Popular firewall technology
designed to boost security on cellular networks can backfire, unwittingly
revealing data that could help a hacker break into Facebook and Twitter
accounts, a new study from the University
of Michigan shows.
The researchers also
developed an Android app that tells phone users when they’re on a vulnerable
Using Android smartphones,
computer science associate professor Z. Morley Mao and doctoral student Zhiyun
Qian revealed how an attacker could hijack a TCP Internet connection by taking
advantage of publicly available information on smartphones; users’ willingness
to download untrusted apps; and network firewall middleboxes, which block data
bundles that don’t appear to be part of the flow of information traffic.
The researchers detected
these middleboxes on 32% of the nearly 150 networks they tested worldwide.
are supposed to protect against this kind of attack, but it turns out they do
the opposite,” Qian said. “Most vendors and carriers that deploy such
firewall middleboxes still believe they are safe and we want them to be aware
of this design flaw.”
Middleboxes monitor the “sequence
numbers” of data packets on their way to mobile devices. When you snap and
share a photo with a friend, for example, it gets chopped into numerous packets
before it’s sent across the network. Your friend’s smartphone looks to the sequence
numbers to put the picture back together. Middleboxes could help hackers use
the process of elimination to home in on a number in the right range.
“An attacker can try
to guess at sequence numbers. It’s usually hard to get feedback on whether a
guessed number is correct, but the firewall middlebox makes this
possible,” Qian said. “The attacker can try a range of sequence
numbers. The firewall will only allow one through if it is in the valid
In their test, the
researchers used a binary search process that can rule out half of the possible
numbers at a time. In 32 rounds, which take just seconds to complete, this
process guarantees that they’ll arrive at a valid number and get a packet
How does the attacker know
he has succeeded? That’s where the Android spyware comes in (smartphone malware
is already very popular, the researchers say, and it wouldn’t be hard for an
attacker to add this capability into an existing program). The intelligence the
spyware needs is not privileged information. It doesn’t need special
administrator or root access. It would just read a couple of the phone’s
publicly available incoming packet counters and let the attacker know when the
Armed with a valid sequence
number, the hacker could spoof Facebook or Twitter’s HTTP (as opposed to the
more secure HTTPS) Web login page and gain the user’s passwords.
The attack Qian and Mao
propose illustrates a susceptibility in the so-called sandboxing safety
mechanism that smartphone platforms utilize. Sandboxing isolates an app to a
certain piece of memory, with the intention of protecting the rest of the phone
from any tampering.
here is that this shows how malware can, in a sense, reach out of its sandbox
and tamper with other legitimate apps such as your browser,” Qian said.