The Browser Exploit Attack on SSL/TLS (B.E.A.S.T), - bet you thougth
it was a rampage hack that launched nukes - it is a practical attack
Thai Duong and
Julian Rizzo at
ekoparty in 2011.
That was the lamest introduction ever, it’s not because the attack
doesn’t deserve it, it’s because it is quite a
BEAST to ride. As a
warning to all
mathemagicians i will try to avoid most if not all math
Shall we begin!?
SSL/TLS Implementation flaw
The core of the attack starts here, with the Secure Socket Layer
(SSL) and Transport Layer Security
(TLS), it’s successor,
both, essential protocols to transmit our data secured by implementing
cryptography, in this case, symmetric cryptography using Cipher Block
Chaining (CBC) mode, plus one single but significant change introduced
CBC mode on
TLS 1.0, the way how
Initialization Vectors (IV) were computed.
Back in 2004, an attack was
SSL 3.0 and
TLS 1.0 could be exploited by a variation
Chosen Plaintext Attack on
CBC, if the
IV is known or can be predicted by the
attacker, attempting to inject plaintext to be encrypted
IV and if the output of the injected plaintext is
identical as the output of one of the packets sent by the
client, then the attack was succesfull!, .
Breathe! I know it’s hard to assimilate, but let’s crumble it into small pieces:
Symmetric encryption with CBC mode
For the sake of simplicity, let’s take the Data Encryption Standard (DES) for this sample. Short explaination, symmetric encryption algorithm, where a plaintext comes in along with a key, the plaintext it’s chopped into a fixed-length, blocks of 8 bytes to be precise. Then each block along the key is encrypted to result in a ciphertext made of 8 bytes blocks, .
CBC mode. Each block of plaintext, is
XORed (Bit a bit
operation) with the previous
ciphertext block before being
encrypted. This way each ciphertext block depends on all plaintext
blocks processed up to that point, .
Almost there, we gotta need to know what is a block cipher. It is a deterministic algorithm operating on fixed-length groups of bits, called a block, . The overall purpose of a block cipher is to ensure the transformation of each block, while on encryption or decryption, it means, giving it more randomness, which in this terms means more security.
Figure 1. CBC Mode - Wikipedia
Last but not least, the
initialization vector (IV) In a few words, it
is the first block cipher which, makes each message unique, the
IV is made entirely of random data.
What is so special about this? let’s join all of that.
Alice (Client) and Bob (Server) are going to chat, but D (Attacker) is an active stalker with feelings for Alice and he wants to learn about her habits.
Figure 2. It was a mistake…
Alice sends a message "I’m hungry, send me pizza" to Bob.
The only thing D knows is that Alice most of the time starts the
message with "I’m hungry". The message encrypted with
Figure 3. Message encrypted with DES
Remeber, encryption is done on the message splitted in 8 bytes blocks, just to make it simple we will take the first two blocks:
Figure 4. DES sample
D somehow can see Alice’s encrypted connection, whilst D does not know Alice’s key, knows how the "I’m hung" looks like after encrypted, after all knows how the message starts. But does not know the rest, D only has to guess and trick Alice into encrypt several messages until it matches the second block.
Notice, that this guess and trick game does not compromise the key, this is pure Chosen-Plaintext Attack, you know barely some portion of the message and you just try to craft more combinations until it mathches.
Here is where
CBC comes in action. Is specially designed to
overcome this kind of situations, the addition of the block cipher
before encrypting each block invalidates the
Chosen-Plaintext Attack, let’s see that with this simple example:
Figure 5. CBC sample
As you may see, it’s quite hard to guess the block that came before it,
specially when the
IV it’s random. You migth think this made it
But hey, if things were easy like that, I wouldn't be writing this article. Remember I mentioned a theorized vulnerabilty that came up in 2004? There is a variation called Blockwise-Adaptive Chosen-Plaintext Attack (BACPA), .
The attack derives from the way
SSL/TLS transmit data: Any plaintext
sent is fragmented into blocks of length less than or equal to
214 bytes, .
