Breaking

How to Own a Router – Fritz!Box AVM Vulnerability Analysis

The below post was originally written on February 9th as a little educational exercise & follow-up to my BinDiff post. (This research was actually triggered by a relative asking about that strange Fritz!Box vulnerability he heard about on the radio). Once we realized the full potential of the bug we decided against publishing the post and contacted several parties instead. Amongst others this contributed to the German BSI press release. Given the cat is out of the bag now anyway, we see no reason to hold it back. We will further take this as an opportunity to lay out our basic vulnerability disclosure principles in a future post. This topic will also be discussed in the panel “Ethics of Security Work & Research” at Troopers

Fritz!Box

Fritz!Box is series of DSL and WLAN routers produced by AVM. They are extremely popular in Germany and are the uncontested market leader for private DSL customers. Recently, a significant number of Fritz!Box owners became victim of an attack that resulted in calls to expensive international numbers. The newspaper “Der Westen” reported about a case where phone calls valued over 4200€ were initiated from a compromised Fritz!Box.  Few days later AVM published a security update for a large number of Fritz!Box models and urged customers to apply the patch as soon as possible.

However, no further details about the vulnerability were published. This blog post describes our analysis of the vulnerability that we performed directly after the first updates were released.

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Events

Bluevoxing

This is a guest post from Graeme Neilson

Reverse engineering is generally thought of as using debuggers, disassemblers and hex editors. Much as I love hex editors, IDA and staring at opcodes for the last few years I have been focused on applying my reverse engineering methodology to larger, composed systems. At Troopers TelcoSec day this year I will be presenting Bluevoxing which demonstrates how this approach works. Bluevoxing is about reverse engineering how web based “audio one time password” systems work. Simply put audio one time password systems use a short audio file as an authentication token. When I discovered these systems I was intrigued as reversing them would involve a range of techniques and tools from web testing, audio tools, signal analysis, phreaking and cryptanalysis. The disassembler would be of no use instead I would have to employ audio tools such as audacity and ruby-processing.

My methodology was:

  1. collect a large sample set of audio one time passwords
  2. extract the raw audio file from the samples
  3. somehow convert the audio file into a number…
  4. analyse the numbers…

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