Security Experts:

New DDoS Attack Method Obfuscates Source Port Data

Recent distributed denial of service (DDoS) attacks showed evidence of a new method being used to bypass existing defenses by obfuscating source port data, Imperva says.

In addition to commonly encountered amplification methods, the observed attacks used payloads with irregular source port data, a vector that only few DDoS defenders considered possible, Imperva claims. The attack method abuses a well-known, unpatched UPnP (Universal Plug and Play) protocol exploit.

The UPnP networking protocol allows for device discovery over UDP port 1900, and for device control over an arbitrarily chosen TCP port. Because of that, many Internet of Things devices use the protocol to discover and communicate to one another over LAN.

However, default settings leaving devices open to remote access, the lack of an authentication mechanism, and UPnP-specific remote code execution vulnerabilities have shown the protocol to pose security risks.

In addition to revealing UPnP related vulnerabilities for nearly two decades, security researchers have also shown how SOAP API calls could be used to remotely reconfigure insecure devices over WAN. SOAP API calls can also be used to remotely execute AddPortMapping commands, which govern port forwarding rules.

While mitigating a SSDP amplification assault on April 11, 2018, Imperva noticed that some of the payloads were arriving from an unexpected source port, and not UDP/1900. The same technique was used in another attack a couple of weeks later.

The investigation into these incidents led to the creation of a “PoC for an UPnP-integrated attack method that could be used to obfuscate source port information for any type of amplification payload,” the security firm says.

To perform DNS amplification attacks using this PoC, one would first have to locate an open UPnP router, which can be done by running a wide-scale scan with SSDP requests using a publicly available online service such as Shodan.

There are over 1.3 million devices that appear in such a search, although not all are vulnerable. Locating an exploitable one is still easy, as scripts can be used to automate the process.

Next, the attacker would need to access the device XML file (rootDesc.xml) via HTTP, which can be done by replacing the ‘Location’ IP with the actual device IP in Shodan.

With the rootDesc.xml file listing all of the available UPnP services and devices, the next step is to modify the device’s port forwarding rules, which can be done via the AddPortMapping command, which is the first on the said list.

“Using the scheme within the file, a SOAP request can be crafted to create a forwarding rule that reroutes all UDP packets sent to port 1337 to an external DNS server ( via port UDP/53,” Imperva notes.

This works because, although port forwarding should only be used for mapping traffic from external IPs to internal IPs and vice versa, most routers don’t verify that a provided internal IP is actually internal, this allowing proxy requests from external IPs to another external IP.

To use this for port-obfuscated DNS amplification, a DNS request issued to the device and received by the UPnP device on port UDP/1337 is proxied to a DNS resolver over destination port UDP/53. The resolver responds to the device over source port UDP/53, and the device forwards the DNS response back to the original requestor after changing the source port back to UDP/1337.

“In an actual attack scenario, however, the initial DNS request would have been issued from a spoofed victim’s IP, meaning that the response would have been bounced back to the victim,” Imperva notes.

The device could be used to launch a DNS amplification DDoS assault with evasive ports, as the payloads would originate from irregular source ports, thus being able to bypass commonplace defenses that identify amplification payloads by looking for source port data. The evasion method can also be used for SSDP and NTP attacks and could work with other amplification vectors as well, including Memcached.

“With source IP and port information no longer serving as reliable filtering factors, the most likely answer is to perform deep packet inspection (DPI) to identify amplification payloads—a more resource-intensive process, which is challenging to perform at an inline rate without access to dedicated mitigation equipment,” Imperva notes.

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