Which is then processed and sent as follow:
Message type (8 bits);
Major/minor version number (16 bits);
Length counter(16 bits);
Plaintext fragment (< = 214 bytes);
Message authentication code (160 bits);
Padding (0-56 bits);
Padding length (8 bits);
But why do we care about how
SSL/TLS sends data? Well, if a message is
splitted into several chunks, following the description above, a 100
bytes message is splitted into 10-10 bytes blocks of messages,
which during transmision each packet will be encrypted, were the
first package will be the only one who’s
IV it’s random,
then the last 8 bytes of each former packet (or n-1 package)
will be the
CBC residue of the first 8 bytes of packet n.
Practically, it means, the residue of the last package
IV of the current package, technique ofently
Chaining IV’s across messages.
Thus, an attacker can perform the Blockwise-Adaptive Chosen-Plaintext
Attack by injecting his own packets into the
SSL stream, he’ll know
CBC will be used to encrypt the beginning of his message.
Perhaps this example will clarify all of this:
Alice and Bob will start their communication, we already know Alice will
send "I’m hungry, send me pizza", here we can see how the process
starts, with a random
IV, Then the message is splitted into 8-bytes
chunks, each block after the first will use the
CBC residue and
lastly will be encrypted:
Figure 6. DES encryption CBC mode
The only thing D can see from there is:
Figure 7. Output
But performing the packet injection rigth there would throw an output
not worth considering, because
XOR the injected plaintext
with the previous residue
CBC, - The next CBC is just a supossition,
for the sake of not extending the example -
as seen here:
Figure 8. Initial injection
The attacker to be able to inject succesfully it’s own packet must
the guessed plaintext with that
CBC Residue as seen here:
Figure 9. Xoring with next block
XOR that output with the second
CBC residue. That remaining
output is then substracted with
XOR properties, the commutativity
propertie to be exact,
A xor B = B xor A
Figure 10. CBC Residue XORing with injected packet
And if the attacker is able to inject it’s packet on the stream Alice would end up encrypting it with her key, thus revealing the message, well, at least a fragment:
Figure 11. Injection succed
Where is the Browser attack?
Perhaps you migth be thinking how this can be exploited? Well, the B
BEAST, stands for Browser if you remember, is not there because
An attacker is entitled to perform a
Man-In-The-Middle-Attack on a
user using an
HTTPS connection, which allows the attacker to get the
ciphered message, splitted as seen previously.
Duong wrote a Java Applet Agent, which purpose was to
HTTPS request and trick the user into visiting their
Java Applet. Once the user were in the
Applet web site they took advantage
Same-Origin Policy (SOP)
vulnerabilty, although it worked only for the time the user was logged
SOP is meant to prevent
cross-site issues, like evil
site trying to access session and cookies from your bank account
stored within the same browser using
browsers were affected by this vulnerability.
To be fair, i will not expand on all the possible ways to exploit it
besides than the mention of
SOP, plus as stated by the authors:
We wanted to focus on more important parts of BEAST such as the actual crypto attack and optimizations, so we stopped looking for alternatives, and used the SOP vulnerability to make an agent.
Besides than the browser vulnerabilties, the exploitation is thanks to
TLS handles communication, where each packet sent requires an
Figure 12. HTTP Request format sample
As we can observe there are values an attacker cannot easily guess, but there a lot of parameters which can be predicted, just by knowing the format of an HTTP request.
What if the last parameters is a password field within its value? Or what if the attacker can predict which block contains cookies?
When the attacker has predicted it, it can act in two ways:
Reassure that certain block has what predicted or not.
Determine the value of the block. Notice, that this values on the stream ranges from 256 characters in ASCII, plus 8 bytes per block, which means 2568 possibilities.
Of course fewer, if special characters are removed and other advanced mechanisms are used, which are out of the scope here.
Although this attack seems dangerous, it only works when the following requirements are met:
Able to packet capture communications.
Able to modify packets sent from you.
Browsing with multiple tabs/sessions.
Attacker must have an idea where you are going to browse.
Attacker must be able to perform their action(s) within the time you are logged in.
Again, although it was dangerous, when both researchers found it and spend several weeks on demonstrating the attack they informed browser vendors and TLS devs about such vulnerability, no harm was done. Sadly, they never released their code nor an official paper describing each phase of the attack.
At least it is unknown if somebody before them took advantage of it.
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