Fill out a PDF form using iTextSharp for .NET core.

iTextSharp is a popular library for working with PDF files. The  unofficial port of the v4.16 is available for .NET core. In this post, I show examples of using the library to programmatically read and fill out a PDF form.

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Async/Await beginner mistake: Using async void in non event handler.

In this post, I share some bits I have learned about the async/await pattern. Specifically, I discuss some of the pitfalls of using async void in non event handlers.

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How to auto start and keep an ASP.NET core web application running on IIS

I have an ASP.NET core web application which hosts a background task via the IHosedService interface. I wrote about it in this post, if you want more info. The task needs to run continuously to poll for messages on an azure queue storage every 5 seconds. I have learned the default settings on IIS do not start the application until it receives the first request. Additionally, if the application has not received a request after a predefined period of time, IIS kills the application.

I could have hosted the application as a Windows service or converted the application into a console application and use the Windows scheduler to have it run continuously. However, I find hosting on a real IIS server convenient and beneficial since we already have other applications running on IIS and we can access the application via HTTP.

In this post, I share how I make the application to auto start and always run on IIS.

If you have an ASP.NET or an ASP.NET core which hosts a background job that needs to always run, want to preload the application for performance instead of waiting for the initial request to hit the app, or just get some tips on IIS, then read on.

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Azure AD B2B vs Azure AD B2C

In this post, I discuss the features of Azure Active Directory  B2B (AAD B2B) and Azure Active Directory B2C  (AAD B2C), the differences between them and when to use one vs the other.

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Encryption in Java with JCA and Bouncy Castle API.

In this post, I cover the basics, what I have learned about encryption while building a module to protect Personal Identifiable Information (PII) data using the Java Cryptography API (JCA) and Bouncy Castle API.

You may find this post helpful if:

• You are new to encryption or not sure how to use the JCA/Bouncy Castle to do encryption in Java.
• You face some issues with key length using the JCA.
• You are not sure which types of encoding to use for encryption.

Secure app settings in ASP.NET Core 2

Update: This post shows how to authenticate to azure key vault using app id/secret. However, this approach is less secure than using managed identity for azure resource and certificate for non-azure resource to grant the resource access to the key vault. For production environment, you should definitely consider using azure managed identity or certificate to authenticate and access azure key vault from your resource. Checkout my other post for more details.

In this blog post, I’ll show you the steps on  how to keep the credentials out of the source code of an ASP.NET Core app using Azure Key Vault.

If you want some convincing examples why leaving secrets in the source code is bad, check out this post. 

I assume you have some familiarity with developing an ASP.NET core 2 app. You also need an Azure subscription to register your application in Azure Active Directory and create an Azure key vault.

Basically the process involves these steps:

1. Register your application in AAD and generate app secret.
2. Set application id/secret using environment variables.
3. Create an Azure Key Vault.
4. Grant your app access to your key vault using access control.
5. Specify URL to your vault in app settings.
6. Load app id and secret from environment variables.
7. Read secrets from Azure Key Vault.

Checkout the sample app for this post from my Git repo.

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Cross Site Scripting (XSS)

In a XSS attack, the attacker’s goal is to inject a malicious script into the user’s browser and have the browser execute the script. The vulnerability of web applications to XSS attacks is because of not validating user’s input and/or not encoding/sanitizing data when rendering into a browser. Don’t confuse Cross Site Scripting  with Cross Site Request Forgery (CSRF).

A successful XSS attack could be devastating. Examples of damages include exposing the victim’s sensitive data, displaying  inappropriate/unintended content, involuntarily transferring of money, impersonating the user’s account etc …

XSS attack is listed under the top ten most critical application security risks for 2017.

Several XSS types of attack describe how a malicious script arrives at a user’s browser: stored XSS attacks, reflected XSS attacks, and server vs client XSS attacks.

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Why OAuth2 is not for authentication.

If you are like me, you might have thought OAuth 2 is for both authentication and authorization. After all, the main OAuth 2 flows ( Authorization Code, Implicit, User Credentials ) all require a resource owner to authenticate against an authorization server.  In this post, I’ll talk about some of the reasons I’ve learned why OAuth 2 is not for authentication.

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OAuth2 – Client Credentials Grant

In this post, I’ll give a high-level overview of the Client Credentials Grant by example.

Client Credentials Grant:

The Client Credentials Grant is for obtaining  an access token based solely on a client’s credentials, without any user’s involvement. Simply put, the Client Credentials Grant is for machine to machine communication.

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