While analyzing the logic behind the “remember” cookie in Grafana 5.2.2, I discovered a bug in the authentication mechanism. It affected users authenticating to Grafana with an external provider (such as Azure AD). By generating a special “remember” cookie, an attacker could sign in as such a user, knowing only her/his username. The bug’s CVE id is CVE-2018-558213.
After I reported the problem to the Grafana team, they fixed the issue on the next day and started rolling out a new release. So if you are vulnerable, don’t hesitate and go update your Grafana.
There are various situations when you need random data in your application. Maybe you want to mix the order of the returned items, or maybe you create nonces for your encrypted messages. Those two sample scenarios require different approaches, and while choosing a non-cryptographic PRNG works just fine in the first situation, using it in the latter is entirely wrong. You may be wondering what a non-cryptographic PRNG is. A PRNG, or pseudorandom number generator, is an algorithm for generating a sequence of numbers whose properties almost equal to the properties of sequences of random numbers. The way how the algorithm creates these sequences could be either cryptographically secure (cryptographic PRNG) or not (non-cryptographic PRNG). A non-cryptographic PRNG cares only about the uniform distribution of random bits and not about their predictability. As we will see in a moment, using the same seed twice in the Non-crypto PRNG, results in two sequences of bytes equal to each other. Cryptographic PRNGs, on the other hand, provide random bits but are also unpredictable. In the coming paragraphs, we will examine in detail the ways we use PRNGs in .NET.
In the previous post we created a sample ASP.NET application, which performs encryption in an old, unsecured way (without signature). Its source code is available in my blog samples repository. To run the application execute the runiis.bat file – you must have IIS Express installed on your machine. If everything starts correctly you should see in your browser this beautiful page:
When we need to deploy an application to Azure from VSTS (Visual Studio Team Services), we use the Azure tasks prepared by Microsoft. These tasks require a contributor account in Azure AD to make changes to your subscription. As this account is not a regular user account but an application account we call it a Service Principal. A very basic build pipeline might look as follows:
The “Azure App Service Deploy” task is an example of a task that will use a Service Principal account to update your App Service in Azure. VSTS makes it easy to create the Service Principal account; it also automatically assigns a contributor role in your subscription to this newly created account. When you want to have full control over your Azure AD you may manually create an App Registration (another name for the Service Principal) in the portal and give it the required rights. You will also need a key to authenticate the service in Azure:
In the next step, you create a new Azure Resource Manager Service Endpoint, providing all the collected information:
While preparing slides and demos for the upcoming BSides Warsaw conference, I spent some time digging through the code of the old ASP.NET Crypto stack. In case you do not remember, six years ago researchers reported multiple cryptographic design flaws in ASP.NET. One of the critical issues was that ASP.NET did not authenticate ciphertexts. Thus they were vulnerable to the padding oracle attack. Microsoft learned its lesson and rewrote the crypto stack in ASP.NET 4.5. If you want to find out more, have a look at those three excellent articles by Levi Broderick: Part 1, Part 2, Part 3. As I plan to demo the padding oracle attack during my presentation I wanted to restore the old behavior using the latest version of the ASP.NET framework. In this post, I am presenting how I achieved that. But to watch the live demo, I invite you to come to my presentation at 10:00, Saturday, October 14th :).
Always Encrypted is a feature of the SQL Server 2016/Azure SQL which allows you to take full control over the encryption process of the sensitive data stored in your SQL databases. Thanks to this mechanism the encryption key is stored only on the client side and is never revealed to the SQL Server. In consequence, data traveling from the server to the client is also encrypted (although I would not rely too much on this fact and always use encrypted connections to the SQL Server). That is a very different approach to Transparent Data Encryption or Cell-level Encryption, in which it is the server role to encrypt/decrypt data received/sent to the client. Server-side encryption is completely transparent to the client and does not impact the way the client builds SQL queries. In Always Encrypted model, any query against an encrypted column will perform comparisons on byte arrays of cipher text. As you can imagine this raises some challenges when building a data model. In this post, I am going to cover some details of how the Always Encrypted feature is implemented and, hopefully, help you use it effectively.
Each build and release definition in TFS has a set of custom variables assigned to it. Those variables are later used as parameters to PowerShell/batch scripts, configuration file transformations, or other tasks being part of the build/release pipeline. Accessing them from a task resembles accessing process environment variables. Because of TFS detailed logging, it is quite common that values saved in variables end up in the build log in a plain text form. That is one of the reasons why Microsoft implemented secret variables.
The screenshot below presents a TFS build configuration panel, with a sample secret variable amiprotected set (notice the highlighted padlock icon on the right side of the text box):
Once the secret variable is saved, it is no longer possible to read its value from the web panel (when you click on the padlock, the text box will be cleared).
And this is how the output log looks like if we pass the secret variable to a PowerShell script and print it:
Let’s now have a look where and how the secret variables are stored.
TestLib, Version=22.214.171.124, Culture=neutral, PublicKeyToken=769a8f10a7f072b4
If the above line means anything to you, you are probably a .NET developer. You also probably know that the hex string at the end represents a public key token, which is a sign that the assembly has a strong name signature. But do you know how this token is calculated? Or do you know the structure of the strong name signature? In this post, I will go into details how strong naming works and what are its shortcomings. We will also have a look at certificate-based signatures and, in the end, we will examine the assembly verification process.
The title mentions ASP.NET 4.5.x, but the encryption algorithm is exactly the same in ASP.NET 4.6.x. It won’t work however in earlier versions of ASP.NET.
Some time ago I published a post entitled “Decrypting ASP.NET identity cookies”. In that post we wrote a Python script to decrypt ASP.NET Identity cookies. You could have also learnt how the derived keys, used to encrypt those cookies, are calculated. If you are interested in details, please have a look at that article. But to summarize, the following steps are performed by ASP.NET:
- Extract the encryption and the validation key from the web.config file
- Calculate the derived keys using the SP800-108 specification, with the context and the label taken from an adequate Purpose class instance
- Validate and decrypt the cipher
The above procedure applies not only to the cookies decryption, but also to many other cryptographic operations, such as ViewState encryption, Forms Authentication, Anti-Forgery tokens creation etc. However, there is still a missing gap in the presented flow. What if the encryption and the validation keys are not explicitly set in the web.config file? Today, we will answer this question.
Few weeks ago my Remote Desktop Connections Manager started to report an access denied while trying to connect to some servers on my list, prompting me for a password. I was pretty sure the password stored in the RDCMan profile was correct, but didn’t really have time to investigate it further. Until today 🙂