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Attackers Using New MS SQL Reflection Techniques

The bad guys are using a fairly new technique to tamper with the Microsoft SQL Server Resolution Protocol (MC-SQLR) and launch DDoS attacks.

In an advisory released this morning, Akamai's Prolexic Security Engineering & Response Team (PLXsert) described it as a new type of reflection-based distributed denial of service (DDoS) attack.

PLXsert first spotted attackers using the technique in October. Last month, researcher Kurt Aubuchon studied another such attack and offered an analysis here. PLXsert replicated this attack by creating a script based on Scapy, an open-source packet manipulation tool.

How it works
The attack manifests in the form of Microsoft SQL Server responses to a client query or request via abuse of the Microsoft SQL Server Resolution Protocol (MC-SQLR), which listens on UDP port 1434.

MC-SQLR lets clients identify the database instance with which they are attempting to communicate when connecting to a database server or cluster with multiple database instances. Each time a client needs to obtain information on configured MS SQL servers on the network, the SQL Resolution Protocol can be used. The server responds to the client with a list of instances.

Attackers abuse SQL servers by executing scripted requests and spoofing the source of the query with the IP address of the intended target. Depending on the number of instances present in the abused SQL server, the amplification factor varies.

The attack presents a specific payload signature, producing an amplification factor of nearly 25x. In this case, the attacker's request totaled 29 bytes, including IP and UDP headers, and triggered a response of 719 bytes including headers. Some servers may produce a larger or smaller response depending on their configuration.

Other tools publicly available on the Internet could reproduce this attack as well. Replicating this attack does not require a high level of technical skill. A scripted attack would only require a list of SQL servers exposed on the Internet that respond to the query. Attackers could use a unicast client request 0x03 or a broadcast request 0x02. Both are requests with a data length of 1 byte that will produce the same type of response from SQL servers.

PLXsert identified a tool on GitHub on January 26, 2015, that weaponizes this type of attack for mass abuse.

Defensive measures
Server hardening procedures should always be applied to servers that are exposed to the Internet. As a general rule, services and protocols that are unnecessary should be disabled or blocked.

This attack can only be performed by querying SQL servers with exposed SQL Server Resolution Protocol ports to the Internet.

The following best practices can help mitigate this type of DDoS attack. These recommendations are by no means exhaustive and affected organizations should refine and adapt them further based on specific infrastructure and exposed services.

  • Follow Microsoft Technet Security Best Practices to Protect Internet Facing Web Servers.
  • The use of ingress and egress filters applied to SQL server ports at firewalls, routers, or edge devices may prevent this attack. If there is a business case for keeping UDP 1434 open, it should be filtered to only allow trusted IP addresses.
  • Block inbound connections from the Internet, if ports are not needed for external access or administration.
  • SQL Server Resolution Protocol service is not needed in servers that have only one database instance. This has been disabled by default since Microsoft SQL Server 2008. It is not disabled in earlier or desktop engine versions. Disable this service to prevent the abuse of SQL server for this type of attack.
  • If the use of SQL Server Resolution Protocol service is needed, add an additional layer of security before the service is accessed, such as authentication via secure methods (SSH, VPN) or filtering as described above.

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