Upstream commit to prevent this exploit is not present in current sumokoin code.
@sumoshi: Can you merge
these commits into sumokoin?
We were not reported as at the time we were still in stealth, we'll need an urgent update. We'll scan the blockchain to see any suspicion and report to you though we doubt it can be easily exploited as our blocksize had increased significantly from original Monero (240KB vs 60KB) and forced high ringsize could be a plus for protection.
Thanks a lot for report.
Fix merged at Github, point release in a couple of hours. No exploitation found.