the current ecosystem of E2EE cloud storage is largely broken.
Several major end-to-end encrypted cloud storage services contain cryptographic flaws that could lead to loss of confidentiality, file tampering, file injection and more, researchers from ETH Zurich said in a paper published this month.
(PDF)
The vulnerabilities range in severity: in many cases a malicious server can inject files, tamper with file data, and even gain direct access to plaintext. Remarkably, many of our attacks affect multiple providers in the same way, revealing common failure patterns in independent cryptographic designs.
previous analyses of MEGA and NextCloud have shown that even the largest providers of E2EE cloud storage are affected by cryptographic vulnerabilities and creating secure E2EE cloud storage is a harder problem than initially thought.
(Summary of the report)
http://brokencloudstorage.info
BleepingComputer reports that Sync is "fast-tracking fixes," while Seafile "promised to patch the protocol downgrade problem on a future upgrade."