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Celo Contracts Audit – Release 6

In another round of auditing, the cLabs team asked OpenZeppelin to review and audit recent changes to the core contracts of the Celo protocol.


This audit was a diff audit, meaning that the scope of the audit was the difference across files in certain pull requests. The pull requests audited in this phase were 8300, 8349, 8360, 8475, 8750, 8751, 8831, and 8854. Note that the audit covered production Solidity files as well as upgrade deployment scripts. A more detailed scope is below.

Within the various pull requests, the following files were considered in-scope for this audit:

Within PR #8300

  • packages/protocol/contracts/stability/GrandaMento.sol

Within PR #8349

  • packages/protocol/contracts/common/Accounts.sol
  • packages/protocol/contracts/common/MetaTransactionWallet.sol
  • packages/protocol/contracts/common/Signatures.sol
  • packages/protocol/contracts/common/linkedlists/AddressLinkedList.sol
  • packages/protocol/contracts/common/linkedlists/AddressSortedLinkedList.sol
  • packages/protocol/contracts/common/linkedlists/AddressSortedLinkedListWithMedian.sol
  • packages/protocol/contracts/common/linkedlists/IntegerSortedLinkedList.sol
  • packages/protocol/contracts/governance/Election.sol
  • packages/protocol/contracts/governance/Governance.sol
  • packages/protocol/contracts/governance/Proposals.sol
  • packages/protocol/contracts/identity/Attestations.sol
  • packages/protocol/contracts/identity/Escrow.sol
  • packages/protocol/contracts/stability/SortedOracles.sol
  • packages/protocol/scripts/bash/
  • packages/protocol/scripts/truffle/make-release.ts

Within PR #8360

  • packages/protocol/contracts/common/Accounts.sol

Within PR #8475

  • packages/protocol/contracts/governance/Validators.sol
  • packages/protocol/contracts/stability/GrandaMento.sol

Within PR #8750

  • packages/protocol/contracts/governance/ReleaseGold.sol

Within PR #8751

  • packages/protocol/contracts/governance/ReleaseGold.sol

Within PR #8831

  • packages/protocol/contracts/stability/ExchangeBRL.sol
  • packages/protocol/contracts/stability/StableTokenBRL.sol
  • packages/protocol/contracts/stability/proxies/ExchangeBRLProxy.sol
  • packages/protocol/contracts/stability/proxies/StableTokenBRLProxy.sol

Within PR #8854

  • packages/protocol/contracts/governance/LockedGold.sol

If a file is not listed above, it should be considered out of scope for this audit. However, some notes were made on auxiliary files.

Overview of the changes

The pull requests are planned to be included in Release 6 of Celo’s Core Contracts. A brief description is given of each pull request:

  • PR #8300 changes the directory of the GrandaMento file for CI purposes.
  • PR #8349 removes the getVersionNumber function from all libraries.
  • PR #8360 introduces the offchainStorageRoots mapping on the Accounts contract that can be used to register new storage roots.
  • PR #8475 implements per-proposal vetoPeriodSeconds and limits it to a hardcoded value.
  • PR #8750 adds a genericTransfer function for rescuing ERC-20 tokens from ReleaseGold contracts.
  • PR #8751 removes the requirement for funding when initializing ReleaseGold and adds an isFunded view function.
  • PR #8831 adds and updates various files for the StableTokenBRL release. Note that PR #8831 was not merged at the time of this audit and we reviewed up to commit 63f75e3. The cLabs team confirmed that some parts of the implementation were still in development.
  • PR #8854 adds a requirement that the reporter of a locked gold slash must also be a registered account.


Below, we list all the vulnerabilities found in this audit.

Critical severity


High severity


Medium severity


Low severity

[L01] Missing error message in require statement

PR #8360 contains a require statement in removeStorageRoot that does not include an error message. Consider including specific and informative error messages in all require statements.

Update: Fixed in PR9010 at commit 093ac05c3b66b8381ae85bfd4eef6b9eacb1c536.

[L02] genericTransfer can fail without error

In PR #8750, the genericTransfer function of the ReleaseGold contract calls the ERC20 member transfer function on the erc20 token, but does not check the returned bool of the transfer.

This means that with certain ERC20 tokens, the transfer could fail and the user would receive no error messages with information on the failing conditions.

Consider updating the token transfer to instead use OpenZeppelin’s SafeERC20 safeTransfer function, which will provide proper logging in the event of a transfer failure.

