user135: How can transactions be maliciously reversed? What resources are required?

user153: There are multiple attack vectors for this.
If the merchant accepts on 0/unconfirmed then there is a race attack in which an attacker (scammer) will send one transaction to the "merchant" (or to a node closer to the merchant) and a separate transaction that double spends those same coins to other nodes, such as mining nodes.  Depending on the merchant's configuration the success ratio of this will vary.  The range where the attacker is successful ranges from near 0% to near 100%, but simple steps taken by the merchant (turn off listener, explicitly connect to well connected nodes) will prevent most of the risks of this attack. 

http://bitcointalk.org/index.php?topic=79090.0

If the merchant accepts on 0/unconfirmed, regardless of protections from a race attack then the Finney attack is possible.  In this, the attacker also has control of a miner that has successfully mined but not yet announced a block with a special transaction that also has not been announced.  As soon as the block is mined, the attacker spends the coin from that special transaction with a merchant, secures delivery of the goods purchased, then broadcasts the block which contains the special transaction which invalidates the payment the merchant already accepted.  A disincentive to this is that for every couple seconds that the block is held, costs the attacker, on average, about $1 so even with this vulnerability there are tools available to a merchant to manage the risk so payment on 0/unconfirmed can still be recognized.
As the number of confirmations required increases the success of a double spend attack drop, and a merchant that requires 6/unconfirmed is essentially secure (based on probability of success) against double spends where the miner attempts to rewrite blocks.  Essentially this is only success for an attacker once 51% majority of network hashing is attained.  The consensus is that while this technically is possible, it doesn't make economic sense (no profit from doing so).  So if "feasible" means it must bring a profit, then this 51% attack method wouldn't be one of the methods you ask about.

user33: This is not a possible thing to do at all without having over 51% of the network. All transaction are irreversable.
With 51% of the network hash power, it is really easy to reverse transactions.
Anyone with that kind of power would make a better profit just mining as attacking the network would cause a collapse in confidence and drop the value (with that kind of power its safe to assume you have a hoard of BTC)

135: <p>How can transactions be maliciously reversed? What resources are required?</p>
 153: <p>There are multiple attack vectors for this.</p>

<p>If the merchant accepts on 0/unconfirmed then there is a race attack in which an attacker (scammer) will send one transaction to the "merchant" (or to a node closer to the merchant) and a separate transaction that double spends those same coins to other nodes, such as mining nodes.  Depending on the merchant's configuration the success ratio of this will vary.  The range where the attacker is successful ranges from near 0% to near 100%, but simple steps taken by the merchant (turn off listener, explicitly connect to well connected nodes) will prevent most of the risks of this attack. </p>

<ul>
<li><a href="http://bitcointalk.org/index.php?topic=79090.0" rel="nofollow">http://bitcointalk.org/index.php?topic=79090.0</a></li>
</ul>

<p>If the merchant accepts on 0/unconfirmed, regardless of protections from a race attack then the Finney attack is possible.  In this, the attacker also has control of a miner that has successfully mined but not yet announced a block with a special transaction that also has not been announced.  As soon as the block is mined, the attacker spends the coin from that special transaction with a merchant, secures delivery of the goods purchased, then broadcasts the block which contains the special transaction which invalidates the payment the merchant already accepted.  A disincentive to this is that for every couple seconds that the block is held, costs the attacker, on average, about $1 so even with this vulnerability there are tools available to a merchant to manage the risk so payment on 0/unconfirmed can still be recognized.</p>

<p>As the number of confirmations required increases the success of a double spend attack drop, and a merchant that requires 6/unconfirmed is essentially secure (based on probability of success) against double spends where the miner attempts to rewrite blocks.  Essentially this is only success for an attacker once 51% majority of network hashing is attained.  The consensus is that while this technically is possible, it doesn't make economic sense (no profit from doing so).  So if "feasible" means it must bring a profit, then this 51% attack method wouldn't be one of the methods you ask about.</p>
 33: <p>This is not a possible thing to do <em>at all</em> without having over 51% of the network. All transaction are irreversable.</p>

<p>With 51% of the network hash power, it is really easy to reverse transactions.</p>

<p>Anyone with that kind of power would make a better profit just mining as attacking the network would cause a collapse in confidence and drop the value (with that kind of power its safe to assume you have a hoard of BTC)</p>
