SANS ISC: AVM Private Key Leak Puts Cable Modems Worldwide At Risk

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In November, Heise, a german technology news publisher, broke a story that AVM cable modems included not only the manufacturer's certificate authority certificate as part of the firmware but also the corresponding private key [1]. The news didn't get a lot of attention back then. AVM is the maker of "Fritz!Box" routers and modems which are almost exclusively sold to german speaking countries only. When I read the news initially, I also considered it only a risk to cable modem's in the area AVM operates. But it turns out, that the leak may have larger repercussions, as pointed out in a note published by CableLabs, the organization in charge of the DOCSIS cable modem standard. A reader forwarded us this note:

"All operators globally may be exposed to a security issue related to the EuroDOCSIS PKI and need to take action. AVM cable modems were shipped with the device private key and the AVM CA private key in the firmware which is now publicly exposed. As a result illegitimate cable modems can be created which could cause network problems such as theft of service. Major CMTS vendors ship their product with both the DOCSIS and EuroDOCSIS Root CA installed as a trust anchor in their CMTSs. As a result, CableLabs recommends all operators blocklist (untrust) the compromised AVM CA in their CMTSs, regardless of whether or not they have AVM equipment deployed in their network or they actively use the EuroDOCSIS Root CA." [2]

So what does this all mean? DOCSIS is the standard cable modem operators adopted to allow manufacturers to create interoperable equipment. EuroDOCSIS is the European version of this standard. I will refer to both standards as "DOCSIS" going forward. 

DOCSIS requires that each cable modem contains a unique certificate to authenticate the cable modem to the provider. The certificate is signed by the manufacturers CA, which in turn is signed using the DOCSIS CA. Cable modem operators trust cable modems that can present a properly signed certificate. 

Due to AVM's mistake, the private key for AVM's is now public, and anybody could create new certificates that will be trusted by cable modem ISPs that trust the EuroDOCSIS CA. This could, for example, allow someone to spoof a cable modem owned by a different provider, or create a cable modem with rogue firmware that will not obey configurations sent by the ISP.

Heise in a follow-up story stated that the key was already used in the wild to sign fake certificates [3]. The fake certificates were apparently used well before Heise broke the story.

As an end user, there is nothing you can do. As outlined by the note from CableLabs, ISPs will have to distrust the AVM CA. This could potentially cut service for users who use modems that provide certificates derived from the AVM CA. But most cable modem ISPs will not allow any modem to connect to their network, but only a subset of DOCSIS certified modems that are compatible with the particular head-end equipment used by the ISP. So the impact is likely small. AVM does not distribute cable-modems outside Europe as far as I know, but I may be wrong.

[1] https://www.heise.de/security/meldung/AVM-entweicht-geheimer-FritzBox-Schluessel-3463752.html (German Only)
[2] https://community.cablelabs.com/wiki/plugins/servlet/cablelabs/alfresco/download?id=de49a18c-a9e8-4bb3-8f35-e949fc538831 (Login Required)
[3] https://www.heise.de/security/meldung/Entfleuchter-FritzBox-Schluessel-zum-Ausstellen-falscher-Zertifikate-missbraucht-3465065.html (German Only)

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Johannes B. Ullrich, Ph.D.
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