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PROPS Token Contracts Audit

The YouNow team asked us to review and audit their PROPS Token contracts. We looked at the code and now publish our results.

The audited commit is eaf0c6fddd320a258a9d7531d72d07a830fae58a and the files included in scope were TokenVesting.sol, ERC865.sol, ERC865Token.sol, PropsSidechainCompatible.sol, PropsTimeBasedTransfers.sol and PropsToken.sol.

While the rest of the project is still a work in progress and was therefore not included in this audit’s scope, additional general notes regarding the project’s setup, documentation and software testability were still included to help improve its overall quality.

Here are our assessment and recommendations, in order of importance.

Update: the YouNow team made some fixes based on our recommendations. We address below the fixes introduced up to commit 7ead8c55da3770c4d39db8c89a8aa20ec62e97b1.

Critical severity


High severity

Incorrect 4-bytes function identifier in ERC865Token contract

According to its docstrings, the function transferPreSignedHashing in the ERC865Token contract is in charge of computing the keccak256 hash of the payload that will be used by the transferPreSigned function. However, the 4-bytes identifier in use (i.e. 48664c16) is obtained from hashing the signature transferPreSignedHashing(address,address,address,uint256,uint256,uint256), whereas the actual signature of the function being used is transferPreSignedHashing(address,address,uint256,uint256,uint256). This mismatch may potentially cause problems in higher application layers or in the interaction with other third-party systems that use the correct signature.

Hence, the 4-bytes identifier must be replaced by the correct one, which is 15420b71. Consider updating this value in line 242 of the ERC865Token, as well as modifying the comment in the previous line so as to match both the correct identifier and function signature.

Update: Fixed. The 4-bytes identifier now matches the signature of the function. Note that the function was renamed, so the signature is now 0x0d98dcb1.

Medium severity

Implementation of the ERC865 interface does not fully match EIP 865 specification

The “Specification” section of the EIP 865 details the process for how users are expected to generate and sign token transactions off-chain and how trusted parties (called delegates) are expected to handle and submit them, in order to be paid fixed fees for their services. However, comparing what is specified in the EIP and the actual implementation in the ERC865Token contract, some inconsistencies were detected.

According to the spec “With their private key, the user generates {V,R,S} for the sha3 of the payload P {N,A,B,D,X,Y,T}”, where N is the nonce, A the sender of the payment, B the recipient of the payment, D is the delegate who pays for the gas, X is the amount of tokens sent from A to B, Y is the fee A pays D, and T is the token. The ERC865Token contract is then expected to reconstruct the hash of the payload P with all data provided by the delegate. Nonetheless, the current implementation in this contract rebuilds payloads placing data in a different order than the spec (the nonce, for instance, is placed last), which might potentially result in differences between on-chain and off-chain generated hashes if the off-chain client is not strictly aligned with the implementation in the ERC865Token contract.

Furthermore, the payload from which the off-chain hash is generated, according to the EIP 865, must contain the payment sender’s address (i.e. the address of the from account). Conversely, the ERC865Token contract does not include such address in the hashed payload, which once again, may result in differences between on-chain and off-chain generated hashes.

Consider applying the necessary changes to fully match the EIP 865 specification, or provide enough end-user documentation that details how off-chain clients are expected to interact with the ERC865Token contract. Should the development team consider their current implementation of the EIP to be more reliable, consider proposing changes to the public EIP while it is still a work in progress.

Potential signature malleability in ERC865Token

As highlighted in the EIP 865’s ongoing discussion, its current full implementation is affected by a signature malleability issue, steaming from the fact that in the current EIP, the recover function:

  • Allows both values 0/1 and 27/28 for v
  • Allows both lower and upper s values

Signature malleability poses a security risk in systems that use these kind of signatures as unique identifiers; as is the case of the ERC865Token contract, where a signatures mapping takes care of tracking signed operations using a signature as the identifier for each operation.

