Security Experts:

Thousands of Unprotected RDP Servers Can Be Abused for DDoS Attacks

Cybercriminals have been abusing unprotected servers running Microsoft’s Remote Desktop Protocol (RDP) service to launch distributed denial-of-service (DDoS) attacks, application and network performance management company NETSCOUT warned this week.

The Windows RDP service is designed to allow users to remotely connect to servers and other devices, often for performing maintenance, deploying updates, and providing help desk support.

Its usage increased significantly as more people work remotely due to the COVID-19 pandemic, which has also resulted in malicious actors increasingly targeting the service to gain access to corporate resources.

However, NETSCOUT warns that RDP has also been abused for UDP reflection and amplification attacks. Windows admins can configure RDP to run on TCP port 3389 or UDP port 3389, and if the latter is enabled, the system can be abused to launch DDoS attacks that have an amplification ratio of 85.9:1.

“The amplified attack traffic consists of non-fragmented UDP packets sourced from UDP/3389 and directed towards the destination IP address(es) and UDP port(s) of the attacker’s choice. In contrast to legitimate RDP session traffic, the amplified attack packets are consistently 1,260 bytes in length, and are padded with long strings of zeroes,” NETSCOUT explained in its alert.

The company has reported seeing roughly 14,000 unprotected RDP servers that can be abused for such attacks.

According to NETSCOUT, DDoS attacks that abuse RDP have already been used by DDoS-for-hire services. The firm has observed attacks ranging between approximately 20 and 750 Gbps.

Organizations whose RDP servers are abused for DDoS attacks may experience partial or full disruption to important remote access services, and blocking traffic on UDP port 3389 may not be a good solution as it can lead to legitimate traffic getting blocked as well.

Enterprises have been advised to identify potentially abusable Windows RDP servers on their own networks and the networks of downstream customers, and take action to mitigate the risk. Administrators should either stop running the RDP service on UDP or place servers behind VPN concentrators to reduce the risk of abuse.

“Organizations with business-critical public-facing internet properties should ensure that all relevant network infrastructure, architectural, and operational BCPs have been implemented, including situationally specific network access policies that only permit internet traffic via required IP protocols and ports. Internet access network traffic from internal organizational personnel should be deconflated from internet traffic to/from public-facing internet properties and served via separate upstream internet transit links,” NETSCOUT said.

UPDATE: NETSCOUT has updated its blog post with more information and it now says there are roughly 33,000 abusable RDP servers. 

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.