Curio — earlyEndGame locks the 0.1 ETH game fee

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 34118-h-01-game-fee-locked-in-earlyendgame-pashov-audit-group-none. Standalone Foundry PoC and full write-up: 34118-h-01-game-fee-locked-in-earlyendgame-pashov-audit-group-none_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/fee-calculation · vuln/dos/frozen-funds

Key info#


Protocol	Curio
Finding	Pashov Audit Group Curio H-01, AuditVault 34118
Vulnerable code	`Game.earlyEndGame`
Harm	0.1 ETH of a 1 ETH pot stays in the game. The treasury receives 0
Chain	local synthetic

TL;DR#

earlyEndGame computes a 10 percent fee on a 1 ETH payout, subtracts 0.1 ETH from the player, and sends the player 0.9 ETH. It never forwards the fee. The game balance is 0.1 ETH and the treasury balance is 0. There is no other withdrawal of that balance. The fixed function sends the 0.1 ETH to the treasury and the game balance ends at 0.

The vulnerable code#

SOLIDITY

uint256 gameFee = (playerPayout.payout * takeRate) / 100;
playerPayout.payout -= gameFee; // @> VULN: gameFee is taken from the player and left in this contract
feeTaken = gameFee;
(bool sent,) = player.call{value: playerPayout.payout}("");

The recommended fix is treasury.call{value: gameFee} after the deduction. That call is absent here.

Root cause#

The fee is an accounting decrease of the player's payout, not a transfer. The ETH that used to back the full payout remains in the contract with no path to the treasury.

Preconditions#

The admin calls earlyEndGame the way the game ends a match. The pot is 1 ETH and takeRate is 10. The player can receive ETH. The treasury is a separate address.

Attack walkthrough#

The game is funded with 1 ETH.
earlyEndGame sets the fee to 0.1 ETH and the payout to 0.9 ETH.
The player receives 0.9 ETH.
The game still holds 0.1 ETH. The treasury holds 0.

The control uses the same numbers and then pays the treasury. The game balance is 0 and the treasury holds 0.1 ETH.

Diagrams#

flowchart TD A["Game holds a 1 ETH pot"] --> B["Take rate is 10 percent"] B --> C["Fee of 0.1 ETH is subtracted from the payout"] C --> D["Player receives 0.9 ETH"] D --> E["0.1 ETH remains in the game"] E --> F["Treasury balance stays 0"]

Impact#

The treasury is owed 0.1 ETH and cannot collect it. The player was paid the net amount, so the missing fee is not the player's loss. It is locked in the game contract. A 10 percent fee on every early end accumulates the same way.

Remediation#

After the deduction, send gameFee to the treasury and revert if the call fails.

How to reproduce#

BASH

cd 34118-h-01-game-fee-locked-in-earlyendgame-pashov-audit-group-none_exp
forge test -vv

Sources#

AuditVault finding 34118
https://github.com/pashov/audits/blob/master/team/md/Curio-security-review.md

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 34118-h-01-game-fee-locked-in-earlyendgame-pashov-audit-group-none_exp (evm-hack-registry mirror).
AuditVault finding: 34118-h-01-game-fee-locked-in-earlyendgame-pashov-audit-group-none.

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