The recover function implementation found in the ERC865Token contract appears to be copied from OpenZeppelin-eth’s ECDSA library. Although a set of lines were commented out in an attempt to patch the signature malleability issue reported in the library, there is still debate around how to best approach the fix and whether only restricting the v value to 27/28 is sufficient to prevent signature malleability.

Taking into account these issues, consider tracking unique operations using a hash of the payload (including a nonce to prevent equal operations from producing the same hash) instead of using potentially malleable signatures. This way, the custom implementation of the recover function in the ERC865Token contract could be removed from the codebase and instead directly imported from OpenZeppelin’s ECDSA library, since the potential signature malleability issue in its recover function would no longer affect the ERC865Token implementation. In addition, the project will be able to benefit from thoroughly tested bug-fixes in future releases of the library.

As EIP 865 is still under discussion, it is highly recommended that the development team closely follows its evolution over time, not only to be aware of this and other security-related issues to be patched, but also of any changes that the contracts’ interfaces may suffer, which could eventually render the audited ERC865 and ERC865Token contracts outdated and non-compliant.

For more details on what signature malleability is and how it can be exploited by attackers, please refer to:

Update: Fixed. The contract is now using the recover function from OpenZeppelin. The hash of the payload and the signer address is now used as the identifier of the operation, for example, see L215. Note that the EIP is still in draft.

Lack of event emission in transferFromPreSigned function of ERC865Token contract

The ERC865Token contract’s purpose is to extend ERC20 transfer and transferFrom operations with pre-signed transfers of tokens. This is achieved via the transferPreSigned and transferFromPreSigned public functions.

Additionally, a new event called TransferPreSigned takes care of logging pre-signed transfers . Even though this event is properly emitted after a successful transferPreSigned operation, TransferPreSigned is not emitted before the execution of the transferFromPreSigned function is completed, which may be difficult tracking this operation via event logs.

To favor consistency and operations traceability, and taking into account that the counterpart ERC20 transferFrom does emit a Transfer event, consider emitting a TransferPreSigned event after a transferFromPreSigned operation is successfully registered.

Update: Fixed. The transferFromPreSigned function now emits the TransferPreSigned event.

Missing test coverage report

There is no automated test coverage report. Without this report it is impossible to know whether there are parts of the code never executed by the automated tests; so for every change, a full manual test suite has to be executed to make sure that nothing is broken or misbehaving.

Consider adding the test coverage report, and making it reach at least 95% of the source code.

Update: Fixed. A coverage report has been added. Note that this report has to be generated every time the code is changed, so consider removing it from the repository and using instead a reporting service like Codecov or Coveralls. Also note that although reported line coverage is 100%, branch coverage is 53%.

Low severity

Transaction token fees may be locked forever in ERC865Token contract

The ERC865Token contract allows the owner of tokens to execute token transfers and approvals without the need to own ETH in the first place. The owner, instead, pre-signs transactions and sends them off-chain to a trusted party that in turn submits the actual transactions to the token contract. As a reward, this trusted party gets paid a pre-arranged fixed amount of tokens.

In the current implementation, fees are paid in tokens to the msg.sender of the transaction, which may in some cases be a contract and not an externally-owned account. In a scenario where the msg.sender is indeed a smart contract without the necessary logic to operate with tokens, the fees may forever be locked in the token contract, resulting in financial losses for the party submitting the actual transactions to the network.

If there are no restrictions on whether contracts can relay transactions to the ERC865Token contract, it is recommended to explicitly warn users about the potential locking of tokens, providing detailed end-user documentation on how to best tackle this issue (e.g. providing a sample contract containing the necessary logic to both relay transactions and operate with received fees).

Update: Partially fixed. A notice comment has been added to the functions to warn users about calling them from smart contracts. For example, see L182.

Unexpected behavior in decreaseApprovalPreSigned function of ERC865Token contract

The function decreaseApprovalPreSigned in the ERC865Token contract decreases the amount of tokens that an owner has allowed a spender. Being an extension of the commonly used decreaseAllowance to support pre-signed operations, it would be expected that both functions behaved exactly the same when decreasing an allowance. Yet, decreaseApprovalPreSigned does not revert when the value to be subtracted is greater than the current allowance, as decreaseAllowance does. Instead, it sets the allowance to zero.

