The technique exploited security flaws in home routers to gain access to the administrator console. Once there, the hackers changed the routers' DNS (Domain Name System) settings, a type of attack known as pharming.
Pharming is tricky to pull off because it requires access to an ISP's or an organization's DNS servers, which translate domain names into the IP addresses of websites. Those DNS systems are typically well-protected, but home routers often are not.
Security firm Proofpoint wrote in a blog post Thursday that launching the attack via email was a novel approach since pharming is normally a network-based attack.
"This case is striking for several reasons, not the least of which is the introduction of phishing as the attack vector to carry out a compromise traditionally considered purely network-based," the company wrote, adding that it showed "the continued pre-eminence of email as the go-to attack vector for cybercriminals."
A successful pharming attack means users can be diverted to a fraudulent website even when they enter a correct domain name. It also means an attacker can perform a man-in-the-middle attack, such as intercepting email, logins and passwords for websites, and hijacking search results, among other things.
Proofpoint said it detected about 100 phishing emails sent mostly to Brazilians who used either UTStarcom or TR-Link home routers. The emails purported to be from Brazil's largest telecommunications company.
They contained malicious links, and clicking one directed the victim to a server that attacked their router. The server was set up to exploit cross-site request forgery (CSRF) vulnerabilities in routers,
If the attack was successful, the hackers gained access to the administrator control panel of the router. They then entered default login credentials for the device, hoping that the user hadn't changed them
If that worked, they changed the router's setting to their own DNS server. Any computer connected to that router "would potentially have their computer query a malicious DNS server to look up any hostname on the Internet."
Although users are dependent on their router manufacturer to issue patches for CSRF flaws, there is another defense, which is old security advice: change the default password on your router.
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