World
ID Identity Manager

Covered the WorldIDIdentityManagerImplV3 update enabling compressed proof verification via the Semaphore Verifier.

report id

0411-0446

1 Executive Summary

This document outlines the security review conducted by Nethermind Security for Worldcoin's WorldID Identity Manager smart contracts. The audit focuses on the updates made to the WorldID Identity Manager Implementation contract, with the goal of processing compressed proofs.

The existing World ID manager will be upgraded to the new WorldIDIdentityManagerImplV3 implementation. This update redefines the verifyProof function, enabling it to utilize the Semaphore Verifier's verifyCompressedProof function when a compressed proof is provided.

Moreover, the same updates have been implemented to the WorldIDBridge contract deployed on Worldchain, enabling it to use the compressed proof function.

The audited code comprises of 138 lines of code written in the Solidity language, and the audit was performed using (a) manual analysis of the codebase, (b) automated analysis tools, (c) simulation of the smart contract.

Audit assessment
Audit Type	Security Review
Draft Report	February 05, 2025
Final Report	February 05, 2025
Methods	Manual Review, Automated analysis
Repository	`world-id-contracts`, `world-id-state-bridge`
Initial Commit Hash	world-id-contracts:
Final Commit Hash	world-id-contracts:
Documentation Assessment	High
Test Suite Assessment	High

Total findings: 1
Fixed: 1
Acknowledged: 0
Mitigated: 0
Unresolved: 0

Findings by severity

Critical0
High0
Medium0
Low0
Informational0
Best Practices1

2 Audited Files

#	Contract	LoC	Comments	Ratio	Blank	Total
1	`src/WorldIDIdentityManagerImplV3.sol`	31	68	219.4%	8	107
2	`src/interfaces/ISemaphoreVerifier.sol`	7	22	314.3%	2	31
	Total	38	90	236.8%	10	138

#	Contract	LoC	Comments	Ratio	Blank	Total
1	`src/abstract/WorldIDBridge.sol`	88	116	131.8%	44	248
	Total	88	116	131.8%	44	248

3 Summary of Issues

#	Finding	Severity	Update
1	Comments/Natspec notes	Best Practices	Fixed

4 System Overview

The Worldcoin team introduced the WorldIDIdentityManagerImplV3 contract to upgrade the existing WorldIDIdentityManagerImplV2 implementation of the World ID identity manager. The primary goal of this upgrade is to enable the contract to process compressed ZK proofs. A proof is considered “compressed” when the last four uint values are zero, effectively reducing the proof size from 8 uints to 4 uints. Similar updates are implemented in the WorldIDBridge abstract contract.

The SemaphoreVerifier contract includes the verifyCompressedProof function that can be called directly to verify compressed proofs. As part of the upgrade, the verifyProof function has been modified to check whether the provided proof is compressed, and it will call the appropriate SemaphoreVerifier function based on the proof type.

function verifyProof(
        uint256 root,
        uint256 signalHash,
        uint256 nullifierHash,
        uint256 externalNullifierHash,
        uint256[8] calldata proof
) public view virtual override onlyProxy onlyInitialized

In addition, a new verifyCompressedProof function has been introduced, allowing to directly pass in a compressed proof for verification:

function verifyCompressedProof(
        uint256 root,
        uint256 signalHash,
        uint256 nullifierHash,
        uint256 externalNullifierHash,
        uint256[4] calldata compressedProof
) public view virtual onlyProxy onlyInitialized

It's important to note that this upgrade does not require reinitializing the contract. As such, the contract's initialized state ($._initialized) will remain set to 2. This poses no issues, as the change does not interfere with any other logic. Additionally, for a future V4 upgrade, simply adding reinitializer(4) will be sufficient.

5 Risk Rating Methodology

The risk rating methodology used by Nethermind Security follows the principles established by the OWASP Foundation. The severity of each finding is determined by two factors: Likelihood and Impact.

