Microsoft announced plans last week to block Flash, Shockwave, and Silverlight content from activating in Office 365.

The block will only apply to Office 365 subscription clients, but not to Office 2016, Office 2013, or Office 2010 distributions, the company said.

This is a full-on block, and not just Microsoft disabling problematic controls with the option to click on a button and view its content. The block means that Office 365 will prevent Flash, Shockwave, or Silverlight content from playing inside Office documents altogether.

Block scheduled for January 2019

The change is set to come into effect starting with January 2019, by the following timeline:

⇨ Controls are blocked in Office 365 Monthly Channel starting in June 2018.

⇨ Controls are blocked in Office 365 Semi Annual Targeted (SAT) Channel starting in September 2018.

⇨ Controls are blocked in Office 365 Semi Annual (SA) Channel starting in January 2019.

Only Flash, Shockwave, and Silverlight content embedded with the "Insert Object" feature are blocked, but not those embedded via "Insert Online Video."

The difference is that the former uses Microsoft's OLE (Object Linking and Embedding) technology, while the latter embed content via an Internet Explorer browser frame.

Security and Flash's EOL cited as reasons

Microsoft cited different reasons for taking this decision. It said that malware authors have abused this mechanism for exploit campaigns, but also that Office users rarely used these features anyway.

In addition, Microsoft said it was also taking this decision after Adobe announced Flash's end-of-life for 2020. Microsoft stopped supporting Silverlight in 2016, with the final end-of-support date for enterprise customers being scheduled for 2021.

In case some companies still need to embed or view Flash or Silverlight-based content in Office 365, Microsoft has published a support page with guidance on how to re-enable Flash, Silverlight, and Shockwave controls.

"We are confident that this will not impact most Office users," Microsoft said.

Flash usage has declined in recent years. Google said the percentage of daily Chrome users who've loaded at least one page containing Flash content per day has gone down from around 80% in 2014 to under 8% in early 2018. Web statistics service W3Techs has also observed Flash going to a market share of 5% today, compared to 28.5% in 2011.