Arkouda pays NFT rewards in whole days, not seconds

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 56425-rewards-calculation-is-done-by-counting-number-of-days-since. Standalone Foundry PoC and full write-up: 56425-rewards-calculation-is-done-by-counting-number-of-days-since_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/reward-calculation · vuln/arithmetic/precision-loss

Key info#

Field	Value
Loss	A holder of 1.5 days is minted 3 ARK. The seconds formula owes 4.5 ARK. The holder is short 1.5 ARK
Vulnerable contract	`Arkouda.getPendingReward`
Attacker EOA	`0x1111111111111111111111111111111111111111` (the NFT holder who claims)
Attack contract	`Exploit`
Attack tx	`Exploit.run()`
Chain / block / date	Ethereum model · block timestamp 1700129600 · 2022-02
Compiler	solc 0.8.27, cancun, via IR
Bug class	Reward accrual truncates elapsed time to whole days

TL;DR#

Zokyo's February 2022 CWBC review says Arkouda.getPendingReward counts days since the last transfer instead of seconds. The daily tier is 3 ARK, the documented minimum. A holder whose NFT last moved at timestamp 1700000000 claims at 1700129600. That is 129600 seconds, 1.5 days. Integer division keeps 1 day, so the claim mints 3 ARK. Paying the same tier for every second mints 4.5 ARK. The 1.5 ARK difference is the half day the division drops. The owner is not called. The control uses the seconds formula and mints 4.5 ARK.

Background#

CryptoBear Watch Club pays Arkouda to NFT holders by tier. Zokyo's companion note says the minimum tier is 3 ARK per day. The recommendation on this finding is to make getPendingReward seconds-based. The stored input is the last transfer time. This run does not move that timestamp and does not call setTokenIdReward, startRewardGeneration, or flipArkoudaClaimStatus.

The vulnerable code#

SOLIDITY

lastTransferTime = LAST_TRANSFER;
elapsedSeconds = block.timestamp - lastTransferTime;
// @> VULN: whole days since the last transfer, not seconds
daysHeld = elapsedSeconds / 1 days;
paid = daysHeld * REWARD_PER_DAY;
fair = elapsedSeconds * REWARD_PER_DAY / 1 days;

REWARD_PER_DAY is 3 ARK. 1 days is 86400 seconds.

Root cause#

The pending amount is elapsed / 1 days * rewardPerDay. The division rounds down. Any hold that is not an exact number of days is paid only for the completed days. The seconds formula is elapsed * rewardPerDay / 1 days, which keeps the fraction.

Preconditions#

The NFT's last transfer is stored as 1700000000. The claim block is 1700129600.
Elapsed time is 129600 seconds, which is 1 day and 12 hours.
The tier is 3 ARK per day. No admin changes that rate in this transaction.

Attack walkthrough#

The claim reads the stored last transfer. It is 1700000000.
The clock is 129600 seconds later.
129600 / 86400 is 1. The extra 43200 seconds are discarded.
1 * 3 ARK is minted. The holder balance is 3 ARK.
129600 * 3 ARK / 86400 is 4.5 ARK. The shortfall is 1.5 ARK.
The control mints 4.5 ARK to the same holder for the same elapsed time.

Diagrams#

flowchart TD A["Last transfer is 1700000000 and the claim block is 129600 seconds later"] --> B["Elapsed time is 1.5 days"] B --> C["VULN divides by 1 day and keeps 1"] C --> D["Claim mints 3 ARK"] B --> E["Seconds formula is elapsed times 3 ARK divided by 1 day"] E --> F["Fair mint is 4.5 ARK"] D --> G["Holder is short 1.5 ARK"] F --> G

Impact#

The holder is minted 3 ARK. The continuous tier for 1.5 days is 4.5 ARK. The holder loses 1.5 ARK of rewards that the seconds formula would have minted. The loss is the truncated half day. It does not depend on the owner rewriting the tier or the start time. The control pays 4.5 ARK and leaves no shortfall.

Remediation#

Compute elapsedSeconds * rewardPerDay / 1 days, and do the multiplication before the division. Do not count only completed days.

How to reproduce#

BASH

cd 56425-rewards-calculation-is-done-by-counting-number-of-days-since_exp
forge test -vv

test_exploit_days_underpay warps to 1700129600 and asserts a 3 ARK mint, a 4.5 ARK fair amount, and a 1.5 ARK shortfall. test_control_seconds_pay_fair asserts the seconds formula mints 4.5 ARK. Captured output is output.txt.

Sources#

AuditVault finding: https://github.com/Auditware/AuditVault/blob/main/findings/56425-rewards-calculation-is-done-by-counting-number-of-days-since.md
Zokyo CWBC 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: 56425-rewards-calculation-is-done-by-counting-number-of-days-since_exp (evm-hack-registry mirror).
AuditVault finding: 56425-rewards-calculation-is-done-by-counting-number-of-days-since.

Alerts & third-party analyses

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