Likelihood measures how likely the finding is to be uncovered and exploited by an attacker. This factor will be one of the following values:

High: the issue is trivial to exploit and has no specific conditions that need to be met;
Medium: the issue is moderately complex and may have some conditions that need to be met;
Low: the issue is very complex and requires very specific conditions to be met.

When defining the likelihood of a finding, other factors are also considered. These can include but are not limited to motive, opportunity, exploit accessibility, ease of discovery, and ease of exploit.

Impact is a measure of the damage that may be caused if an attacker exploits the finding. This factor will be one of the following values:

High: the issue can cause significant damage, such as loss of funds or the protocol entering an unrecoverable state;
Medium: the issue can cause moderate damage, such as impacts that only affect a small group of users or only a particular part of the protocol;
Low: the issue can cause little to no damage, such as bugs that are easily recoverable or cause unexpected interactions that cause minor inconveniences.

When defining the impact of a finding, other factors are also considered. These can include but are not limited to data and state integrity, loss of availability, financial loss, and reputation damage. After defining the likelihood and impact of an issue, the severity is determined according to the table below.

Impact / Likelihood	Low	Medium	High
High	Medium	High	Critical
Medium	Low	Medium	High
Low	Info / Best Practices	Low	Medium
Undetermined	Undetermined	Undetermined	Undetermined

To address issues that do not fit a High, Medium or Low severity, Nethermind Security also uses three more finding severities: Informational, Best Practices, and Undetermined.

Informational findings do not pose any risk to the application, but they carry some information that the audit team intends to pass to the client formally;
Best Practice findings are used when some piece of code does not conform with smart contract development best practices;
Undetermined findings are used when we cannot predict the impact or likelihood of the issue.

6 Issues

6.1 Comments/Natspec notes

File	Fix commit	Type	Status
`WorldIDIdentityManagerImplV3.sol`	`effe834` `64bdbb4`	Best Practices	Fixed

The following are observations about the comments and Natspec on the WorldIDIdentityManagerImplV3 file:

WorldIDIdentityManagerImplV3
    - Comment on L21 stats that the contract inherits from `WorldIDIdentityManagerImplV1`. 
      However, it actually inherits from `WorldIDIdentityManagerImplV2`.

WorldIDIdentityManagerImplV3.verifyProof
    - Comment on L61: "The of the Merkle tree" should be corrected to  "The root of the Merkle tree".

WorldIDIdentityManagerImplV3.verifyCompressedProof
    - Comment on L89: "The of the Merkle tree" should be corrected to "The root of the Merkle tree".

Recommendation

Consider addressing the comment and Natspec observations made above.

7 Documentation Evaluation

Software documentation refers to the written or visual information that describes the functionality, architecture, design, and implementation of software. It provides a comprehensive overview of the software system and helps users, developers, and stakeholders understand how the software works, how to use it, and how to maintain it. Software documentation can take different forms, such as user manuals, system manuals, technical specifications, requirements documents, design documents, and code comments. Software documentation is critical in software development, enabling effective communication between developers, testers, users, and other stakeholders. It helps to ensure that everyone involved in the development process has a shared understanding of the software system and its functionality. Moreover, software documentation can improve software maintenance by providing a clear and complete understanding of the software system, making it easier for developers to maintain, modify, and update the software over time. Smart contracts can use various types of software documentation. Some of the most common types include:

Technical whitepaper: A technical whitepaper is a comprehensive document describing the smart contract’s design and technical details. It includes information about the purpose of the contract, its architecture, its components, and how they interact with each other;
User manual: A user manual is a document that provides information about how to use the smart contract. It includes step-by-step instructions on how to perform various tasks and explains the different features and functionalities of the contract;
Code documentation: Code documentation is a document that provides details about the code of the smart contract. It includes information about the functions, variables, and classes used in the code, as well as explanations of how they work;
API documentation: API documentation is a document that provides information about the API (Application Programming Interface) of the smart contract. It includes details about the methods, parameters, and responses that can be used to interact with the contract;
Testing documentation: Testing documentation is a document that provides information about how the smart contract was tested. It includes details about the test cases that were used, the results of the tests, and any issues that were identified during testing;
Audit documentation: Audit documentation includes reports, notes, and other materials related to the security audit of the smart contract. This type of documentation is critical in ensuring that the smart contract is secure and free from vulnerabilities.

