Block height attack

gistfile1.txt

// Assume difficulty retargets every 5 blocks for illustrative purposes
// Assume difficulty retargeting is done with block pairs (0, 5), (5, 10), (10, 15), ...
// (i.e. assuming we don't have to worry about time warp bug)

Block #     Timestamp (relative to start)
0           0
1           1 
2           2
3           3 
4           4 
5           5 * target block time
6           6
7           7
8           8
9           9 
10          10 * target block time
11          11

This still meets the block timestamp > median(past 11 blocks' timestamp) requirement along the whole chain, but allows someone to create an extremely long chain while still keeping the tip of the chain's time stamp sufficiently far back in time to be accepted. To fix this, you would need at least