Attackers are now abusing exposed LDAP servers to amplify DDoS attacks

LDAP adds to the existing arsenal of DDoS reflection and amplification techniques that can generate massive attacks

Attackers are abusing yet another widely used protocol in order to amplify distributed denial-of-service attacks: the Lightweight Directory Access Protocol (LDAP), which is used for directory services on corporate networks.

DDoS mitigation provider Corero Network Security has recently observed an attack against its customers that was reflected and amplified through Connectionless LDAP (CLDAP), a variant of LDAP that uses the User Datagram Protocol (UDP) for transport.

DDoS reflection is the practice of sending requests using a spoofed source IP address to various servers on the Internet, which will then direct their responses to that address instead of the real sender. The spoofed IP address is that of the intended victim.

The requests are sent to various services that work over UDP, because unlike the Transmission Control Protocol (TCP), this transport protocol does not validate source addresses. Services that have been abused for DDoS reflection so far include the Domain Name System (DNS), the Network Time Protocol (NTP), the Simple Network Management Protocol (SNMP), the Simple Service Discovery Protocol (SSDP) and the Character Generator Protocol (CHARGEN). CLDAP is just the latest addition to the list.

DDoS reflection has the property of hiding the real source of the attack from the victim, as the traffic is reflected through third-party servers, but there is another more important reason why attackers love it: its amplification effect.

Most of the protocols used for reflection also allow attackers to trigger large responses using small queries. This means that attackers can amplify the amount of traffic they could otherwise generate.

While LDAP is widely used inside corporate networks, its use directly on the internet is considered risky and is highly discouraged. This doesn't mean that there are no publicly accessible LDAP servers: The SHODAN search engine shows over 140,000 systems responding to requests over port 389, which is used for LDAP -- almost 60,000 of them are located in the U.S.

It's not clear how many of these servers accept connections over both TCP and UDP and could therefore be abused for DDoS amplification, but even a small portion of them would be able to generate large attacks. That's because according to Corero, CLDAP (LDAP over UDP) has an average amplification factor of 46x and a peak of 55x.

This means that attackers can generate responses that are 50 times larger in size than the queries that triggered them and servers typically have larger bandwidth than home computers and consumer devices that typically make up DDoS botnets.

Also, today's DDoS attacks combine multiple techniques. For example, an attacker in control of a large botnet could direct a portion of it to reflect its traffic through LDAP servers, another portion to abuse DNS servers, another one to perform direct SYN floods or TCP floods and so on. According to an Akamai report from June, over 60 percent of DDoS attacks observed this year used two techniques or more.

The problem with a new, zero-day amplification vector like LDAP is that it isn't diffused, said Dave Larson the CTO of Corero Network Security. Since only a small number of attackers know about it, they can use the full capacity of these exposed LDAP servers to launch attacks. That's not the case with DNS servers for example, which have been mapped and are used for reflection and amplifications by many attackers at the same time, limiting the size of their individual attacks, he explained.

Another thing is that blacklists already exist for DNS, NTP and other type of servers that have constantly been abused in DDoS attacks. Such lists don't likely exist yet for LDAP servers.

The size of DDoS attacks has reached unprecedented levels in recent months, partially because of large numbers of compromised internet-of-things devices. Last month, the blog of cybersecurity reporter Brian Krebs was hit with a 620Gbps DDoS attack launched from a botnet of thousands of hacked routers, IP cameras and digital video recorders. A few days later, French hosting company OVH was hit with a 799Gbps attack from a similar botnet.

Last week, a DDoS attack launched against managed DNS provider Dynamic Network Services (Dyn) rendered many popular websites inaccessible to users on the U.S. East Coast.

Corero's Larson believes that increasing numbers of insecure IoT devices combined with new amplification vectors could lead to multiterabit attacks over the next year and even attacks that reach 10Tbps in the future.

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