Zapper Hack

TOTAL LOST $2.5M
Medium Other

Summarize with AI

ChatGPT Perplexity Claude Grok
Affected Chain 2021 Incident surface
Recovered $2.5M 100.0% returned
All-Time Rank #592 By amount stolen
Protocol Type Exploit/Other Target category

Incident Overview

The team of whitehat hackers (AndreiKei and VV) identified a loophole in the old Polygon bridge smart contract, which could lead to the possible exploit and lost of the users funds.

The vulnerability allows possible attacker to transfer tokens from users’ accounts, for which they had previously granted unlimited approval, via the Zap. This was possible due to a function, which executes external swaps and Zaps (via 0x, Paraswap, etc), accepting non-validated calldata:

(bool success, ) = swapTarget.call.value(valueToSend)(swapCallData)

The Zap lacked an appropriate pause function which meant that the Zapper team was unable to prevent the vulnerability from being exploited.

Zapper team together with the team of whitehat hackers (AndreiKei and VV) executed whitehat hack to withdraw funds from the old contract to the safe distribution contract, where users could successfully reclaim their funds.

Deprecated Polygon bridge:

https://etherscan.io/address/0x1f0d1927…9527ec

Smart contract used for the exploit:

https://etherscan.io/address/0x7284e5cd…d7eff6

Received funds were transferred to the safe smart contract for further distribution to the affected users:

https://etherscan.io/address/0x63cd1f35…d94b84

Incident Report

Protocol / Project Zapper
Date of Incident
Attack Technique Other
Classification Other
Primary Source View Post-Mortem #1 View Post-Mortem #2

Protocol Information

Protocol Type Exploit/Other
Official Website zapper.fi/
Protocol Twitter/X @zapper_fi
Team Anonymous
Source Code Unverified

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 other and Solidity and EVM internals
Capital Required Seed capital to cover gas and initial position setup
On-Chain Access Ability to interact with smart contracts and deploy a custom exploit contract
Protocol Analysis Identification of the exploitable vulnerability in Zapper's contract logic - root cause: other
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 Other audit checklist and test coverage

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

  • Verify all logic paths related to 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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Funds Recovery

100.0%

Recovered

$2.5M

Net Loss

0

Sources & References

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