How much was lost in the Visor Finance hack?

The Visor Finance hack resulted in $973K in losses on November 26, 2021.

What attack technique was used in the Visor Finance exploit?

The Visor Finance exploit utilized Flash Loan Attack. 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.

Visor Finance Hack

TOTAL LOST $973K

Low Nov 26, 2021 Flash Loan Attack

Affected Chain 2021 Incident surface

Recovered - No recovery reported

All-Time Rank #848 By amount stolen

Auditors 1 Prior security audit

Incident Overview

The attacker's address 1:

https://etherscan.io/address/0x89640eb6…0e3ae1

The attacker's address 2:

https://etherscan.io/address/0xf434edf6…d3a98e

The transactions behind the flash loan attack (exploiter 1):

https://etherscan.io/tx/0x5c9ac39c…afb6ab

https://etherscan.io/tx/0xc2104896…3f7316

https://etherscan.io/tx/0x07f39ed1…099639

https://etherscan.io/tx/0x2f49b436…0d8f55

https://etherscan.io/tx/0xca682696…3f3a7a

Stolen funds (91 ETH) were deposited into Tornado cash mixer at:

https://bloxy.info/txs/calls_from/0x89640eb6…0e3ae1?signature_id=994162&smart_contract_address_bin=0x722122df…5b6967

The transactions behind the flash loan attack (exploiter 2):

https://etherscan.io/tx/0x4208ef77…984f67

https://etherscan.io/tx/0x6e5be7b8…f8f461

https://etherscan.io/tx/0x42c91595…ab5fb8

Stolen funds (124 ETH) were deposited into Tornado cash mixer at:

https://bloxy.info/txs/calls_from/0xf434edf6…d3a98e?signature_id=994162&smart_contract_address_bin=0x722122df…5b6967

The exploit was executed according to the following scheme:

1. Take a flash loan of asset X.

2. Swap X for Y to pump the price of Y, dump the price of X.

3. Deposit Y into XY Visor Pool to get outsized shares based on the manipulated value of Y.

4. Withdraw shares to get the equal ratio of XY back from Visor.

5. Rebalance the pool, and pay back the flash loan.

This is classified as a Flash Loans Attacks . Compare similar hacks →

Incident Report

Protocol / Project Visor Finance

Date of Incident November 26, 2021

Attack Technique Flash Loan Attack

Classification Yield Aggregator

Primary Source View Post-Mortem

Protocol Information

Protocol Type Lending

Affected Token VISR

Official Website www.visor.finance/

Protocol Twitter/X @visorfinance

Team Anonymous

Source Code Unverified

Market Context at Time of Hack

Token Categories

BNB Chain Ecosystem

Protocol Links

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 flash loan attack and Solidity and EVM internals

Capital Required Flash loan capital (borrowed atomically, zero upfront cost)

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

Protocol Analysis Identification of the exploitable vulnerability in Visor Finance's contract logic - root cause: yield aggregator

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? Yes — skilled auditors routinely flag Flash Loan Attack vulnerabilities in code review

Audited by Audit Report 1 — still lost $973K. Prior audits don't guarantee safety, especially after post-audit code changes.

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

Verify all logic paths related to Flash Loan Attack are guarded by proper access controls and input validation - see the Flash Loans Attacks attack class for patterns
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

Audit Report 1 Report

Related Attack Classes

The technique used in this hack maps to these vulnerability classes in our security curriculum:

See all Flash Loans Attacks examples →

Flash Loans Attacks

Sources & References

01
Source 1 https://www.visor.finance/

Threat Assessment

Severity Level

LOW

Attack Vector Flash Loan Attack
Root Cause Yield Aggregator
Year 2021

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