  Proof of Authority (PoA) is a type of permissioned consensus algorithm with a
fixed committee. PoA has been widely adopted by communities and industries due
to its better performance and faster finality. In this paper, we explore the
\textit{unfairness} issue existing in the current PoA implementations. We have
investigated 2,500+ \textit{in the wild} projects and selected 10+ as our main
focus (covering Ethereum, Binance smart chain, etc.). We have identified two
types of order manipulation attacks to separately break the transaction-level
(a.k.a. transaction ordering) and the block-level (sealer position ordering)
fairness. Both of them merely rely on honest-but-\textit{profitable} sealer
assumption without modifying original settings. We launch these attacks on the
forked branches under an isolated environment and carefully evaluate the
attacking scope towards different implementations. To date (as of Nov 2021),
the potentially affected PoA market cap can reach up to $681,087$ million USD.
Besides, we further dive into the source code of selected projects, and
accordingly, propose our recommendation for the fix. To the best of knowledge,
this work provides the first exploration of the \textit{unfairness} issue in
PoA algorithms.
