Thank you Steve for your quick response.
I have just gone through and ensured all my 30 or so sites have version 2.3.1 if they have PP Capabilities installed. I can also confirm that the compromised sites were also only those with PublishPress Capabilities on them. I do realise this might be coincidence but I’ve not seen a counterexample yet.
I have checked the security plugins and procedures as well as scanning for malware using my server tools, and found nothing yet. In all cases I have reset all users’ passwords and notified them.
It appears to me as if the user is being added, and then that user’s privileges are being used to upload the wp-striplple plugin. I am looking at the code of that plugin now. I am intrigued that seems to have experienced it the other way round, but this way makes sense that it could potentially be a flaw in the Capabilities plugin. (I am not out to apportion blame at all here, by the way – just to diagnose so I can stop it!)
I believe the malicious activity is happening manually rather than fully automatically. This is because on the couple of sites where I acted within a minute to delete the user once added, and before the user had reset their password, those sites did not have the wp-striplple plugin installed. So it appears I stopped the user before they had time to do it.
I shall file a report shortly on the link you mention once I’ve finished firefighting.
Thanks again both!
Tim