Windows Defender blocking all PDF attachments from opening.

[GerJMG]

Hi everybody,

I am currently experiencing an issue with a client where a User can't open any PDF attachments from Outlook because it is being blocked by Windows Defender with this message:"Your IT administrator caused Windows Security to block this action. Ask your Helpdesk for further information".  

What Intune policy could cause this? I've been searching forever, and I'll continue on searching, but perhaps someone can immediately identify the policy causing this issue.

Thanks in advance!

[#2]

@GerJMG do you have any safe attachment policy configured in Defender for O365?

[#3]

Just pdfs or all attachments?

Moe

[#4]

We do indeed, but it only blocks "Current and future E-Mails with detected Malware-Attachments".

[#5]

I'll double check on that, but I'm 90% confident that only PDFs are being blocked.

[#6]

@GerJMG 

Did Microsoft provide an explanation or root cause, because it's also happening in our company O365 email.