CWBC — Arkouda.claimReward mints before rewardGenerationStartTime is set

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 56419-user-can-claim-rewards-before-the-start-zokyo-none-cwbc-mark. Standalone Foundry PoC and full write-up: 56419-user-can-claim-rewards-before-the-start-zokyo-none-cwbc-mark_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/wrong-condition · vuln/logic/reward-calculation · vuln/logic/missing-check

Reproduction: Local Foundry reproduction. rewardGenerationStartTime stays 0. No fork. Trace: output.txt.

Root cause#

rewardGenerationStartTime defaults to 0 and is written by a separate startRewardGeneration call. claimReward never rejects 0, so block.timestamp >= rewardGenerationStartTime is true before rewards are started. A holder whose NFT was acquired at timestamp 1, observed in the block at timestamp 1 + 10 days, mints 10 * BASE_REWARD = 100 ARK.

SOLIDITY

require(block.timestamp >= rewardGenerationStartTime, "before start");
// @> VULN:
lastReward = daysHeld * BASE_REWARD;

The block timestamp is set once for the whole transaction. run() does not warp.

Attack path#

flowchart TD A["NFT acquired at timestamp 1"] --> B["This block is 10 days later"] B --> C["rewardGenerationStartTime is still 0"] C --> D["timestamp >= 0 passes"] D --> E["claim multiplies 10 days by 10 ARK"] E --> F["100 ARK is minted before the start"]

PoC#

The exploit asserts the start time is still 0, lastDays is 10, and the mint is 100 ARK. Those 100 ARK are transferred to the holder.

The fixed claim reverts with "not started" and mints 0.

BASH

cd 56419-user-can-claim-rewards-before-the-start-zokyo-none-cwbc-mark_exp
forge test -vv

Remediation#

Require rewardGenerationStartTime != 0 before the timestamp comparison, and set the start once.

References#

AuditVault finding 56419
Report: https://github.com/solodit/solodit_content/blob/main/reports/Zokyo/2022-02-27-CWBC.md

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 56419-user-can-claim-rewards-before-the-start-zokyo-none-cwbc-mark_exp (evm-hack-registry mirror).
AuditVault finding: 56419-user-can-claim-rewards-before-the-start-zokyo-none-cwbc-mark.

Alerts & third-party analyses

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