The Treacherous POODLEHow does the SSL fallback's works
By Daniel Yepes | May 02, 2018
A gas vendor, each week receives
gas, which he stores in pipes
and discretely refills them
with water. Each day sells this
gas to his clients,
unbeknown to an "auditor" in
black robes - aka Poodle - paying
attention to this situation. One
day the "auditor" undercover, tells
the vendor he will only offer gas
with less octanage, thus his pipes
should met certain old requeriments,
ignorant of the situation he downgrades
it’s pipes standards until the "auditor"
agrees on it.
Next day, the "auditor" goes to the station, disguised as a client and ask the vendor to refill his car, next he reveals himself telling the vendor that he knows what he is doing plus that he has outdated pipes which represent a threat, but it won’t talk if he pays him a commision for each sell.
Following act, the auditor was just a regular dude impersonating someone else. And the vendor in it’s infortune, kept an old standard storage pipes without realizing the harm it could make to it’s clients or to himself.
I hope that doesn’t end in the worst analogy in human history, moreover i hope it helps to understand certain technicisms we will see later. But please, bare in mind that i’m not an expert on oil industry.
The Padding Oracle On Downgraded
Legacy Encryption SSL 3.0 (POODLE v3),
it is a protocol vulnerability on
Secure Socket Layer (SSL 3.0),
which can make any *Transport Layer Secure
(TLS)*` version to fallback to `(SSL 3.0)
plus it takes advantage on weak
encryption using a mechanism to
check message authencity using Cipher
Block Chaining Message Authentication Code
(CBC-MAC), allowing an
attacker to steal cookies from an user
on the same network.
Discovered on September, 2014,
Bodo Möller ,
Thai Duong and
Krzysztof Kotowicz from the
If you are not aware what this terms are
(CBC-MAC) will be explained here,
Petting the POODLE
Have you clean yourself from that awful analogy? From now on, guess you will see why i wrote it.
Whenever client and server
are going to start
an exchange using
use a protocol called
a process that is explicity
to settle a ground of standards
between the client and the server,
where both agrees which
version protocol will be used,
the cryptography method used,
key exchange, plus more.
All with the final intention
to send messages while speaking
on the same language.
But why does this matter? Let’s take a look at that process step by step:
Although i will not describe each step in depth, it is necesary to show the overall process, to understand how does the downgrading process is and where the attack finally lands.
In simple terms, the attack relies on the fact, that whenever an attempt to establish a secure connection, it fails (Step 1 to Step 2), then the server will fallback to an older protocol, .
Remeber the gas vendor downgrading it’s pipes until the "auditor" waits till he has that old pipes standard? Good…
Notice that whilst
is the sucessor of
SSL 3.0 has more precedence
TLS 1.0 - 1.2,
mostly to guarantee an
smooth user experience and
legacy machines, thought,
it was obsolete and insecure.
Moreover the attack will even
work if both have
Well then, let’s see how
does the downgrade works.
If in the first step, the client offers it’s highest version of the protocol, let’s say,
TLS 1.2, the server, while not
having such version, will negotiate the usage
TLS 1.0 by downgrading it’s own
protocol version, but notice, that such
offering can be repeated several times
as long it fails.
Quite like this:
Hope you didn’t miss the
CBC-MAC under the
protocol version, because this is the most
interesting part of this attack.
The reason behind the fallback advantage, is to force the client and the server to use weak encryption. It won’t reveal the key, but it will allow an attacker to eventually recreate cookies/session parameters by intercepting the downgraded exchange channel, .
How does this works in
SSL 3.0? Well,
CBC works as
expected, just with one subtle difference, the
is done to check the authencity of a message,
works like a charm on fixed-length messages.
But! not on variable-length messages
as the padding will fail to be fully
verified when decrypting.
Remeber from that stupid analogy the gas vendor
refilling it’s gas pipes with water? Well,
that’s what the padding is, although the
CBC-MAC algorithm won’t have any
economic advantage, it just tries
to handle variable-length messages, by
adding extra random characters to met
the needed multiple block-length,
to get 8 or 16 bytes blocks
depending on the algorithm (DES,
3DES, AES). Taking advantage on this
is ofently called
Padding Oracle Attack.
So, here is where the version downgrade lands. But let’s see it in action with this simple example, i’m gonna use 8-bytes blocks to make it short.
Let’s suppose we have already intercepted
a client-server communication and we
have forced the use of
SSL 3.0, now we
are going to reveal the encrypted messages
while both are at the last step of the handshake.
Without too much details, the client sends a request to the server:
A message request from the client which is
going to be encrypted with
First at all, the
MAC of the message
should be computed:
IVis full of 0’s
XORwith each block of plaintext
And append it to the end of the message, then check if the length of the message is a multiple of 8 (block-size), if not, add random padding characters at the end until it hits a multiple of the block-size and the final byte becomes the length of the padding, which output is like this:
|I will represent the random padding characters by "-", don’t get confused.|
Then the process of encryption with
as described in the
article. Which output is:
The message is then sent to the server, and now consider the decryption process on the server:
Taking on account that the exchange
is being made with
SSL 3.0 and the
fact that when a
algorithm is used,
SSL 3.0 does
not cover padding with
Which means, that the mechanism
used to verify the authenticity of
a message won’t be able to fully
verify it while decrypting it.
Did you spot something weird with this mechanism, but besides than the stated flaw? No? Perhaps on encryption? Or before?
Well, if you didn’t, here is a clue: Authentication should be done after encryption, NOT before. The Authenticate-then-Encrypt poses a problem, which by that time wasn’t that evident.
So, to process each block of the ciphertext, denoted as C, the recipient determines each block of the plaintext, denoted as P, using the following mathematical formula, :
Pi = Dk(Ci) XOR Ci-1
Where C0 is the
Initialization Vector (IV)
C ranges from C1 to Cn.
P ranges from P1 to Pn.
Dk the block-cipher decryption using per-connection key K or
This in simple words means, that each current block is
with the previous block, then checks and removes the padding
at the end, and finally checks and removes the
So how does the attack use decryption to get the plaintext without the key?
Considering our padding block
And the block we want to decrypt, Ci.
Replace Cn by Ci, usually this block modification will be rejected, but only once on 256 request, it won’t, the attacker will conclude that the last byte of Cn-1 XORed with Ci will yield, 7.
Mathematically speaking Dk(Ci) XOR Cn-1 = 7
SSL 3.0 doesn’t care for the rest
of bytes on the padding block, less
for the block-length, it will accept it.
And thus that Pi = 7 XOR Cn-1 XOR Ci-1
a calculation which will reveal the bytes unknown on
the block the attacker wanted.
This can be seem like a duplication of
certain block on the stream, which will replace
the last block, thus, the last byte
XORed with the last byte of the
previous block, resulting in 7, .
This is possible as the block is on the same
stream, thus when the message authentication
is performed it will take it as a valid block.
As stated before, this trick will be performed almost 256 request until it’s accepted, each fail means the last byte has to be shifted.
Plus it has to be done byte by byte on the cipher stream or at least, in each byte of the block the attacker wants to know.
Although the attack seems quite similar
BEAST attack, it relies
enterely on a flaw on
The only requirements are:
Man-In-The-Middle Attackagainst the victim.
Perform the Downgrade if
Once an attacker has done it, it can steal the cookies/session from a user.
Well, there is a funny quote by the researchers:
disabling the SSL 3.0 protocol in the client or in the server (or both) will completely avoid it. If either side supports only SSL 3.0, then all hope is gone, and a serious update required to avoid insecure encryption.
But there was and still exist an iniciative to disable ssl from all browsers and on any servers using it.