Accessing referenced file MasterPage.master is not allowed because the reference is outside of the App Web.

​​​​​​​​​Masterpage inheritance will break your apps. Setting you want to reset or change the masterpage of the site and all its subsite. This works all fine like you expect to, until you open a SharePoint Add-in and you get errors.​

​Web Error

​ULS Error

​Steps to re-produce

1. Create a new site collection;
2. Install any app from the app store;
3. Check the app and confirm its working;
4. Change the system masterpage, i.e. the default oslo, and check “Reset all subsites to inherit this system masterpage setting”;
5. Try the app and see the (expected) error message.

​Explanation what happend

• ​Change the system masterpage and reset all subsites to inherit this system masterpage;
• Or a stapled event handler that changes the masterpages.

Fixing with ​PowerShell​​

$prefix = "spapp"

$allappurls = Get-SPSite | Get-SPWeb -Limit All | where {$_.url -like "http://$prefix*"}
foreach ($app in $allappurls)
{
 $name = $app.name
 $app.CustomMasterUrl = "/$name/_catalogs/masterpage/app.master"
 $app.MasterUrl = "/$name/_catalogs/masterpage/app.master"
 $app.Update()
}

​Fixing with CSOM​

​add-type -Path 'Microsoft.SharePoint.Client.dll'

$siteurl = 'https://mavention-27798620f05bf1.sharepoint.com/publication/'
$username = 'admin@mavention.onmicrosoft.com'
$password = Read-Host –Prompt ‘Enter Password’ -AsSecureString

try
{
 $context = New-Object Microsoft.SharePoint.Client.ClientContext($siteurl)
 $credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)

 $context.Credentials = $credentials
 $site = $context.Site
 $web = $site.OpenWeb('Publication')
 $web.CustomMasterUrl = "/Publication/_catalogs/masterpage/app.master" 
 $web.MasterUrl = "/Publication/_catalogs/masterpage/app.master" 
 $web.Update() 

 $context.ExecuteQuery()
}
catch
{
 Write-Host -ForegroundColor Red 'Error when trying to set app master url on ' $siteurl, ':' $Error[0].ToString()
}
​

No-code fix​

You can also redeploy all your apps to reset the masterpage back to the (original) app.master.