Microsoft 365 was quarantining some image-containing emails (EX873252)


Microsoft's Exchange Online was suffering from a false positive filtering issue that led it to flag some emails as malware and quarantine them based on a false positive signature match for some percentage of emails containing images.

Things I don't know:

  • Was it all images, or just certain ones that happen to match a certain string or signature?
  • Was it embedded images sent from M365, inbound HTML emails referencing external emails, both, or something else?

If you were attempting to send an image via email from Microsoft 365 and seeing that falsely flagged as malware, know that the issue is since resolved. According to the Microsoft incident page for EX873252, the issue occurred from 9:09 am through 1:10 pm US central time on August 26th.

Find more info here and here.

(Note that you must be an Microsoft 365 admin to view the incident page linked above.)

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