What happened
On December 5, 2022, an attacker exploited the buy lottery in the Roast Football (RFB) token on BNB Chain and took about 12 BNB. RFB gave buyers a chance at a bonus reward, but it picked winners with a pseudorandom number built from values the buyer could know in advance. The attacker bought only when the draw would win and reverted every other attempt. BlockSec flagged the attack; SolidityScan reports two wins in about fifty tries.
Attack transaction: 0xcc8fdb3c6af8bb9dfd87e913b743a13bbf138a143c27e0f387037887d28e3c7a.
How it happened
- The attacker took a 20 WBNB flash loan from a DODO pool and unwrapped it to BNB.
- In a loop of about 50 attempts, the attack contract bought RFB with BNB through PancakeSwap and immediately sold the RFB back for BNB.
- After each buy-and-sell, it checked whether its BNB balance had grown. If not, it reverted that attempt, so a losing draw cost only gas.
- Buys that won the lottery paid out the bonus reward and were kept.
- The attacker repaid the flash loan and kept about 12 BNB.
Protocol details
Classification
Input Validation
Protocol Type
DeFi Protocol
Implementation language
Solidity
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