The Bitcoin network was subject to a strange attack this week. Up to 25% of the recorded transactions were modified using a technique called transaction malleability . By examining the Bitcoin blockchain, I've created an hour-by-hour look at the attack.

For details on how transaction malleability works, see my article Bitcoin transaction malleability: looking at the bytes. As a quick summary, the attacker takes a new Bitcoin transaction, modifies it in a trivial way that changes the transaction hash, and sends it back into the Bitcoin system. The modified transaction functions exactly the same (transferring the bitcoins between the same addresses), but results in two slightly different versions of the transaction in the system. However, if client software or exchange software depends on the transaction hash, temporarily having two different hashes for the transaction can cause a variety of problems.

The reason malleability is possible is that inside a Bitcoin transaction is a tiny program that provides the signature data. This script pushes the (hex) 48 byte signature by using the instruction 48. An attacker can change the script to use the OP_PUSHDATA2 instruction (4d) followed by a two-byte length (48 00). The modified transaction is still valid, since the script has exactly the same action.

Tracking the malleability attack

I created the graph below, which shows the hourly progress of the attack: the blue line is the total number of Bitcoin transactions, and the green line is the number of transactions that were modified by the malleability attack.

Graph of Bitcoin transactions suffering from malleability attack, Feb 2014.

The attack started off affecting a fairly small number of transactions on Feb 9. The malleability attack itself appears to have started in block 284980 (Feb 9, 8:12 PST), which contains 36 modified transactions. Since the number of affected transactions in this block and following blocks was small, I wonder if this was a test phase for the attack.

The attack really took off the morning of February 10. At the peak, up to 25% of Bitcoin transactions were modified.

The attack ended fairly abruptly the morning of Feb 11. I made a bunch of transactions that evening, hoping to see a modified one, but I was disappointed that they all went through untouched.

A few modified transactions continued to trickle in for the next few days, with some even today (Feb 14). Some of these are older transactions that were mined very slowly because they didn't include any fees. For example, this transaction was modified on Feb 10, but not mined until Feb 14.

History of OP_PUSHDATA2 usage

OP_PUSHDATA2

I wanted to find out if there were any precursors to the malleability attack, or any similar attacks earlier. I scanned the entire blockchain looking for transactions using theopcode, which is used in the malleability attack. (As an aside, the Bitcoin data is a pain to parse for several reasons.)

Up until the attack, OP_PUSHDATA2 was very rare. I saw OP_PUSHDATA2 used in July 2013 here for a strange - but not modified (malleated?) - transaction. OP_PUSHDATA2 was used again in November 5 (here)when someone used OP_PUSHDATA to include a joke signature in the transaction: I should not run the washing machine while listening to WZBC. I managed to convince myself that the machine was slowly failing -- that a rythmic, squeaking noise it had been making had gotten a little worse. Ten minutes later, though, the machine had paused. But the noise was still there. All that text is stored inside the Bitcoin transaction. There are a bunch of ways to "hide" text messages in the blockchain, and this transaction used an unusual one.

On Feb 4, this transaction used OP_PUSHDATA2 in a strange broken transaction that wasted 0.001 BTC. Interestingly, the sibling transaction wasted 0.03201 BTC in a broken MULTISIG transaction with the "correct horse battery staple" public key. I conclude that someone was trying out strange things on Feb 4, including the rare OP_PUSHDATA2 instruction. Was this debugging for the malleability attack a few days later or was this unrelated experimentation?

Some conclusions

There has been some speculation that the malleability attack directed the modified transactions to a specific miner. However, I looked at the blocks containing these transactions, and they come from a variety of well-known miners. So there's nothing miner-specific about this attack. The attackers don't have their own mining pool.

There's a 100-millisecond sleep in the Bitcoin peer's message processing loop. There has been speculation that the attacker could beat regular peers by avoiding this loop: regular peers would wait 100ms to pass along messages, while the attacker could get a transaction, modify it, and send it to a miner immediately. This seems plausible to me.

One puzzle is that Mt.Gox announced their difficulties on Feb 7, and then explained Feb 10 that they were stopping withdrawals due to a malleability attack. Since the OP_PUSHDATA2 attack didn't start until Feb 9, this attack can't be responsible for the Feb 7 problems. One possibility is there was a different type of malleability attack that affected Mt.Gox. It would be interesting to get the hash for one of the affected transactions from before Feb 7, to see what was going on.

Around the same time as the malleability attack, many people received tiny payments from 1Enjoy and 1Sochi addresses. I believe all these payments were rejected by miners as junk and remain unconfirmed. As far as I know, there is no connection between these tiny spam payments and the malleability attack, but the timing is suspicious.