  Possible manipulation of user transactions by miners in a permissionless
blockchain systems is a growing concern. This problem is a pervasive and
systemic issue, known as Miner Extractable Value (MEV), incurs highs costs on
users of decentralised applications. Furthermore, transaction manipulations
create other issues in blockchain systems such as congestion, higher fees, and
system instability. Detecting transaction manipulations is difficult, even
though it is known that they originate from the pre-consensus phase of
transaction selection for a block building, at the base layer of blockchain
protocols. In this paper we summarize known transaction manipulation attacks.
We then present L{\O}, an accountable base layer protocol specifically designed
to detect and mitigate transaction manipulations. L{\O} is built around
accurate detection of transaction manipulations and assignment of blame at the
granularity of a single mining node. L{\O} forces miners to log all the
transactions they receive into a secure mempool data structure and to process
them in a verifiable manner. Overall, L{\O} quickly and efficiently detects
reordering, injection or censorship attempts. Our performance evaluation shows
that L{\O} is also practical and only introduces a marginal performance
overhead.
