The $140,000 Arbitrum exploit discovered on March 10, 2025, by CertiK exposed a class of vulnerabilities that continues to plague decentralized finance protocols: signature bypass through arbitrary external calls. For experienced smart contract developers and security auditors, this incident provides a detailed case study in how seemingly robust access control mechanisms can be undermined through creative attack vectors. This advanced walkthrough examines the technical mechanics of signature bypass attacks and provides a comprehensive framework for detection and prevention.
The Objective
By the end of this guide, you will understand how signature verification bypass attacks work at the EVM level, how to identify vulnerable patterns in Solidity code, and how to implement defensive patterns that prevent these attacks. The techniques covered go beyond basic audit checklists and address the nuanced attack surfaces that CertiK’s investigation revealed.
Signature verification in smart contracts serves as the foundation for access control. When a contract verifies that a transaction was authorized by a specific address, it relies on cryptographic signatures that prove ownership of a private key. A signature bypass attack circumvents this verification through indirect means—typically by exploiting how contracts handle external calls and delegate operations.
Prerequisites
This guide assumes familiarity with Solidity development, the Ethereum Virtual Machine, and common smart contract security patterns including reentrancy protection, access control modifiers, and the checks-effects-interactions pattern. You should also understand how ERC-20 permit functions, EIP-712 typed data signing, and meta-transactions work, as these are common vectors for signature-related vulnerabilities.
Required tooling includes Foundry or Hardhat for local testing, Slither for static analysis, and access to a forked mainnet environment for testing against real contract states. Familiarity with CertiK’s Skynet monitoring platform will also be helpful for understanding real-time threat detection concepts.
Step-by-Step Walkthrough
Step 1: Understanding the Attack Vector
The Arbitrum exploit used what CertiK classified as an “arbitrary smart contract call vulnerability.” The attacker deployed a malicious contract that appeared to be a legitimate swap adapter. When users approved this contract for token spending, the contract executed external calls that bypassed the signature verification logic in the target protocol. The key insight is that the vulnerability was not in the signature verification code itself but in how the protocol handled interactions with untrusted external contracts.
Step 2: Identifying Vulnerable Patterns
Examine your contracts for patterns where external calls are made to addresses that can be controlled by users or external actors. The dangerous pattern looks like this: a function accepts an address parameter, makes an external call to that address, and then performs state changes based on the result. If the signature verification happens after the external call—or if the external call can modify state that affects the verification—an attacker can bypass the check.
Use Slither’s “unchecked-lowlevel-calls” and “reentrancy” detectors to identify these patterns. Pay special attention to functions that combine delegatecall or call operations with signature validation.
Step 3: Implementing Defensive Patterns
The most effective defense is to ensure that signature verification happens before any external calls and that the verified parameters cannot be modified by subsequent operations. Implement a “verify-then-execute” pattern where all authorization checks complete before any state changes or external interactions begin.
For protocols that must interact with external contracts, implement a whitelist of approved contract addresses. The Safe{Wallet} exploit that led to the $1.4 billion Bybit hack demonstrated what happens when contracts blindly trust external code—a single compromised developer laptop was enough to inject malicious JavaScript that replaced the wallet’s implementation contract.
Step 4: Testing Against Attack Scenarios
Create Foundry test cases that simulate signature bypass attacks. Deploy mock contracts that attempt to circumvent your verification logic through external calls, delegatecall, and state manipulation. Test with modified operation types—the Bybit/Safe hack succeeded by changing operation=0 to operation=1, converting a simple CALL to a DELEGATECALL that replaced the wallet’s implementation.
Step 5: Continuous Monitoring
Deploy monitoring infrastructure that watches for anomalous transaction patterns on your contracts. CertiKAIAgent’s detection of the Arbitrum exploit demonstrates the value of AI-powered monitoring that can identify attack patterns in real time. Implement event logging for all approval changes, external calls, and signature verification failures.
Troubleshooting
If your audits reveal potential signature bypass vulnerabilities, the most common fix is to add explicit checks before external calls. However, be careful not to introduce new vulnerabilities through overly complex verification logic. The most secure patterns are the simplest ones: verify the caller, verify the parameters, execute the operation, and never allow external code to influence the verification process.
