The alert functionality in bitcoind and Bitcoin-Qt before 0.7.0 supports different character representations of the same signature data, but relies on a hash of this signature, which allows remote attackers to cause a denial of service (resource consumption) via a valid modified signature for a circulating alert.

The Bitcoin protocol, as used in bitcoind before 0.4.4, wxBitcoin, Bitcoin-Qt, and other programs, does not properly handle multiple transactions with the same identifier, which allows remote attackers to cause a denial of service (unspendable transaction) by leveraging the ability to create a duplicate coinbase transaction.

wxBitcoin and bitcoind before 0.3.13 do not properly handle bitcoins associated with Bitcoin transactions that have zero confirmations, which allows remote attackers to cause a denial of service (invalid-transaction flood) by sending low-valued transactions without transaction fees.