A team of four Danish security researchers has disclosed this week a security flaw that impacts cable modems that use Broadcom chips.

The vulnerability, codenamed Cable Haunt, is believed to impact an estimated 200 million cable modems in Europe alone, the research team said today.

Cable Haunt impacts Broadcom spectrum analyzers

The vulnerability impacts a standard component of Broadcom chips called a spectrum analyzer. This is a hardware and software component that protects the cable modem from signal surges and disturbances coming via the coax cable. The component is often used by internet service providers (ISPs) in debugging connection quality.

On most cable modems, access to this component is limited for connections from the internal network.

The research team says the Broadcom chip spectrum analyzer lacks protection against DNS rebinding attacks, uses default credentials, and also contains a programming error in its firmware.

Researchers say that by tricking users into accessing a malicious page via their browser, they can use the browser to relay an exploit to the vulnerable component and execute commands on the device.

Using Cable Haunt, an attacker could:

Change default DNS server

Conduct remote man-in-the-middle attacks

Hot-swap code or even the entire firmware

Upload, flash, and upgrade firmware silently

Disable ISP firmware upgrade

Change every config file and settings

Get and Set SNMP OID values

Change all associated MAC Addresses

Change serial numbers

Be exploited in botnet

While the research team estimated that the number of vulnerable devices is around 200 million across Europe, they believe the total number of exploitable devices to be impossible to quantify.

"The reason for this, is that the vulnerability originated in reference software, which have seemingly been copied by different cable modems manufacturers, when creating their cable modem firmware," researchers said. "This means that we have not been able to track the exact spread of the vulnerability, and that it might present itself in slightly different ways for different manufacturers."

Proof-of-concept code available

The four-man research team published a white paper and a dedicated website this week with information about Cable Haunt.

"The purpose of this website, is to inform as many affected users and providers as possible, in order to improve their ability to protect themselves," they said.

The idea is to have ISPs test their devices and then release a firmware updates to patch the CAble Haunt attack vector. At the time of writing, four ISPs across Scandinavia have released patches (Telia, TDC, Get AS, and Stofa), but many others across Europe have not, or aren't even aware of this security flaw.

Furthermore, due to the reasons explained above, the research team wasn't able to test all Broadcom-based cable modem models in use today. Although they confirmed that some cable modems are vulnerable (see table below), many cable modem models remain untested.

The researchers have published proof-of-concept code that ISPs and tech-savvy users can use and test their cable modem and see if it's vulnerable to a Cable Haunt attack.

A Broadcom spokesperson told ZDNet that they released fixes for the reported vulnerability back in May 2019, altough, it is unclear if these fixes made their way downstream to user devices.

One point of emphasis the ZDNet team wants to relay about Cable Haunt is that this attack is extremely complex to pull off, primarily because the vulnerable spectrum analyzer component is only available on the cable modem's internal network, and not directly exposed to the internet.

Exploiting Cable Haunt requires that an attacker go through several hoops in a multi-step process, which makes this attack highly improbable ever to be used by botnet operators. However, the attack is not out of the range of a determined attacker looking to compromise a high-value target.

All in all, it's clever research, but your cable modem will most likely get hacked because you forgot to change its default password or is vulnerable to other security flaws that are directly exploitable from the internet because you forgot to update its firmware.

a super interesting vulnerability uncovered by some researchers at @lyrebirds_dk! affecting cable modem reference software, it's exploited by gaining access to the vulnerable endpoint and then hitting it with a buffer overflow attack. #cablehaunthttps://t.co/JJwJgZWv59 — gabsmashh (@gabsmashh) January 9, 2020

Article updated on January 17 with Broadcom statement about patch availability.