Android Stagefright Vulnerability – Beyond The Hype

You may have heard in the news about a recently announced vulnerability in the Stagefright media library in Android. The good people at Zimperium proudly announce on their blog that they’ll be unveiling “the worst Android vulnerability in the mobile OS history!” at a conference. This has garnered a lot of press, in large part because the media loves big headlines like “95% of android devices vulnerable”. This is of course great for Zimperium, because their recommended fix is “buy our shit” (FYI, don’t buy their shit – mobile security and anti-virus is almost entirely snake oil).

But what is the vulnerability, and is it as serious as claimed?

Based on the patches submitted by Joshua Drake, we can take an educated guess at what the vulnerability is, and how it could be exploited. The patches fix a series of integer overflow and underflow bugs, in bounds checks and allocation size calculations, in various common media codecs. This would suggest that the exploit forces malloc to allocate an undersized buffer. There are various standard techniques they can use to exploit a heap buffer overflow, although it’s not clear that standard techniques alone are enough, since Google have been actively researching mitigations against heap corruption vulnerabilities. It’s unclear how Zimperium’s researchers dealt with these mitigations.

So how serious is this? Well, remote code execution is always bad news. Google were quick to point out that Android’s sandboxing model would limit the damage the exploit could do – so if the exploit were triggered in the Facebook app, then it would only have access to the same features Facebook does. But this may not be much comfort, since the example Zimperium give uses the Hangouts app, and Google have a tendency to give their own apps privileged access to the system.

But is it the “WORST EXPLOIT EVAR!”? From an Android perspective, maybe. The Samsung SwiftKey vulnerability is a contender, but isn’t exploitable without user interaction (connecting to a compromised WiFi hotspot). The Stagefright vulnerability appears to be exploitable without user intervention, with only hints that you’ve been compromised, and could affect a wider range of devices. However, it’s probably not the worst ever vulnerability, since Windows and Linux have suffered comparable breaches before. Also, Android’s device fragmentation may help it here, since attacks may need to be tweaked to work on different devices, so in practice penetration may be lower than feared.

It also goes without saying that not all the details of this exploit have been published, and it may transpire that Zimperium have oversold the power of their attack.

The key question, as always, will be how fast handset manufacturers can push out security updates. Google have made patches available to handset manufacturers (although interestingly they don’t appear to be in AOSP yet, although they are in CyanogenMod), so with luck, updates should be available shortly (for devices whose manufacturers still provide updates, at least).

In the meantime, be very suspicious of MMS messages from strangers.


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