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ASP.NET Core - Who stole my cookie?

I stumbled over a strange issue I had in ASP.NET Core. A cookie created in ASP.NET MVC didn’t show up in my requests in ASP.NET Core.

This cookie was used to share some user preferences between 2 subsites in the same domain. The original cookie was created without setting a SameSite value but was also not marked as secure or httponly.

I first updated the cookie generation logic to set both the Secure and HttpOnly value to true. This is not strictly required but as I’m only reading the cookie in the backend this is already a step in the right direction.

Let’s now focus on the SameSite value. SameSite is an IETF draft standard designed to provide some protection against cross-site request forgery (CSRF) attacks. Cookies without SameSite header are treated as SameSite=Lax by default. If you know that we need SameSite=None to allow this cookie for cross-site usage, that should certainly be our next step.

Remark: Cookies that assert SameSite=None must also be marked as Secure.

Although I could now finally see using the Web Developer tools that the cookie certainly is available as part of my request, it didn’t show up yet inside my ASP.NET Core application when checking the Cookies property on my Http request.

There was one other problem with my cookie and this was related to the data in the cookie.Let’s have a look:

Do you notice the ‘\’ in the value? This is causing the problem; the original code didn’t URL encode the cookie values. Although the old ASP.NET MVC application didn’t seem to bother, ASP.NET Core certainly does and didn’t want to load and parse my cookie.

I had to update the original code to start url encoding the value:

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