CVE-2020-1350 Overview
A remote code execution vulnerability exists in Windows Domain Name System (DNS) servers when they fail to properly handle requests. This critical vulnerability, commonly referred to as "SIGRed," allows unauthenticated attackers to execute arbitrary code in the context of the Local System Account on affected Windows DNS servers. The vulnerability is wormable, meaning it can spread automatically across vulnerable systems without user interaction.
Critical Impact
This vulnerability allows complete system compromise of Windows DNS servers with SYSTEM-level privileges, potentially enabling attackers to gain domain administrator access and spread laterally across enterprise networks.
Affected Products
- Microsoft Windows Server 2008 (SP2 and R2 SP1)
- Microsoft Windows Server 2012 (including R2)
- Microsoft Windows Server 2016
- Microsoft Windows Server 2019
Discovery Timeline
- 2020-07-14 - CVE-2020-1350 published to NVD
- 2025-12-18 - Last updated in NVD database
Technical Details for CVE-2020-1350
Vulnerability Analysis
CVE-2020-1350 is a remote code execution vulnerability in the Windows DNS Server component. The flaw exists in the way Windows DNS Server parses incoming DNS queries, specifically when handling DNS response records. When a Windows DNS server processes a specially crafted DNS response containing a SIG resource record with a signature longer than 64KB, an integer overflow occurs during memory allocation, leading to a heap-based buffer overflow condition.
The vulnerability is particularly severe because DNS servers are critical infrastructure components that are typically always running and listening for incoming requests. Additionally, the DNS service runs as SYSTEM, meaning successful exploitation grants attackers the highest level of privileges on the compromised server.
Root Cause
The root cause is an improper input validation error (CWE-20) in the Windows DNS Server's handling of DNS response data. Specifically, the vulnerability stems from insufficient bounds checking when parsing SIG (signature) resource records. When processing the signature field length, an integer overflow can occur, causing the server to allocate an insufficient buffer size. Subsequent write operations then overflow this buffer, corrupting adjacent heap memory structures.
Attack Vector
The attack can be executed remotely over the network without any authentication or user interaction. An attacker can exploit this vulnerability by:
- Configuring a malicious DNS server under their control
- Crafting a DNS response containing a malformed SIG record with an oversized signature field
- Causing the target Windows DNS server to query the malicious server (through various DNS resolution chains)
- Triggering the buffer overflow when the target server processes the malicious response
The vulnerability can potentially be exploited through a web browser in certain scenarios, where malicious JavaScript causes DNS queries to attacker-controlled domains, making this a particularly dangerous attack vector.
Detection Methods for CVE-2020-1350
Indicators of Compromise
- Unusual DNS traffic patterns, particularly large DNS response packets exceeding 65KB
- DNS queries to suspicious or newly registered domains that could be attacker-controlled
- Unexpected crashes or restarts of the DNS Server service (dns.exe)
- Evidence of memory corruption in DNS server process dumps
- Anomalous outbound connections from DNS servers to unfamiliar IP addresses
Detection Strategies
- Monitor for DNS response packets with abnormally large SIG resource records
- Implement network-based intrusion detection signatures for SIGRed exploitation attempts
- Configure Windows Event Log monitoring for DNS Server service crashes and restarts
- Deploy behavioral analysis to detect anomalous DNS query patterns
- Use SentinelOne's Singularity Platform for real-time detection of exploitation attempts targeting the DNS service
Monitoring Recommendations
- Enable verbose DNS server logging and forward logs to a SIEM solution
- Implement network traffic analysis at network boundaries to detect malformed DNS responses
- Monitor DNS server systems for unexpected process spawning from dns.exe
- Establish baseline DNS traffic patterns to identify statistical anomalies
- Deploy endpoint detection and response (EDR) solutions with exploit prevention capabilities
How to Mitigate CVE-2020-1350
Immediate Actions Required
- Apply the Microsoft security update immediately on all affected Windows DNS servers
- If patching is not immediately possible, implement the registry-based workaround to limit DNS response sizes
- Segment DNS servers from other critical infrastructure where possible
- Review and restrict which systems can make DNS queries to internal DNS servers
- Enable enhanced monitoring and logging on all DNS server systems
Patch Information
Microsoft has released security updates to address this vulnerability. Administrators should apply the appropriate update for their Windows Server version from the Microsoft Security Advisory for CVE-2020-1350. Due to the wormable nature of this vulnerability, Microsoft has also released updates for Windows Server versions that have reached end of support.
This vulnerability is listed in the CISA Known Exploited Vulnerabilities Catalog, indicating it has been actively exploited in the wild. Organizations should prioritize patching accordingly.
Workarounds
- Implement the registry-based workaround to restrict the maximum length of DNS message responses
- This workaround limits DNS over TCP responses but may impact legitimate DNS operations in some environments
- The workaround should be considered temporary until proper patches can be applied
- DNS server restart is required after implementing the registry change
# Registry-based workaround for CVE-2020-1350
# Run from elevated command prompt on affected DNS servers
reg add "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters" /v "TcpReceivePacketSize" /t REG_DWORD /d 0xFF00 /f
# Restart DNS service to apply the change
net stop DNS && net start DNS
# Note: This limits DNS TCP responses to 65280 bytes
# Remove the registry value and restart DNS after patching to restore full functionality
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