For protocols that use meta-transactions or gasless flows, ensure that the relayer cannot modify transaction parameters between the user’s signature and the contract execution. Use EIP-712 typed data to bind signatures to specific operations and parameters.
Mastering the Skill
Signature bypass vulnerabilities represent one of the most subtle and dangerous attack classes in smart contract security. The March 2025 incidents—from the $140,000 Arbitrum exploit to the $1.4 billion Bybit hack—demonstrate that even sophisticated teams can fall victim to these attacks. Mastering the art of signature verification security requires continuous learning, regular auditing, and a deep understanding of how EVM execution semantics interact with access control patterns. With Bitcoin trading near $78,500 and the DeFi ecosystem holding hundreds of billions in value, the stakes have never been higher. Every developer building on Ethereum and its Layer 2 networks must treat signature verification as a critical security boundary that demands the same rigor as private key management.
Disclaimer: This article is for informational purposes only and does not constitute financial or investment advice. Always conduct your own research before making any financial decisions.
finally a proper technical breakdown of the arbitrum exploit. most articles just said ‘signature bypass’ with zero detail. the EVM level explanation here is solid
the EVM trace showing where the signature check got skipped was useful. most security writeups skip the actual bytecode level detail
most coverage of that arbitrum exploit was surface level at best. the EVM explanation of how the signature got bypassed was exactly what I needed
the defensive patterns section is genuinely useful. checking for arbitrary external calls in your own contracts should be step one of any audit. bookmarking this.
bookmarked the defensive patterns section too. checking for arbitrary external calls before signature verification should be audit 101
140K exploit on Arbitrum and the same pattern exists on mainnet protocols with 100x TVL. auditors need to add arbitrary call path detection to their standard checks not just sig verification
the $140K loss on Arbitrum was a proof of concept. if someone deployed this against a protocol with $500M TVL the same vulnerability would drain it in one tx
Beatrix L. exactly. the technique scales linearly with TVL. $140K is the audit fee these protocols pay and they still ship vulnerable code
signature verification is the foundation of access control and most devs copy paste OpenZeppelin without understanding the edge cases. the arbitrary external call vector is old but still works because nobody audits the calldata path
fn_selector_ the calldata path is where most auditors stop reading. they check the signature verification function itself but skip tracing where the external call data flows before it hits ecrecover
$140k is a small exploit by 2025 standards but the technique scales. same pattern could drain millions on a larger protocol
same signature bypass on a lending protocol with $500M TVL would be catastrophic. small exploit, massive attack surface
nullsig_ the technique scaling is the real concern. a $140k loss on Arbitrum is a warning shot. same pattern on Aave or Compound would be 9 figures easy
same bypass on a $500M lending protocol would be catastrophic. this needs to be in audit checklists immediately
byte_mask_ a 500M lending protocol with this bug would be drained in a single tx. the fact that it was only 140K on a small target is pure luck not good security
the $140k arbitrum exploit seems small but this bypass pattern could scale to millions on larger protocols
finally a technical explanation that doesn’t just say ‘signature bypass’ without details. the EVM level breakdown actually helps
defensive patterns section is genuinely useful. checking arbitrary external calls should be step one of any audit now
140k loss from ecrecover bypass is honestly low for an Arbitrum target. the real issue is most auditors skip arbitrary call injection paths because the test cases are non-trivial to construct
fn_selector_ most auditors check if ecrecover is called correctly but skip tracing where the calldata originates. the arbitrary external call before sig check is the actual vulnerability not the signature logic itself
ecrecover_ghost_ the calldata origin tracing is tedious but necessary. most audits I have seen stop at checking the sig verification logic and never trace upstream
ecrecover_ghost_ yes the calldata path is the issue. protocols import OpenZeppelin ECDSA.recover thinking that covers them but the bypass happens before the data even reaches that function
140K on Arbitrum was the warning shot. same bug class on a 500M TVL lending market and the attacker walks away with everything in one tx