A third party component called “Guzzle” is affected by HTTPoxy vulnerability (as filed as CVE-2016-5385 for PHP). This component, which handles http requests on behalf of ownCloud can be tricked into passing inbound requests to a proxy server controlled by a third party. in combination with the ajax cron feature, the third party can potentially see external storage credentials and data. We recommend to use system cron whenever possible, which also significantly improves reliability and experience.
If possible, we recommend an immediate update to 9.0.4, 8.2.7 or 8.1.9 respectively, which each contain a patch for Guzzle. ownCloud 8.0 is shipping an older version of Guzzle and is not affected. However, 8.0.14 fixes a number of other issues and we encourage everyone on older versions of 8.0 to update right away as well.