Vibe — another minter can occupy an id the sale is selling

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w. Standalone Foundry PoC and full write-up: 27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/dos/frozen-funds · vuln/logic/missing-check

Key info#


Protocol	Vibe
Finding	MixBytes, June 2023, AuditVault 27820
Vulnerable code	`buyNFT` calls `nft.mintWithId(recipient, id)`
Harm	Id 7 is taken, so the 1 ETH sale collects 0. The exclusive-minter control collects 1 ETH.
Chain	local synthetic (audited Ethereum contracts)

TL;DR#

NFTMintSaleMultiple.buyNFT mints one predetermined id with mintWithId. The ERC721 is shared. Any other minter can create that id first. The sale's call then reverts because the id exists, and that 1 ETH listing cannot complete.

The vulnerable code#

The report points at NFTMintSaleMultiple.sol L77.

SOLIDITY

function buyNFT(address recipient) external payable {
    require(msg.value == price, "price");
    uint256 tokenId = id;
    // @> VULN: mintWithId reverts when another minter already took this id.
    nft.mintWithId(recipient, tokenId);
    sold += 1;
    collected += msg.value;
}

Root cause#

The sale does not own the id space. mintWithId is a global write on the NFT. The sale assumes the id is still free at purchase time. Nothing in the sale stops a second minter from using the same id.

Preconditions#

The NFT allows a minter other than this sale. The sale is offering id 7 at 1 ETH. The external minter gets there first. One transaction is enough for the occupying mint. The buy is attempted in the same transaction here and reverts.

Attack walkthrough#

The external minter calls mintWithId for id 7.
ownerOf(7) is that minter.
A buyer sends 1 ETH into buyNFT.
nft.mintWithId(recipient, id) reverts because id 7 exists.
The sale records the unsold amount as 1 ETH.
sold stays 0 and collected stays 0. The 1 ETH is refunded to the buyer.

Control: mintWithId is restricted to the sale. The external mint reverts. buyNFT collects 1 ETH and the buyer owns id 7.

Diagrams#

flowchart TD A["Sale is offering id 7 for 1 ETH"] --> B["Another minter calls mintWithId for id 7"] B --> C["VULN: buyNFT calls mintWithId and the id exists"] C --> D["The buy reverts"] D --> E["sold is 0 and collected is 0"] E --> F["Control exclusive minter collects 1 ETH"]

Impact#

The buyer cannot purchase id 7. The sale collects 0 of the 1 ETH price. With minting limited to the sale, that same purchase collects 1 ETH and the buyer receives the NFT.

Remediation#

Use only mint or only mintWithId on a given VibeERC721, which is the MixBytes recommendation. The sale should be the only account allowed to mint ids inside its range.

How to reproduce#

BASH

cd 27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w_exp
forge test -vv

Sources#

AuditVault finding: https://github.com/Auditware/AuditVault/blob/main/findings/27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w.md
MixBytes report: https://github.com/mixbytes/audits_public/blob/master/Vibe/README.md
Audited file: contracts/mint/NFTMintSaleMultiple.sol at d08057edbaf83b00d94dcaca2a05e3c44a45e4d9 L77

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w_exp (evm-hack-registry mirror).
AuditVault finding: 27820-nftmintsalemultiple-wont-work-if-other-minter-mints-tokens-w.

Alerts & third-party analyses

Web3Sec X hacked database: search.
Rekt leaderboard: search.
Solodit incident search: search.

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.

Source-level debugger — step opcodes and Solidity in sync