Update: Fixed in PR9025 at commit 6e5a9b26b3d4b34f7370f465ea8ee4fc28183cc3.

[L03] Validators does not increment PATCH version

In PR #8349, the getVersionNumber function is removed from AddressLinkedList.sol.

Validators.sol does not increment the PATCH version even though it is using AddressLinkedList.sol. Consider incrementing the PATCH version of the Validators contract to reflect the change.

Update: Fixed in PR8475 as of commit 4bd0dea1a9a24282c107527175442de879230ced.

[L04] Contract registry has no entry for BRL

In PR #8831, the UsingRegistryV2 contract provides various getters returning addresses for contracts comprising the Celo protocol. For each featured protocol contract, a constant pointer is defined and used to inform the corresponding getter. While this mechanism isn’t too complex, duplicating this process in code for a particular contract can lead to errors.

The problem is that the EUR token has corresponding getters for its registry and exchange, while the BRL token does not. So retrieval of corresponding BRL contract address would require code that may be error prone and not follow the standard defined by the UsingRegistryV2 contract.

Consider defining within the UsingRegistryV2 contract getters corresponding to the BRL token.

Update: Fixed as of commit 0afc15a6dac109ada329e12c0d40fec7dcb9f40c. cLab’s statement for this issue:

“we were indeed missing the getters, but the contract UsingRegistryV2 is not actually used anywhere and the contract to update should have been UsingRegistry.”

Notes & Additional Information

[N01] addStorageRoot can add duplicate url‘s

In PR #8360, the addStorageRoot function of the Accounts contract adds url‘s to an offchainStorageRoots array for the msg.sender‘s account.

This function does not check whether the url already exists in the array so there can be duplicate entries added.

This does not pose a security risk. However, it can result in unnecessary gas expenditure. Furthermore the resulting redundant data returned by the getOffchainStorageRoots function can confuse offchain clients and add computational complexity to the processing of this data.

Consider using OpenZeppelin’s EnumerableMap as a more practical solution than an array.

Update: Acknowledged, and will not fix. cLab’s statement for this issue:

These duplicates can be checked for on the client side easily, both as a check before adding a duplicate or as a filter on the returned storage roots. To avoid complicating and inflating the cost of the storage root logic, we will not make a change to address this at this time.

[N02] ExchangeBRL.sol lacks Solidity version pragma

In PR #8831, the ExchangeBRL.sol file does not define a Solidity version pragma.

In this case, it does not pose a security threat since the ExchangeBRL contract has trivial logic and its base contract does have its Solidity version pragma set. But it is considered good practice to always define the Solidity version pragma, since it can be a security concern when the contract contains non-trivial operations which can have different effects across Solidity versions.

Consider defining Solidity version pragmas for all Solidity source files in this project.

Update: Fixed as of commit 8e0caecc0554d5a3f6fb25b6d024f29ff7562029.

[N03] Not using SafeMath

PR #8360 contains a getOffchainStorageRoots function which concatenates the url bytes of an account using the help of a local totalLength variable. The totalLength is the aggregated length of all the offchainStorageRoots elements for a specified account.

Consider using OpenZeppelin’s SafeMath to perform the addition to totalLength to prevent overflow and maintain consistency with the method in batchGetMetadataURL.

Update: Fixed in PR9026 as of commit 6af9bf0371550f723731080929f9d8946d78d524.

[N04] Incorrect token initialization data

In PR #8831, the migrationsConfig.js file is updated to include initialization data for the deployment of the StableTokenBRL.

This initialization data is incorrectly set to be a copy of the StableTokenEUR initialization data. Specifically the tokenName and tokenSymbol are set incorrectly to Celo Euro and cEUR.

This incorrect token initialization will not affect the inner logic and accounting of the token itself. However, this mislabelling can cause errors in indexing this token in exchanges, and will likely confuse users which can greatly affect its utility and adoption.

Consider updating the migrationsConfig.js to have appropriate tokenName and tokenSymbol set specific to the StableTokenBRL.

Update: Fixed as of commit 92441e39e3b5cbb29bb8af6ed914b90a874d9afe. cLabs comments on the issue:

As a clarification, wanted to point out that file is only used for testing proposals and not for mainnet (not even testnets).


No critical or high severity issues were found. Some changes were proposed to follow best practices and reduce potential attack surface.