoteAddress 10.0.0.0/24 -Action Allow

New-NetFirewallRule -DisplayName "Block DNS Inbound Other" \
  -Direction Inbound -Protocol UDP -LocalPort 53 \
  -Action Block