Bird Bridge - Smart Contract Audit Report

Summary

Bird.Money has built an off-chain Farm, analytics, and lending platform. We previously reviewed the project's token contract here, lending platform here, Oracle here, and Farm here.

For this audit, we analyzed the project's on-chain bridge logic at commit 047b317d47e252149c377b7b6d6fa12452602694 on GitHub.

Notes on the Contract:
The Bird team has developed a cross-chain bridge which will allow users to transfer their tokens across the Ethereum and Binance Smart Chain blockchains.
The two contracts serve as the on and off ramp for moving tokens from chain to chain.
Users can deposit tokens into the contract on the Ethereum blockchain to obtain them on the Binance Smart Chain, and vice versa.
transferOwnership() and transferTokenOwnership() in the both contracts could be declared external instead of public to save some execution cost when called.
Both contracts declare nonce and feepayer as a global variables; though they are never used. These can be safely removed to save on deployment costs.
Cross-chain swaps involve some off-chain logic run by the team. Please note we have not reviewed the off-chain logic related to the bridge.
Compiler version 0.8.x is used, preventing any overflow-related issues.

Audit Findings Summary:
Users must trust the team as they will operate off-chain logic to power the bridge used for cross-chain transfers.
Date: August 4th, 2021

Combined External Threat Results

Vulnerability Category	Notes	Result
Arbitrary Storage Write	N/A	PASS
Arbitrary Jump	N/A	PASS
Delegate Call to Untrusted Contract	N/A	PASS
Dependence on Predictable Variables	N/A	PASS
Deprecated Opcodes	N/A	PASS
Ether Thief	N/A	PASS
Exceptions	N/A	PASS
External Calls	N/A	PASS
Integer Over/Underflow	N/A	PASS
Multiple Sends	N/A	PASS
Suicide	N/A	PASS
State Change External Calls	N/A	PASS
Unchecked Retval	N/A	PASS
User Supplied Assertion	N/A	PASS
Critical Solidity Compiler	N/A	PASS
Overall Contract Safety		PASS

ETH Bridge

Inheritence Chart

Smart Contract Inheritance

Function Graph

Smart Contract Graph

Functions Overview


 ($) = payable function
 # = non-constant function
 
 Int = Internal
 Ext = External
 Pub = Public

 + [Int] IERC20 
    - [Ext] totalSupply
    - [Ext] balanceOf
    - [Ext] transfer #
    - [Ext] allowance
    - [Ext] approve #
    - [Ext] transferFrom #

 + [Int] IToken 
    - [Ext] mint #
    - [Ext] burn #
    - [Ext] changeOwnership #

 +  BridgeEth 
    - [Pub]  #
    - [Pub] transferOwnership #
    - [Pub] transferTokenOwnership #
    - [Ext] transferToContract #
    - [Ext] transferFromContract #

BSC Bridge

Inheritence Chart

Smart Contract Inheritance

Function Graph

Smart Contract Graph

Functions Overview


 ($) = payable function
 # = non-constant function
 
 Int = Internal
 Ext = External
 Pub = Public

 + [Int] IERC20 
    - [Ext] totalSupply
    - [Ext] balanceOf
    - [Ext] transfer #
    - [Ext] allowance
    - [Ext] approve #
    - [Ext] transferFrom #

 + [Int] IToken 
    - [Ext] mint #
    - [Ext] burn #
    - [Ext] changeOwnership #

 +  BridgeBsc 
    - [Pub]  #
    - [Pub] transferOwnership #
    - [Pub] transferTokenOwnership #
    - [Ext] burn #
    - [Ext] mint #

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