How much was lost in the Unlock Protocol hack?

The Unlock Protocol hack resulted in $416K in losses on November 21, 2021.

What attack technique was used in the Unlock Protocol exploit?

The Unlock Protocol exploit utilized Access Control. 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.

Unlock Protocol Hack

TOTAL LOST $416K

Low Nov 21, 2021 Access Control

Affected Chain 2021 Incident surface

Recovered - No recovery reported

All-Time Rank #1038 By amount stolen

Protocol Type Payments Target category

Incident Overview

The attackers' addresses:

https://polygonscan.com/address/0xcc06dd34…5b2165

https://polygonscan.com/address/0x8c769a59…c4daac

The private key of Unlock Founder & CEO was compromised. This key had been used to deploy the Unlock contract on xDAI and Polygon previously and was able to upgrade them.

Ownership was transferred at:

xDAI:

https://blockscout.com/xdai/mainnet/tx/0x12f0a54b…08bda8

Polygon:

https://polygonscan.com/tx/0xe3b852c9…14a1a3

xDAI:

1. 20,000 Tokens were stolen from Unlock contract:

https://blockscout.com/xdai/mainnet/tx/0x6e9cbe95…a13904

2. Out of the 20,000 tokens that were stolen, 19,980 UDT were transferred by the attacker to the xDAI bridge:

https://blockscout.com/xdai/mainnet/tx/0x9241dcbf…724448

3. Tokens were burnt on xDAI, transferred on Ethereum, and sold on Uniswap:

https://etherscan.io/tx/0x966c9cda…9efd54

Polygon:

1. 30,000 tokens were stolen from the Unlock contract:

https://polygonscan.com/tx/0x91a74c7b…bbd5a4

2. 10,000 UDT were transferred to the Polygon Bridge:

https://polygonscan.com/tx/0xc5527097…5cb6b6

3. The attacker transferred another 20,000 tokens, the token recipient then transferred them to the Polygon Bridge:

https://polygonscan.com/tx/0x687e7742…f91fe1

This is classified as a Access Control Attacks . Compare similar hacks →

Incident Report

Protocol / Project Unlock Protocol

Date of Incident November 21, 2021

Attack Technique Access Control

Classification Other

Primary Source View Post-Mortem #1 View Post-Mortem #2

Protocol Information

Protocol Type Payments

Affected Token UDT

Official Website unlock-protocol.com/

Protocol Twitter/X @UnlockProtocol

Team Anonymous

Source Code Unverified

Market Context at Time of Hack

Token Categories

Gaming Ethereum Ecosystem

Protocol Links

Discord

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 Operational-security tradecraft (phishing, malware, leaked seed phrases, or insider access) to obtain treasury signing authority

Capital Required Minimal capital - only enough to cover gas while draining the compromised accounts

On-Chain Access Valid signing authority over the compromised wallets / multisig signers, allowing direct transfer of funds or stake authorization

Target Reconnaissance Identification of Unlock Protocol's high-value treasury accounts and the authority / multisig structure controlling them

Execution Speed Speed to drain the compromised accounts before the team detects the breach and revokes signing authority or freezes the assets

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 Access Control audit checklist and test coverage

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

Verify all logic paths related to Access Control are guarded by proper access controls and input validation - see the Access Control 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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Related Attack Classes

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

See all Access Control Attacks examples →

Access Control Attacks

Sources & References

01
Source 1 https://unlockprotocol.notion.site/unlockprotocol/Sunday-Novembe…
02
Source 2 https://twitter.com/UnlockProtocol/status/1462568384910999555

Threat Assessment

Severity Level

LOW

Attack Vector Access Control
Root Cause Other
Year 2021

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