How much was lost in the Thorchain DEX hack?

The Thorchain DEX hack resulted in $8.0M in losses on July 22, 2021.

What attack technique was used in the Thorchain DEX exploit?

The Thorchain DEX exploit utilized Refund Logic Exploit / Other. Understanding this attack vector is critical for smart contract auditors and developers building secure DeFi protocols.

How can developers prevent similar smart contract attacks?

Preventing smart contract hacks requires a combination of secure coding practices, thorough security audits, and understanding common attack vectors like reentrancy, oracle manipulation, and access control flaws. Smart Contract Hacking (SCH) provides hands-on training to help developers and auditors identify and prevent vulnerabilities before deployment.

Thorchain DEX Hack

TOTAL LOST $8.0M

Medium Jul 22, 2021 Refund Logic Exploit / Other ethereum

Affected Chain ethereum Incident surface

Recovered - No recovery reported

All-Time Rank #344 By amount stolen

Auditors 1 Prior security audit

Incident Overview

Quick Summary

Thorchain Bifrost component was exploited through the ETH Router contract, resulting in the loss of various tokens including 966.62 ALCX, 20,866,664.53 XRUNE, 1,672,794.010 USDC, 56,104 SUSHI, 6.91 YFI, and 990,137.46 USDT.

Details of the Exploit

The attacker created a fake router and emitted a deposit event by sending ETH. They then passed returnVaultAssets() with a small amount of ETH, but the router was defined as an Asgard vault. On the Thorchain Router, it forwarded ETH to the fake Asgard.

This action created a fake deposit event with a malicious memo. Thorchain Bifrost interpreted this as a normal deposit and refunded to the attacker due to a bad memo definition.

Block Data Reference

The attacker's address:

https://etherscan.io/address/0xd95e6eab…982072#tokentxns

The transaction behind the attack:

https://etherscan.io/tx/0x9db403ad…4f564b

Other relevant addresses:

Router: https://etherscan.io/address/0xc145990e…a8c2ce

Vault: https://etherscan.io/address/0xf56cba49…36e370

Attack contract: https://etherscan.io/address/0x700196e2…658805

Attack wallet (spawned from Tornado Cash): https://etherscan.io/address/0x8c1944fa…f76d62

Incident Report

Protocol / Project Thorchain DEX

Date of Incident July 22, 2021

Affected Chain(s) ethereum

Attack Technique Refund Logic Exploit / Other

Classification Protocol Logic / Exchange (DEX)

Primary Source View Post-Mortem

Protocol Information

Protocol Type Dexs

Affected Token RUNE

Smart Contract Language Solidity

Official Website thorchain.org/

Protocol Twitter/X @thorchain

Team Anonymous

Source Code Unverified

Market Context at Time of Hack

Token Categories

Cosmos Ecosystem Decentralized Exchange (DEX) Token DeFi DEX Multicoin Capital Portfolio Exnetwork Capital Portfolio Injective Ecosystem THORChain Ecosystem

Protocol Links

GitHub Discord Reddit

What the Attacker Needed to Succeed

Understanding the prerequisites for this type of attack helps auditors identify protocols that are most at risk and helps developers build better defenses.

Technical Knowledge Deep understanding of refund logic exploit / other and Solidity and EVM internals

Capital Required Seed capital to cover gas and initial position setup

On-Chain Access Ability to interact with ethereum smart contracts and deploy a custom exploit contract

Protocol Analysis Identification of the exploitable vulnerability in Thorchain DEX's contract logic - root cause: protocol logic / exchange (dex)

Execution Speed Precise transaction ordering and timing to exploit the vulnerability within a single atomic block

Obfuscation Plan A strategy to launder and move stolen funds - typically through mixers, cross-chain bridges, or decentralized DEX swaps to resist tracing

What Auditors Should Check

Could this have been caught in audit? Likely — with a thorough Refund Logic Exploit / Other audit checklist and test coverage

Audited by Certik — still lost $8.0M. Prior audits don't guarantee safety, especially after post-audit code changes.

If you're auditing a protocol with similar architecture to Thorchain DEX, these are the critical security checks that could have prevented this incident (July 2021).

Verify all logic paths related to Refund Logic Exploit / Other are guarded by proper access controls and input validation
Review privileged functions (owner, admin, governance) for potential abuse vectors - centralization risks should be documented and bounded with timelocks or multi-sigs

Master these auditing techniques with hands-on labs and real exploit scenarios in the Smart Contract Hacking course.

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Security Audit History

Certik 2020-04-06 Report

Sources & References

01
Source 1 https://rekt.news/thorchain-rekt2/
02
Reference https://medium.com/thorchain/post-mortem-eth-router-exploits-1-2…
03
Reference https://www.rekt.news/thorchain-rekt2/

Threat Assessment

Severity Level

MEDIUM

Attack Vector Refund Logic Exploit / Other
Root Cause Protocol Logic / Exchange (DEX)
Year 2021

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