On June 14, 2023, Beijing time, HashFlow fell victim to a hacker attack, resulting in an estimated profit of around $600,000 for the attackers.

SharkTeam conducted a prompt technical analysis of the incident and has summarized security measures to be taken as a precautionary approach. It is our hope that this incident serves as a lesson for future projects, contributing to the strengthening of security defenses within the blockchain industry.

1. Incident analysis

Attacker address: 0xBDf38B7475Ff810325AA39e988fb80E0aA007E84

Attack contract: 0xDDb19a1Bd22C53dac894EE4E2FBfdB0A06769216

Attacked contract: 0x79cdFd7Bc46D577b95ed92bcdc8abAba1844Af0c

Attack transactions:

0xdedda493272b6b35660b9cc9070d2ea32ee61279b821184ff837e0a5752f4042

0xb08f6d3fc70b95223cfffc2c905d9c0467a589e5f652cd193e5c00b4ad329b99

0x08b5f35076beb363a7206b8f9b4a6460f42aa9f998b561582fb4e4cdd6f05dce

1. After deploying the attack contract (0xDDb19a1B), the attacker (0xBDf38B74) proceeded to call the Wooooo function within the attack contract (0xDDb19a1B).

2. The attack contract (0xDDb19a1B) called the function 0x0031b016 of the target contract (0x79cdFd7B) during the attack.

3. The function directly transferred the user's USDT tokens to the attack contract.

2. Vulnerability Analysis

The target contract (0x79cdFd7B) that was attacked is a deprecated HashFlow contract, which was abandoned in May of the previous year and was not open-source. Through reverse engineering, it can be observed that the contract transfers tokens from the "from" address to the "to" address. Based on analysis, it is highly likely that users had granted significant authorization to this contract before May of the previous year. However, after the contract was deprecated, these authorizations were not revoked, and due to potential issues with the restriction logic after deprecation, attackers were able to call functions in the deprecated contract to transfer user assets.

3. Subsequent Developments

After carrying out the attack, the attacker (0xBDf38B74) open-sourced the attack contract and left a message stating, "Before use recover, please revoke first. Your funds are not safe." This message serves as a reminder to users to revoke their authorizations to the targeted contract (0x79cdFd7B) before transferring their funds elsewhere.

The hacker left behind two functions. One function allows users to withdraw all their funds, while the other function leaves 10% of the assets as a reward for the attacker. Currently, users have started withdrawing their funds one by one.

4. Security Recommendations

The occurrence of this incident was due to the fact that the targeted contract (0x79cdFd7B) had received significant user authorizations in the past, and these authorizations were not revoked after its deprecation, resulting in user asset losses. To prevent similar attacks in the future, it is important to follow these precautions during the development process:

(1) Project developers should thoroughly validate and address any potential logic issues that may arise after deprecating a contract.

(2) Users should regularly review their account authorizations for different protocol contracts and promptly revoke authorizations for contracts they no longer interact with or have been upgraded.

(3) Before deploying contract upgrades, it is crucial to collaborate with professional third-party auditing teams to ensure security.

About us

SharkTeam’s vision is to comprehensively protect the security of the Web3 world. The team is composed of experienced security professionals and senior researchers from all over the world. They are proficient in the underlying theory of blockchain and smart contracts, and provide services including smart contract auditing, on-chain analysis, and emergency response. It has established long-term cooperative relationships with key players in various fields of the blockchain ecosystem, such as Polkadot, Moonbeam, polygon, OKC, Huobi Global, imToken, ChainIDE, etc.
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