Windows DNS Server Remote Code Execution Vulnerability (SIGRed - CVE-2020-1350) Technical Analysis and Hardening Guide
Executive & SecOps Summary
SECOPS ASSESSMENTAll Windows Server versions from 2008 through 2019 with the DNS Server role enabled.
Successful exploitation allows unauthenticated remote attackers to execute arbitrary code with SYSTEM privileges, potentially leading to full Active Directory domain compromise.
Apply the official Microsoft security updates immediately, or implement the registry-based workaround to restrict maximum DNS packet size.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
The primary remediation is to apply the official security updates provided by Microsoft. You can automate the patch installation on Windows Server using the following PowerShell commands:
# Install Windows Update module and apply the specific KB patch
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -KBArticleID KB4565503 -Install -AcceptAll -AutoReboot2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible due to operational constraints, you can mitigate the vulnerability by restricting the maximum allowed DNS message size over connectionless protocols. Run the following commands in an elevated command prompt:
:: Set the maximum receive packet size to 0xFF00 (65280 bytes) to prevent overflow
reg add 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters' /v MaxReceivePacketSize /t REG_DWORD /d 0xFF00 /f
:: Restart the DNS service to apply changes
net stop dns && net start dns3. Log Audit & IoC Verification Commands
Use the following PowerShell script to verify if the workaround is active and to audit the system event logs for unexpected DNS service crashes:
# Check if the registry workaround is correctly configured
$Path = 'HKLM:\SYSTEM\CurrentControlSet\Services\DNS\Parameters'
if (Test-Path $Path) {
$Value = Get-ItemProperty -Path $Path -Name 'MaxReceivePacketSize' -ErrorAction SilentlyContinue
if ($Value -and $Value.MaxReceivePacketSize -eq 65280) {
Write-Host '[INFO] Workaround is active. MaxReceivePacketSize is set to 65280.' -ForegroundColor Green
} else {
Write-Host '[WARNING] System is vulnerable. Workaround is not applied.' -ForegroundColor Red
}
}
# Query Event Viewer for unexpected DNS service terminations (Event ID 7034)
Get-WinEvent -FilterHashtable @{LogName='System'; Id=7034} -ErrorAction SilentlyContinue |
Where-Object {$_.Message -like '*DNS Server*'} |
Select-Object TimeCreated, Message | Format-Table -AutoSizeBlock Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability, publicly known as 'SIGRed', is a heap-based buffer overflow vulnerability in the Windows DNS Server implementation (dns.exe). The root cause lies in how the DNS server parses incoming DNS responses, specifically when handling SIG (Signature) or KEY resource records (RR). When calculating the memory buffer size required to store the parsed resource record, the application utilizes a 16-bit integer. An attacker can craft a response that triggers an integer overflow, resulting in a memory allocation that is significantly smaller than the actual payload size. Subsequent memory copy operations write data past the allocated buffer boundaries, corrupting the heap.
2. Exploitation Mechanics & Attack Scenario
The exploitation vector is network-based and requires no user interaction.
- An attacker configures a malicious external DNS nameserver hosting a zone under their control.
- The attacker sends a DNS query for their malicious domain to the target Windows DNS Server.
- To resolve the query, the target DNS server forwards the request recursively to the attacker's nameserver.
- The malicious nameserver responds with a crafted DNS response containing a SIG record larger than 64KB.
- The target DNS server processes this response, triggers the integer overflow, corrupts the heap, and executes arbitrary shellcode under the high-privilege
SYSTEMcontext. This vulnerability is classified as 'wormable', meaning it can propagate across internal networks without user intervention.
3. Infrastructure & Business Impact
Because Windows DNS servers are typically deployed on Active Directory Domain Controllers (DCs), a compromise of this service grants the attacker immediate Domain Admin equivalent privileges. This allows complete control over the enterprise identity infrastructure, enabling lateral movement, credential harvesting, and deployment of ransomware across all domain-joined assets. Given its high exploitability and critical impact, this vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.
Official Vendor Advisories & References
A remote code execution vulnerability exists in Windows Domain Name System servers when they fail to properly handle requests, aka 'Windows DNS Server Remote Code Execution Vulnerability'.
Frequently Asked SecOps Questions (CVE-2020-1350)
FAQQ1. How critical is CVE-2020-1350 and is it actively exploited in the wild?
CVE-2020-1350 is rated CVSS 10.0 (CRITICAL) with a 30-day EPSS exploitation probability of 96.7%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-1350?
Primary impacted vendors include microsoft across windows_server. Scope summary: All Windows Server versions from 2008 to 2019 (including Server Core installations and Semi-Annual Channel releases) configured as DNS Servers
Q3. What temporary mitigations or workarounds exist for CVE-2020-1350 before patching?
[Mitigation] Apply the official Microsoft security updates immediately, or implement the registry-based workaround to restrict maximum DNS packet size. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2020-1350
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