These types of documentation are essential for smart contract development and maintenance. They help ensure that the contract is properly designed, implemented, and tested, and they provide a reference for developers who need to modify or maintain the contract in the future.

8 Remarks & Test Suite

The documentation for the World identity manager implementation upgrade is contained in the provided PR description in addition to the README file in the project's GitHub repository.

The PR description is thorough and informative. It includes a clear breakdown of the changes made, the impact on the existing codebase, and specific actions required for deployment.

The README file delivers an in-depth explanation of the WorldID Semaphore Contracts, clearly outlining the key components and how they interact. It also includes a section dedicated to explaining the development dependencies, as well as step-by-step instructions on how to build and test the code.

The added functions are thoroughly documented. Each function includes detailed comments that explain the logic behind its implementation and the significance of each parameter.

The test suite for the World Identity Manager contracts is comprehensive, covering both public and internal functions. For the audited changes, new tests have been added to upgrade the implementation to version 3 (v3), as well as to cover the flow of validating compressed proofs.

Test suite

96Tests passed

0Failed

The complete test output is in the full report.

General advisory to clients

As auditors, we recommend that any changes or updates made to the audited codebase undergo a re-audit or security review to address potential vulnerabilities or risks introduced by the modifications. By conducting a re-audit or security review of the modified codebase, you can significantly enhance the overall security of your system and reduce the likelihood of exploitation. However, we do not possess the authority or right to impose obligations or restrictions on our clients regarding codebase updates, modifications, or subsequent audits. Accordingly, the decision to seek a re-audit or security review lies solely with you.

Disclaimer

This report is based on the scope of materials and documentation provided by you to Nethermind in order that Nethermind could conduct the security review outlined in 1. Executive Summary and 2. Audited Files. The results set out in this report may not be complete nor inclusive of all vulnerabilities. Nethermind has provided the review and this report on an as-is, where-is, and as-available basis. You agree that your access and/or use, including but not limited to any associated services, products, protocols, platforms, content, and materials, will be at your sole risk. Blockchain technology remains under development and is subject to unknown risks and flaws. The review does not extend to the compiler layer, or any other areas beyond the programming language, or other programming aspects that could present security risks. This report does not indicate the endorsement of any particular project or team, nor guarantee its security. No third party should rely on this report in any way, including for the purpose of making any decisions to buy or sell a product, service or any other asset. To the fullest extent permitted by law, Nethermind disclaims any liability in connection with this report, its content, and any related services and products and your use thereof, including, without limitation, the implied warranties of merchantability, fitness for a particular purpose, and non-infringement. Nethermind does not warrant, endorse, guarantee, or assume responsibility for any product or service advertised or offered by a third party through the product, any open source or third-party software, code, libraries, materials, or information linked to, called by, referenced by or accessible through the report, its content, and the related services and products, any hyperlinked websites, any websites or mobile applications appearing on any advertising, and Nethermind will not be a party to or in any way be responsible for monitoring any transaction between you and any third-party providers of products or services. As with the purchase or use of a product or service through any medium or in any environment, you should use your best judgment and exercise caution where appropriate. FOR AVOIDANCE OF DOUBT, THE REPORT, ITS CONTENT, ACCESS, AND/OR USAGE THEREOF, INCLUDING ANY ASSOCIATED SERVICES OR MATERIALS, SHALL NOT BE CONSIDERED OR RELIED UPON AS ANY FORM OF FINANCIAL, INVESTMENT, TAX, LEGAL, REGULATORY, OR OTHER ADVICE.