More than 4.5 million DSL modems have been compromised as part of a sustained hacking campaign in Brazil, with the devices spreading malware and malicious web address redirects.
The attacks focused on a vulnerability in modem firmware that was largely ignored by users and the security community, security vendor Kaspersky said, allowing attackers to enter modem configuration settings and change the DNS server used to browse the internet.
The vulnerable modems, made by six different manufacturers, all use a chip set made by Broadcom.
A similar vulnerability was used by Estonian hackers to harness control of modems with its DNSChanger attack.
According to Kaspersky, the Brazilian attackers sought to steal users' banking credentials by redirecting users to false versions of popular sites — like Facebook or Google — and prompting them to install malware.
Some 40 DNS servers were set up outside Brazil too in order to serve forged requests for domain names belonging to Brazilian banks.
The country's cyber emergency and response team (CERT) estimated that roughly 4.5 million modems had been compromised as of March this year. It met with banks, ISPs, device makers and government agencies to work out a solution to the massive attack.
After manufacturers issued firmware updates to plug the security hole, the number of compromised modems reduced. However, some 300,000 modems are still thought to be controlled by attackers.