ESET researchers have discovered that
CepKutusu.com, a Turkish alternative Android app store, was spreading malware
under the guise of all offered Android apps.
When users browsed the Turkish alternative app
store CepKutusu.com and proceeded to download an app, the “Download now” button
led to banking malware instead of the desired app.
A few weeks after ESET researchers turned to the
store’s operator with the discovery of the attack, the store’s malicious
activity ceased.
Interestingly, although ESET researchers found the
redirection from a legitimate app to the malicious one to be general – meaning
that every single app was set to be replaced with the banking malware – the
crooks behind the campaign added an exception.
Probably to increase their chances to stay under
the radar longer, they introduced a seven-day window of not serving malware
after a malicious download. In practice, after the user downloads the infected
app, a cookie is set to prevent the malicious system from prevailing, leading
to the user being served clean links for the next seven days. After this period
passes, the user gets redirected to the malware once they try to download any
application from the store.
The malicious app distributed by the store at the
time of the investigation was a remotely controlled banking malware capable of
intercepting and sending SMS messages, displaying fake activity, as well as
downloading and installing other apps.
When installed, the malware doesn’t mimic the app
the user intended to install. Instead, it imitates Flash Player.
Figure 1 –
The malicious app served to a user who thinks they are downloading the Clash of
Clans game and the legitimate game served to the same user within the seven day
period, respectively.
To gain more insight into this attack and its wider
implications, we turned to Lukáš Štefanko, a
malware researcher at ESET, who specializes in Android malware and who
discovered the malware-distributing app store.
An app store serving its customers with malware on
a mass scale – that sounds like a big threat. On the other hand, serving Flash
Player instead of whatever customers wanted – that’s a rather thin disguise.
What’s your take on this?
First, let me say that this is the first time I’ve
seen an entire Android market infected like that. Within the Windows ecosystem
and in browsers, this technique is known to have been used for some time. In
the Android ecosystem, however, it’s really a new attack vector.
As for the impact, what we saw in this particular
case was most probably a test. The crooks misused their control of the app
store in the simplest manner. Replacing the links to all apps with a link to a
single malicious app requires virtually no effort – but it also gives the
store’s customers a fair chance to detect the scam. If you got lured into
downloading a popular game and ended up with Flash Player instead … I think
you’d uninstall it straight away and report the issue, right?
This might explain why we have seen only a few
hundred infections.
From this point of view, it doesn’t sound like a
big deal …
Well, like I said, it was probably a test. I can
imagine a scenario in which the crooks who control the store’s back end append
a malicious functionality to each of the apps in the store. Serving those
interested in a particular game with a trojanized version of that game – that
would remove the biggest red flag and the number of victims might rise
significantly.
As for the attribution of this attack – have you
found any traces?
There are three possible scenarios here: an app
store built with the intention to spread malware; a legitimate app store turned
malicious by an employee with bad intentions; and a legitimate app store
becoming a victim of a remote attacker.
As for scenarios two and three, I would think that
such an attack wouldn’t go unnoticed by a legitimate store. User complaints,
suspicious server logs and changes in code should be sufficient indicators for
its operators – especially if it occurs over a prolonged period of time. Also
of interest in this regard is that we contacted the store operators with our
findings but haven’t received any reaction.
How to protect yourself
Recommendations by Štefanko:
·
If possible,
always favor downloading apps from official app stores. This piece of advice is
infinitely repeated for a good reason – there’s no guarantee of any security
measures in alternative app stores, making them a great place for malware
authors to spread their “work”, and not just via single malicious apps, but
also on a mass scale, as illustrated in this case.
·
Be cautious
when downloading content from the internet. Pay attention to anything
suspicious in file name, size and extension – this is where many threats can
still be recognized and avoided in time.
·
Use a
reliable mobile security solution to protect you from the latest threats. As
for this threat hidden in the CepKutusu.com alternative app store, ESET detects
it as Android/Spy.Banker.IE and prevents it from getting downloaded.