While this issue does not pose a security risk, since the allowance is indeed decreased, the differences in behavior between functions is not documented at all and might be entirely unexpected for developers and users, potentially causing errors in off-chain clients interacting with it. Thus, consider not implementing custom behavior in interfaces that have become de-facto standard. If strictly required, documenting differences in behavior is highly recommended.

Update: Fixed. The decreaseApprovalPreSigned function now reverts when the value subtracted is greater than the current allowance.

Code repetition in PropsSidechainCompatible contract

Following code reusability best practices, the settle function of the PropsSidechainCompatible contract may benefit from internally calling its public transfer function (instead of calling super.transfer) and on success only emit the Settlement event, as the TransferDetails event will already be emitted by the transfer function.

Furthermore, consider explicitly documenting the rationale behind including two extremely similar functions such as transfer and settle, where the only difference between the two is the emission of a Settlement event in the latter. Off-chain clients accidentally calling transfer instead of settle to transfer tokens may expect a Settlement event that is never going to be emitted. Additionally, this behavior is inconsistent, considering that the settle function only works with transfer and not with transferFrom.

Update: The PropsSidechainCompatible contract has been removed.

Missing error messages in require statements

There are several require statements in the ERC865Token contract without error messages. Consider including specific and informative error messages in all require statements.

Update: Fixed. All the require statements of the ERC865Token contract now have an error message.

Missing docstrings for public functions

There are several public functions without docstrings in both PropsSidechainCompatible and PropsTimeBasedTransfers contracts. Consider adding Natspec docstrings to everything that is part of the contracts’ public API.

Update: Fixed. The PropsSidechainCompatible contract has been removed and now all the functions of the PropsTimeBasedTransfers contract have docstrings.

Outdated OpenZeppelin vesting contract in use

The vesting contract TokenVesting is outdated and has two known low-severity issues, thoroughly described in the last LevelK’s OpenZeppelin audit report (refer to page 15).

In particular, these issues were fixed in newer versions of the TokenVesting contract and now include additional safety checks during construction and restricted private functions that are not yet intended to be part of the contract’s public API (such as releasableAmount and vestedAmountfunctions).

As these changes are already ported to newer versions of the openzeppelin-eth package, consider upgrading the TokenVesting contract to include these fixes, or at least properly documenting the rationale behind not including them.

Update: The TokenVesting contract has been removed.

OpenZeppelin MIT license violation

The contract TokenVesting in the TokenVesting.sol file was copied from the openzeppelin-eth public repository. However, it does not include the MIT license required by the project. As stated by the this license:

“The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.”

When not importing the contracts directly from installed packages and instead copying them into your project (which is not the recommended way of use), consider including all necessary licenses in the contract’s code, together with documentation explaining developers and auditors the rationale behind copying the contracts.

Update: The TokenVesting contract has been removed.

Untested Solidity optimizations may cause unpredictable behavior

In the truffle.js configuration file, Solidity optimizations are enabled. Solidity has some default optimizations always executed, and some others that are optional. Enabling the optional ones increases the risk of unexpected behavior, since they are not as battle-tested as the default optimizations. Consider adding full test coverage without optimizations before enabling them, so as to verify that the introduced optimizations preserve expected behavior.

Update: Partially fixed. Reported line coverage is 100%, but branch coverage is 53%.

Commented out code in several functions

Several functions in the ERC865Token contract include commented out lines of code without giving developers enough context on why those lines have been discarded, thus providing them with little to no value at all. The same issue was identified in the contract’s import statements.

As the purpose of these lines is unclear and may confuse future developers and external contributors, consider removing them from the codebase. If they are to provide alternate implementation options, consider extracting them to a separate document where a deeper and more thorough explanation could be included.

