There is a code path that can cause NetworkJob::storeCredentials to be called with empty credentials, causing the existing credentials to be overwritten even though authentication succeeded. (This happens when the proxy credentials are read from the system settings in the network layer rather than in webkit.) Internal PR 287791 has been filed to investigate why this happens; in the meantime, ignore empty credentials as a workaround.
Created attachment 185585 [details] workaround
Comment on attachment 185585 [details] workaround r+ based on Leo's review
Comment on attachment 185585 [details] workaround Clearing flags on attachment: 185585 Committed r141336: <http://trac.webkit.org/changeset/141336>
All reviewed patches have been landed. Closing bug.