Revisions to What is a recursive calling vulnerability?

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edited Apr 13, 2017 at 13:01

Community Bot

1

Chasing the DAO Attacker’s Wake - "you cannot assume anything about the state of your contract after the external call is executed", and "do not call external contract code in your contract using Solidity’s call construct, ever if you can avoid it. If you can’t, do it last and understand that you lose all guarantees as to the program flow of your contract at that point".
Analysis of the DAO exploit - this link provide a good analysis of the attack, including the method the attacker managed to repeatedly call splitDAO(...) by transferring away their The DAO tokens before their token balance is set to 0 and then transferring it back to repeat the attack. (Thanks @eth for the answer in What was the second vulnerability used in The DAO attack on 17 June 2016?What was the second vulnerability used in The DAO attack on 17 June 2016?).
The big theDAO heist FAQ
Deconstructing theDAO Attack: A Brief Code Tour
Which accounts are involved in mounting the recursive call vulnerability attacks on The DAO?Which accounts are involved in mounting the recursive call vulnerability attacks on The DAO?
Which split proposal was used to mount the recursive call vulnerability attack on The DAO?Which split proposal was used to mount the recursive call vulnerability attack on The DAO?
How was the recursive call vulnerability attack conducted via proposal #59 when there were no Yes votes?How was the recursive call vulnerability attack conducted via proposal #59 when there were no Yes votes?
Is there any way to determine how long it took for the DAO attacker to deploy the attack?Is there any way to determine how long it took for the DAO attacker to deploy the attack?
Can I execute a contract function by sending a regular transaction to the contract?Can I execute a contract function by sending a regular transaction to the contract?
How much computation can be done in a fallback function?How much computation can be done in a fallback function?

Thinking About Smart Contract Security
Smart Contract Security
Scanning Live Ethereum Contracts for the "Unchecked-Send" Bug (from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?How did “the DAO” attack happen on 17 June 2016?).
Thoughts on The DAO Hack (also from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?How did “the DAO” attack happen on 17 June 2016?).

Chasing the DAO Attacker’s Wake - "you cannot assume anything about the state of your contract after the external call is executed", and "do not call external contract code in your contract using Solidity’s call construct, ever if you can avoid it. If you can’t, do it last and understand that you lose all guarantees as to the program flow of your contract at that point".
Analysis of the DAO exploit - this link provide a good analysis of the attack, including the method the attacker managed to repeatedly call splitDAO(...) by transferring away their The DAO tokens before their token balance is set to 0 and then transferring it back to repeat the attack. (Thanks @eth for the answer in What was the second vulnerability used in The DAO attack on 17 June 2016?).
The big theDAO heist FAQ
Deconstructing theDAO Attack: A Brief Code Tour
Which accounts are involved in mounting the recursive call vulnerability attacks on The DAO?
Which split proposal was used to mount the recursive call vulnerability attack on The DAO?
How was the recursive call vulnerability attack conducted via proposal #59 when there were no Yes votes?
Is there any way to determine how long it took for the DAO attacker to deploy the attack?
Can I execute a contract function by sending a regular transaction to the contract?
How much computation can be done in a fallback function?

Thinking About Smart Contract Security
Smart Contract Security
Scanning Live Ethereum Contracts for the "Unchecked-Send" Bug (from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?).
Thoughts on The DAO Hack (also from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?).

Chasing the DAO Attacker’s Wake - "you cannot assume anything about the state of your contract after the external call is executed", and "do not call external contract code in your contract using Solidity’s call construct, ever if you can avoid it. If you can’t, do it last and understand that you lose all guarantees as to the program flow of your contract at that point".
Analysis of the DAO exploit - this link provide a good analysis of the attack, including the method the attacker managed to repeatedly call splitDAO(...) by transferring away their The DAO tokens before their token balance is set to 0 and then transferring it back to repeat the attack. (Thanks @eth for the answer in What was the second vulnerability used in The DAO attack on 17 June 2016?).
The big theDAO heist FAQ
Deconstructing theDAO Attack: A Brief Code Tour
Which accounts are involved in mounting the recursive call vulnerability attacks on The DAO?
Which split proposal was used to mount the recursive call vulnerability attack on The DAO?
How was the recursive call vulnerability attack conducted via proposal #59 when there were no Yes votes?
Is there any way to determine how long it took for the DAO attacker to deploy the attack?
Can I execute a contract function by sending a regular transaction to the contract?
How much computation can be done in a fallback function?

