Beetle — marking a beetle safe after a loss blocks the opponent's NFT

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 62314-c-01-user-can-deny-opponent-nft-rewards-by-marking-safe-post. Standalone Foundry PoC and full write-up: 62314-c-01-user-can-deny-opponent-nft-rewards-by-marking-safe-post_exp in the evm-hack-registry mirror.

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

Reproduction: a self-contained Foundry PoC that compiles and runs offline with only forge-std — no fork and no RPC. Trace: output.txt. PoC: test/62314-c-01-user-can-deny-opponent-nft-rewards-by-marking-safe-post_exp.sol.

Key info#


Impact	CRITICAL — the opponent receives 0 beetles and 1 beetle stays locked on the loser's stake
Protocol	Beetle `BeetleBattleGame`
Vulnerable code	`markBeetleSafe` writes `safeBeetle[msg.sender] = _tokenId` with no battle check
Bug class	Post-battle shield blocks `claimBeetleFromVictory`
Finding	Shieldify — Beetle, August 2025 · id 62314, report C-01
Report	Beetle-Security-Review.md
Source	AuditVault finding
Status	Team fixed the timing. Reproduced here as a local synthetic.
Compiler	solc 0.8.27, cancun, via IR

TL;DR#

markBeetleSafe checks that the caller staked the token and then stores it as that player's safe beetle. It does not look at whether a battle for that token has already been lost. claimBeetleFromVictory will not transfer a safe beetle.

The loser waits until battle 1 names the opponent as the winner, calls markBeetleSafe(1), and the opponent's claim returns without a transfer. The opponent's balance stays 0. The game still holds 1 beetle, staked to the loser.

The vulnerable code#

SOLIDITY

function markBeetleSafe(uint256 _tokenId) public {
    if (isStaked[_tokenId] != msg.sender) revert InvalidTokenOwner();
    safeBeetle[msg.sender] = _tokenId; // @> VULN: allowed after the battle is already lost
    emit SafeBeetleUpdated(msg.sender, _tokenId);
}

The claim then hits if (safeBeetle[loser] == tokenId) and returns. The transfer to the winner does not run.

The fix reverts markBeetleSafe when battleOf[tokenId] is already set.

Root cause#

Safety is only an ownership check. The battle result is not an input, so the loser can change safeBeetle after the winner is known and before the claim.

Preconditions#

The attacker has staked beetle 1.
A battle already records the opponent as the winner and beetle 1 as the stake.
claimBeetleFromVictory refuses to move a beetle that is the loser's safeBeetle.

Attack walkthrough#

Mint beetle 1 to the attacker and stake it. The game holds the NFT.
Finish the battle. The opponent is the winner.
The attacker calls markBeetleSafe(1). The assignment stores token 1.
The opponent calls claimBeetleFromVictory(1). The safe-beetle check returns early.
The opponent's balance is 0. The game's balance is 1. The stake still names the attacker.

Control: markBeetleSafe reverts with TooLate once the battle exists. The claim then transfers beetle 1 to the opponent. The opponent's balance is 1 and the game's balance is 0.

Diagrams#

flowchart TD A["Attacker stakes beetle 1"] --> B["Battle ends with the opponent as winner"] B --> C["Attacker calls markBeetleSafe on token 1"] C --> D["safeBeetle is stored with no timing check"] D --> E["Opponent claim sees the safe beetle and returns"] E --> F["Opponent balance stays 0 and the game still holds the beetle"]

Impact#

The winner cannot take the 1 beetle the battle awarded. It stays locked in the game under the loser's stake. The opponent's balance remains 0.

Remediation#

Allow markBeetleSafe only while staking, or reject it when a battle for that token is already recorded. The fixed function reverts with TooLate in that case, and the claim transfers the beetle to the winner.

Sources#

AuditVault finding 62314
Shieldify Beetle review
Vulnerable function: BeetleBattleGame.sol at 4ab13a82

Taxonomy#

severity/high

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 62314-c-01-user-can-deny-opponent-nft-rewards-by-marking-safe-post_exp (evm-hack-registry mirror).
AuditVault finding: 62314-c-01-user-can-deny-opponent-nft-rewards-by-marking-safe-post.

Alerts & third-party analyses

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