Bug Bounty Report: natspec documents wrong onERC721Received magic value (0xf0b9e5ba vs implemented 0x150b7a02)

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调研方向

错误的签名位于 src/contracts/tokens/erc721.sol（约第 49-50 行）和 src/contracts/tokens/nf-token.sol（约第 116-117 行）的 natspec 注释中。请将它们与 nf-token.sol 第 32-35 行的常量 MAGIC_ON_ERC721_RECEIVED 以及 src/contracts/token/erc721-token-receiver.sol 中的接收者接口进行比较。当两处注释都写明 onERC721Received(address,address,uint256,bytes)，并与选择器 0x150b7a02 一致时即完成；你可以用 issue 中的 Python 代码片段确认该哈希。

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描述

Found while reviewing src/contracts under the bug bounty program (latest release, @nibbstack/erc721 2.6.1).

What

The natspec on safeTransfer / safeTransferFrom documents the required receiver magic value with the wrong function signature:

src/contracts/tokens/erc721.sol:49-50 and src/contracts/tokens/nf-token.sol:116-117:

* function checks if `_to` is a smart contract (code size > 0). If so, it calls
* `onERC721Received` on `_to` and throws if the return value is not
* `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`.

The implementation does not check that value. src/contracts/tokens/nf-token.sol:32-35:

* Equal to: bytes4(keccak256("onERC721Received(address,address,uint256,bytes)")).
bytes4 internal constant MAGIC_ON_ERC721_RECEIVED = 0x150b7a02;

The two selectors differ:

$ python3 -c "from Crypto.Hash import keccak
for s in ['onERC721Received(address,uint256,bytes)','onERC721Received(address,address,uint256,bytes)']:
    k=keccak.new(digest_bits=256); k.update(s.encode()); print(s, '0x'+k.hexdigest()[:8])"
onERC721Received(address,uint256,bytes) 0xf0b9e5ba
onERC721Received(address,address,uint256,bytes) 0x150b7a02

So the documented value is 0xf0b9e5ba while the code requires 0x150b7a02. The interface natspec at src/contracts/token/erc721-token-receiver.sol states the correct four-argument signature, which makes the two in-repo documents contradict each other.

Impact

A developer implementing a receiver from the main safeTransferFrom natspec returns 0xf0b9e5ba; every safeTransferFrom to that contract then reverts with "unsafe recipient". The failure looks like a contract bug rather than a documentation mismatch, and counterparties reading the same natspec draw the wrong conclusion about what an implementation requires. This falls under "any undocumented and unintuitive behavior" in the program scope.

Suggested fix

Update the natspec string in erc721.sol and nf-token.sol to onERC721Received(address,address,uint256,bytes) so it matches MAGIC_ON_ERC721_RECEIVED and the receiver interface.

Tested against tag 2.6.1 (commit 198dec9, npm tarball byte-identical with the repo).

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