How-to resolve peculiarity with .aspx file upload from automated client context

The capabilities (powers) of SharePoint as underlying business applications platform can be utilized in multiple ways. Example of a pragmatic one is to utilize SharePoint as authorized web-distribution platform for content created elsewhere. The added value it brings here are that the origin of the content itself does not need to be (made) accessible for the readers, no need to (web) serve content, the permission handling of SharePoint can be utilized to only make the content available for authorized persons.

This simple application usage is for instance applied to continuously publish and distribute system monitoring dashboard report on infra level from Linux servers to the monitoring people. They do not / are not allowed access to the Linux servers in the datacenter, but are granted access to SharePoint as application platform. This worked perfectly, until we recently migrated the hosting site from SharePoint on-prem to SharePoint Online.

The problem symptom is that the uploaded .aspx file on selecting it in the SharePoint Online UI, does not open in browser, but instead starts the ‘Download / Save As’ behavior. Which clearly obstructs the SharePoint role as host of the published infra dashboard. Other .aspx files in the same library that were migrated from the source site on-prem, all do open in the browser. That rules out document library settings. So it must be directly tied to the upload of the file. The particular upload is via Curl – which gave us some challenges to authenticate against SharePoint Online, but I will post on that separately -, but once uploaded nothing can be identified what clarifies why this file behaves different from the other .aspx files in the library. Inspected the document item properties, even up to detailed level via SharePoint Designer: all the same. The only noticeable difference is when trying to resolve in SharePoint Designer via file item properties the url to document: for the troublesome document this returns in ‘file not found’.

Strange, as the file is clearly present; and as such accessible both in the browser via the SharePoint listview UI, as when opening the library in Windows Explorer via ‘Open with Explorer’. Heck, even with sync via OneDrive, the file is included in the synced library content.

So this really kept us puzzled. Until business user self-remembered an action we did on restoring the upload via Curl: as good SharePoint citizen, I reduced the permission level of the automated client account from ‘Full Control’ to ‘Contributor’. This turned out to be the key to explaining and next resolving the issue. On SharePoint level, also uploaded .aspx files are treated as (content) page. And for completed upload + administration, the account uploading an .aspx file must have ”Add and Customize Pages - Add, change, or delete HTML pages or Web Part Pages, and edit the Web site using a Microsoft SharePoint Foundation-compatible editor”. And that permission is missing from “Contributor” permission level. It does have "Add items to library", and therefore the upload itself succeeds from the automated client context. But the next processing on SharePoint (Online) side after the file upload to convert it into a browsable page context is not allowed when only 'Contributor'. The needed permission is included in ‘Full Control’, but that gives away too much control to the automated client account. Applying ‘Least Privilege’security principle, I therefore configured a new Permission Level “Upload ASPX page”, included the needed permission, and assigned this permission level to the automated client account.