Cable Haunt flaw exposes 200M+ Broadcom-based cable modems at remote hijacking

Pierluigi Paganini January 11, 2020

A flaw, dubbed Cable Haunt, in Broadcom’s cable modem firmware exposed as many as 200 million home broadband gateways in Europe alone, at risk of remote hijackings.

Hundreds of millions of Broadcom-based cable modems are at risk of remote hijacking due to the presence of a vulnerability dubbed Cable Haunt, CVE-2019-19494.

The flaw resides in the hardware and software component of Broadcom chips called a spectrum analyzer that protects the cable modem from signal surges and disturbances coming via the coax cable.

“There are an estimated 200 million cable modems in Europe alone. With almost no cable modem tested being secure without a firmware update, the number of modems initially vulnerable in Europe is estimated to be close to this number.” reads the website set up by the researchers. “However, it is tough to give a precise estimate of the reach of Cable Haunt. The reason for this is that the vulnerability originated in reference software, which has seemingly been copied by different cable modems manufacturers when creating their cable modem firmware. 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.”

Four Danish researchers (Alexander Dalsgaard Krog (Lyrebirds), Jens Hegner Stærmose (Lyrebirds), Kasper Terndrup (Lyrebirds), Simon Vandel Sillesen (Independent)) have demonstrated how to exploit the flaw by tricking a victim into opening a specially crafted web page containing malicious JavaScript or a malicious email. Then the code connects to the web server built into the vulnerable modem on the local network and the script alters the contents of the modem’s processor registers, by overwriting the stack and triggering a buffer overflow. With this attack, it is possible to redirect execution to malicious code provided with the request and perform a broad range of malicious activities, including:

  • 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

For security reasons, on most cable modems, access to the spectrum analyzer is allowed only for connections from the internal network.

The research team discovered that the Broadcom chip spectrum analyzer lacks protection against DNS rebinding attacks, uses default credentials, and its firmware contains programming bugs.

A‘DNS Rebinding’ attack allows any website to create a DNS name that they are authorized to communicate with, and then make it resolve to localhost.

This attack could allow remote attackers to take over vulnerable Broadcom-based cable modems in a stealth way.

“Cable Haunt is exploited in two steps. First, access to the vulnerable endpoint is gained through a client on the local network, such as a browser. Secondly the vulnerable endpoint is hit with a buffer overflow attack, which gives the attacker control of the modem.” the researchers explained.

The four researchers published a white paper and the dedicated website also contains a PoC exploit for the issue working with the sagemcom F@st 3890 modem.

The Cable Haunt vulnerability affects cable modems using Broadcom chipset running on the open-source Embedded Configurable Operating System (eCos). The experts published a list of known affected broadband gateway models here.

Experts pointed out that firewalls could block this attack only using very specific configuration which they suspect nearly nobody to have set up.

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Pierluigi Paganini

(SecurityAffairs – Cable Haunt, hacking)

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