Update: Fixed. Commented out lines have been removed from the ERC865Token contract.

Erroneous docstrings in codebase

A first instance of this issue can be found at the inline documentation provided before the PropsTimeBasedTransfers contract declaration, where the current docstrings correspond to the PropsSidechainCompatible contract. Similarly, contract ERC865 docstrings correspond to the ERC865Token contract.

Moreover, there are references in the PropsToken contract docstrings to an integration with the TPL protocol that is not currently implemented in the token. Additionally, in this same contract, the docstrings state that the PropsToken is pausable, but no such behavior was found in the code’s logic.

If the PROPS token is based on the ZEP token implemented by the ZeppelinOS team, as docstrings in PropsToken highlight, please consider including a link referencing the ZEP token’s public code.

Furthermore, an inline comment in the ERC865Token contract mistakenly describes the signatures mapping as a data structure to hold nonces, while it is actually used to store signatures.

As inaccurate docstrings may confuse users, developers and auditors alike, consider fixing all instances of this issue.

Update: Partially fixed. The docstring of the IERC865 contract is still wrong.

Erroneous and complex project set up

The setup instructions found in the README file require developers to do a minor change in the openzeppelin-eth ERC20 contract at node_modules/openzeppelin-eth/contracts/token/ERC20.sol. For a more developer-friendly experience while setting up the project’s dependencies, consider directly including the modified contract in the contracts folder (changing the import paths were needed), instead of instructing to modify a package in the node_modules folder, which requires developers to apply the modifications every time a fresh npm install command is run. Also, consider fixing the path specified in the README to node_modules/openzeppelin-eth/contracts/token/ERC20/ERC20.sol.

Additionally, one step in the testing instructions in the the README file mentions a logic-contract-deployer-address, from which contracts are expected to be deployed. However, it is never explained to contributors what that address is nor how to obtain it.

Finally, further hindering the developers and potential contributors’ experience, the openzeppelin-eth version in the package.json file is not fixed to 2.0.2, but to ^2.0.2, which currently leads to version 2.1.3 of the openzeppelin-eth package being installed after npm install is run, a version of the package meant to be used with Solidity compiler’s version 0.5, while the rest of the project’s contracts are to be compiled with a version of Solidity up to 0.4.25. Until the rest of the contracts in the PROPS Token project is upgraded to Solidity 0.5, consider pinning this dependency to 2.0.2, or ~2.0.2 to benefit from future patch versions.

Update: Fixed. The README has been simplified and the openzeppelin-eth version has been pinned to 2.0.2.

Erroneous test suite

The testing suite does not run after following the instructions in the README file of the project.

As the test suite was left outside the audit’s scope, please consider thoroughly reviewing it to make sure all tests run successfully after following the instructions in the README file.

Update: Fixed. The test suite now can be run after following the instructions in the README. However, note that the tests Name should be correct and Symbol should be correct are failing.

Erroneous Truffle configuration

An error in the truffle.js configuration file prevents Truffle 5 from running with the specified solc version (0.4.24). Current configuration is:

solc: { 
  version: '0.4.24',
  optimizer: { 
    enabled: true, 
    runs: 500,

According to Truffle docs, it should be:

compilers: {
  solc: { 
    version: '0.4.24',
    optimizer: {
        enabled: true,
        runs: 500,

Updated: Fixed. The truffle configuration is now correct.

Outdated solc version in use

An outdated solc version, 0.4.24, is currently in use. Consider upgrading to the latest Solidity v0.5 or at least to the latest version of the 0.4 branch, currently 0.4.25.

Updated: Fixed. The truffle configuration now specifies solidity v0.4.25.

Notes & Additional Information


No critical and one high severity issue were found. Some changes were proposed to follow best practices and reduce the potential attack surface.

Note that as of the date of publishing, the above review reflects the current understanding of known security patterns as they relate to the PROPS Token’s contracts. The above should not be construed as investment advice. For general information about smart contract security, check out our thoughts here.