Security Experts:

Attackers Change DNS Settings of DrayTek Routers

Attackers have been targeting a zero-day vulnerability in routers made by DrayTek to change their DNS settings and likely abuse them in future attacks.

The Taiwan-based manufacturer of broadband Customer Premises Equipment (CPE) has already acknowledged the problem and has issued a firmware update to address it.

According to the company, the security vulnerability impacts the web administration feature, allowing for an attacker “to intercept or create an administration session and change settings on your router.”

Checking whether a device has been hit is quite easy, as it would show a different DNS server than the one set by the user (or the default blank). The attackers are changing the DNS settings to at least one rogue server, 38[.]134[.]121[.]95, an IP located on the network of China Telecom.

The altering of DNS settings on routers is likely the initial phase of a larger attack, where users would be redirected to rogue DNS servers and fake websites. Thus, cybercriminals can harvest usernames and passwords, steal sensitive information such as banking credentials, or serve malicious applications to unsuspecting users.

“Shodan shows there are nearly 800,000 Draytek routers worldwide, so the vulnerability provides a big opportunity for malicious redirections which could result in people and businesses losing credentials, data and ultimately money,” Sion Lloyd, Researcher at Nominet, told SecurityWeek in an emailed comment.

“Given DNS is basically the underlying protocol that directs traffic around the internet, it often enjoys certain privileges on the corporate firewall. Attackers know this, which is why it is often seen as a weak spot and hijacked and abused,” Lloyd continued.

The rogue address observed on impacted DrayTek routers is not responding to DNS queries, suggesting that the attackers might have not activated the server yet, or took it offline. The issue might not be visible on affected devices if the hackers set a secondary (legitimate) address as a fallback.

Researchers who noticed the altered DNS settings on DrayTek suggest that the attackers indeed used an exploit and didn’t abuse default login credentials. The manufacturer hasn’t provided specific details on the targeted issue, but apparently did confirm that a zero-day was being abused.

DrayTek has issued a couple of advisories to inform users on the flaw, and one of them also contains a list of all impacted router models and the updated firmware versions released for them.

The company also notes that, in addition to the router’s DNS and DHCP settings, users should also check the settings for each subnet, if the router supports multiple LAN subnets. The attackers might have also disabled the DHCP server on affected routers, which should cause errors on LAN, thus making the issue more obvious.

“Specific improvements have been identified as necessary to combat this and we are in the process of producing and issuing new firmware. You should install that as soon as possible,” DrayTek says.

Users who have been compromised are advised to restore a configuration backup or manually correct all settings. They are also advised to change the admin password, check whether other admin users have been added, and disable remote access to the router, unless it is needed.

“The best defense against this type of attack is always to make sure you have the latest firmware installed; note that similar attacks on other devices have used default passwords - so changing these is also advised. Connected hardware is constantly being picked apart by attackers, so monitoring security alerts and patching the holes they discover is crucial,” Lloyd said.

Additionally, keeping an eye on monitoring DNS traffic could help organizations understand whether requests are redirected to rogue servers or are resolving at the intended host.

“Monitoring DNS traffic for anomalies or behavioral changes, as well as comparing it against known bad identifiers, can provide a useful way for security teams to stop this kind of attack occurring before it is a problem. There is also a mechanism to validate that a DNS response is correct, known as DNSSEC. Owners of valuable domains can use this to make it possible to spot when a DNS response has been altered, although in the case where your DNS server is compromised this may not help,” Lloyd pointed out.

Related: Flaws Affecting Top-Selling Netgear Routers Disclosed

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