Hardening IIS

Draft - Work In Progress

Basic configuration

Common changes that should be part of all IIS installations.

Disable directoryBrowsing

Directory browsing gives the user the ability to just navigate to http://server/directory/ and get a list of all files in the directory. This was useful when web servers were primarily file servers, but is clearly a security problem now.

To disable directory browsing in IIS 10.0 (and several earlier versions, either:

1) Alter the web.config to set the directoryBrowse feature to false

  <configuration>
     <system.webServer>
        <directoryBrowse enabled="false" />
     </system.webServer>
  </configuration>

2) or Navigate to IIS in the Server Manager, and uncheck Directory Browsing under Common HTTP Features.

Avoid wildcard host headers

IIS 10.0 has added wildcard host headers. This means that if there is a website hosted for a domain, the server will handle requests for any subdomain, allowing the developer to make decisions based on the request as how to respond.

In general, this is a bad idea and shouldn't be used. There are very specific reasons to use them, but it is almost guaranteed that your situation isn't one of them.

Certainly, do not use wildcard domains, like http://* for example. But in general avoid using them at all. Instead use site bindings to solve the same problem.

Ensure applicationPoolIdentity is configured for all application pools

applicationPoolIdentity configured the Active Directory user that the applications in the pool impersonate.

To assure that this value is set, navigate to an Application Pool in IIS Manager, right click and select Application Pool defaults. Then select the appropriate user.

Use an unique applicationPool per site

Application bools are designed to create a collection of sites that can be restarted together, and have a common max memory limit, and some other features. With today's applications, it is best if there is a unique application pool for each site. Perhaps if there is a separate project for services and the front end of an application, then they could go together in one pool but for the majority of applications, one pool per app.=

There are two ways to configure application pools for IIS.

1)In IIS Manager, expand Sites in the Connections pane. Then click Advanced Settings, then the ellipsis button next to Application Pool. Select a unique pool there.

2) Using the command prompt, run appcmd to set up new command pools.

 appcmd.exe set config -section:system.applicationHost/applicationPools /+"[name='NewCommandPool',autoStart='True',managedPipelineMode='Integrated']" /commit:apphost

Disable IIS detailed error page from displaying remotely

When debugging a production application that is misbehaving, we would like to see detailed errors when using the server at the console, but show remote users custom pages.

To handle this, use the IIS Console and select Exceptions. In the Actions column, select Edit Feature Settings and then select Detailed Errors For Local Requests and Custom Errors For Remote Requests.

Request filtering

Configure maxAllowedContentLength

The maxAllowedContentLength is a part of the requestLimits collection, and it is too long by default for most requests (around 26 MB). On an application bases it can be altered using the requestFiltering node in the Security collection. Here is an example in the web.config:

  <configuration>
     <system.webServer>
        <security>
           <requestFiltering>
              <requestLimits>
                 <headerLimits>
                    <add header="Content-type" sizeLimit="100" />
                 </headerLimits>
              </requestLimits>
           </requestFiltering>
        </security>
     </system.webServer>
  </configuration>

In C#, it is possible to change the content length for a particular request with the ServerManager:

     using (ServerManager serverManager = new ServerManager())
     {
        Configuration config = serverManager.GetWebConfiguration("Default Web Site");
        ConfigurationSection requestFilteringSection = config.GetSection("system.webServer/security/requestFiltering");
        ConfigurationElement requestLimitsElement = requestFilteringSection.GetChildElement("requestLimits");
        ConfigurationElementCollection headerLimitsCollection = requestLimitsElement.GetCollection("headerLimits");
   
        ConfigurationElement addElement = headerLimitsCollection.CreateElement("add");
        addElement["header"] = @"Content-type";
        addElement["sizeLimit"] = 100;
        headerLimitsCollection.Add(addElement);
   
        serverManager.CommitChanges();
     }

Transport Encryption

A list of recommendations for IIS

Ensure TLS cipher suites are correctly ordered

https://cloudblogs.microsoft.com/microsoftsecure/2017/09/07/new-iis-functionality-to-help-identify-weak-tls-usage/

HSTS support

IIS recently (Windows Server 1709) added turnkey support for HSTS

https://docs.microsoft.com/en-us/iis/get-started/whats-new-in-iis-10-version-1709/iis-10-version-1709-hsts

CORS support

If you choose not to handle CORS in your application, we ship an IIS an IIS module to help configure CORS

https://blogs.iis.net/iisteam/getting-started-with-the-iis-cors-module

Authors

Sourabh Shirhatti (Microsoft)

Bill Sempf ([email protected])