Thinking About Smart Contract Security
Smart Contract Security
Scanning Live Ethereum Contracts for the "Unchecked-Send" Bug (from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?).
Thoughts on The DAO Hack (also from @Roland Kofler's answer in How did “the DAO” attack happen on 17 June 2016?).

Grammar

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edited Jan 20, 2017 at 8:37

BokkyPooBah

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The attacker creates a wallet contract (0xc0ee9db1a9e07ca63e4ff0d5fb6f86bf68d47b89 in the 17/06/2016 attack) with a default (or fallback) function () to call The DAO's splitDAO(...) function a number of times. Following is a simple default function ():
```
 function () {
    // Note that the following statement can only be called recursively
    // a limited number of times to prevent running out of gas or
    // exceeding the call stack
    call TheDAO.splitDAO(...)
 }
```
The attacker creates (or joins) a split proposal (#59 in the 17/06/2016 attack) with the recipient address begin~~begin~~ set to the wallet contract created above.
The attacker votes Yes on the split proposal.
After the split proposal expires, the attacker calls The DAO's splitDAO(...) function.

a. The splitDAO(...) function calls the wallet contract's default function () as part of sending the ethers to the recipient.

b. The wallet contract's default function () calls The DAO's splitDAO(...) again, which repeats the cycle from a. above.

c. The wallet contract's default function () must ensure that an error is not thrown as the transactions will be rolled back if the call stack or gas is exceeded.

The attacker creates a wallet contract (0xc0ee9db1a9e07ca63e4ff0d5fb6f86bf68d47b89 in the 17/06/2016 attack) with a default (or fallback) function () to call The DAO's splitDAO(...) function a number of times. Following is a simple default function ():
```
 function () {
    // Note that the following statement can only be called recursively
    // a limited number of times to prevent running out of gas or
    // exceeding the call stack
    call TheDAO.splitDAO(...)
 }
```
The attacker creates (or joins) a split proposal (#59 in the 17/06/2016 attack) with the recipient address begin the wallet contract created above.
The attacker votes Yes on the split proposal.
After the split proposal expires, the attacker calls The DAO's splitDAO(...) function.

a. The splitDAO(...) function calls the wallet contract's default function () as part of sending the ethers to the recipient.

b. The wallet contract's default function () calls The DAO's splitDAO(...) again, which repeats the cycle from a. above.

c. The wallet contract's default function () must ensure that an error is not thrown as the transactions will be rolled back if the call stack or gas is exceeded.

The attacker creates a wallet contract (0xc0ee9db1a9e07ca63e4ff0d5fb6f86bf68d47b89 in the 17/06/2016 attack) with a default (or fallback) function () to call The DAO's splitDAO(...) function a number of times. Following is a simple default function ():
```
 function () {
    // Note that the following statement can only be called recursively
    // a limited number of times to prevent running out of gas or
    // exceeding the call stack
    call TheDAO.splitDAO(...)
 }
```
The attacker creates (or joins) a split proposal (#59 in the 17/06/2016 attack) with the recipient address ~~begin~~ set to the wallet contract created above.
The attacker votes Yes on the split proposal.
After the split proposal expires, the attacker calls The DAO's splitDAO(...) function.

a. The splitDAO(...) function calls the wallet contract's default function () as part of sending the ethers to the recipient.

b. The wallet contract's default function () calls The DAO's splitDAO(...) again, which repeats the cycle from a. above.

c. The wallet contract's default function () must ensure that an error is not thrown as the transactions will be rolled back if the call stack or gas is exceeded.