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🐞 Missing Cookie in iframe

·653 words·4 mins
Hong Konger.

😨 Problem #

There is a internal web application which showing an article from another website in an <iframe>. In order to read the full article the user has to login to the website (i.e. a special Form cookie is required.)

A user reported:

I login to the website in a separated browser tab. I can read the full article. However, when I browing the same item in the internal web app, it requires me to login and I am failed to login (in that <iframe>).

I cannot reproduce this using my browser (Firefox v85). This issue happens on Chrome v88.


It’s empty:Β Document.cookie #

I suspect this is something related to my cookie. I check the cookie using document.cookie in Dev tool’s console. It’s empty when I load the website inside the <iframe>.

However, document.cookie should contain something after I login to the website, right?

Here and here show the exact same issue reported by other chromium user. Turns out it’s about the updated cookie policy in Chrome.

The cookie policy #

I inspected the cookie of the website: it’s just a cookie with HttpOnly attribute and othing else. So, something else is missing in the cookie response, right?

SameSite #

The modern browsers (e.g. latest Chrome and coming versions of Firefox) require the developer to define an explicit SameSite policy. You can see SameSite cookies explained for details and choose the suiteable value.

Secure and Β HttpOnly #

The cookie has to be served via HTTPS. Hence, those attributes are also required.

I found that Set-Cookie - HTTP | MDN is a good documentation for you to check all the available attributes related to the Set-CookieHTTP response header.

I know what might be missing now but how can I fix it? The websites are running on IIS and there are multiple sites has the same cookie problem. A quick fix is required and better don’t alter the code.

πŸ˜€ Solution: Enforcing cookie responses on IIS #

Advanced: change the config file directly #

If you feel confident to modify the server config directly, open this file:


It’s a XML file containing the server-level configs.

Find the section under system.webServer > rewrite If no, create one:

    <rule name="AddCookiePolicy">
      <match serverVariable="RESPONSE_Set-Cookie" pattern="^(.*)(AUTH)(=.*)$" />
      <action type="Rewrite" value="{R:0};SameSite=None;Secure;HttpOnly">

You need to change the regex of the pattern attribute. I use ^(.*)(AUTH)(=.*)$ because there are multiple websites and all the auth cookie names are ended with AUTH.

Make sure that your regex should have a matching group so that you can make use of it in the action section (i.e. the {R:0})

Save the file and it’s done.

Verify the config #

Launch the IIS manager and choose the Configuration Editor on the Server level (not website level). Check the configuration collection under system.webServer/rewrite/outboundRules . You should see the config named AddCookiePolicy.

Easy: Add an outbound rule in IIS Manager #

If you want to be safe, launch the IIS Manager and set the rule.

  1. Select the URL Rewrite option at the Server level:

    URL Rewrite

  2. Choose Blank rule under Outbound rules section. Click OK.

    Blank rule

  3. Set the matching criteria:

    • Matching scope: Server Variable.

    • Variable name: RESPONSE_Set-Cookie.

    • Variable value: Matches the Pattern using Regular Expressions.

    • Pattern: ^(.*)(AUTH)(=.*)$" />. You should change this regex in order to match your use case.

    • Ignore case.

    Matching Criteria

  4. Set the Action:

    • Value: {R:0};Secure;SameSite=None;HttpOnly

    • Replace existing server variable value


  5. Click Apply to enable the rule.

  6. Done.

After I set this outbound rewrite rule for the cookies, I can see the full article loaded inside the <iframe>. Problem solved.

🎯 Conslusion #

When you dealing with cookies in modern browsers, set the correct cookie response attributes including an explicit SameSite policy, a HttpOnly and Secure:

Set-Cookie: {name}={value}; SameSite=None;HttpOnly;Secure

In order to add those attributes in a .NET Framework web app running on IIS, you can simply define an outbound rule rewrite rule on the server level of the IIS Manager.


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