A new Trojan program that displays rogue advertisements during browsing sessions uses a DNS-based email validation protocol called the Sender Policy Framework (SPF) in order to receive instructions from attackers without being detected, according to security researchers from Symantec.
However, the most interesting aspect of this malware is the way in which it receives updated URLs from attackers to use in the rogue HTML script elements.
The malware periodically generates a domain name according to a predefined algorithm and makes an SPF lookup for it. Knowing in advance which domain will be generated, the attackers register it and configure its SPF record to contain IP (Internet Protocol) addresses or host names that will be used by the malware to construct new malicious URLs.
SPF was designed to detect email spoofing and is implemented using the DNS (Domain Name System).
A domain name owner can specify an SPF policy -- a number of IP addresses or host names that are allowed to send emails from that particular domain -- inside a DNS TXT or SPF record. Email servers can then perform SPF lookups via DNS in order to check that email messages appearing to have been sent from that domain actually came from an IP address authorized by the domain administrator.
If the sender IP address or host specified in an email's header is not listed in the SPF policy for the corresponding domain name then the email sender's address was probably spoofed.
In the case of Trojan.Spachanel, the SPF policy for the domain name is not used to validate emails, but to provide a new list of malicious host names to be used by the malware.
Using this technique, attackers can hide the malicious traffic from firewalls and other security products that might otherwise block direct connections to known malware command-and-control servers.
That's because in order to perform SPF lookups, the malware queries a trusted DNS server located on the local network or the Internet service provider's network. This server then queries other DNS servers up the chain until the request reaches the authoritative DNS server for the domain name, which responds with a TXT or SPF record containing the SPF policy.
"In some cases, specific domains are blocked by a local DNS server, but this malware generates a domain that is rarely filtered," Katsuki said.