Sqwid raffle drops the fractional REEF and leaves it in the contract

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy. Standalone Foundry PoC and full write-up: 56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/arithmetic/precision-loss · vuln/dos/frozen-funds

Key info#

TL;DR#

Zokyo's June 2022 Sqwid review says enterRaffle divides msg.value by 1e18, and endRaffle multiplies the integer by 1e18. Their example is 2500000000000000000 wei, which is 2.5 REEF. The division keeps 2. The payout is 2000000000000000000 wei, which is 2 REEF. The contract still holds 0.5 REEF. The control forwards msg.value with no division, pays the seller 2.5 REEF, and leaves a zero balance.

Background#

SqwidMarketplace records raffle entries in whole REEF so a later pseudo-random draw can weight them. Solidity division drops the remainder. The team kept the division and documented it. A direct caller who sends a fractional REEF amount still loses the fraction: it is not part of the payout.

The vulnerable code#

entered = msg.value;
// @> VULN: msg.value / 1e18 drops the fractional REEF
wholeReef = msg.value / 1e18;
payout = wholeReef * 1e18;
stuck = entered - payout;

Root cause#

The entry is stored as msg.value / 1e18. The settlement pays stored * 1e18. The product is the largest whole-REEF amount that does not exceed what was sent. The remainder never leaves the contract in this settlement.

Preconditions#

• The entrant sends exactly 2.5 REEF, the amount in the report.
• endRaffle pays wholeReef * 1e18 to the seller.
• No fee is taken on top. The only missing amount is the truncated fraction.

Attack walkthrough#

1. The marketplace receives 2.5 REEF.
2. 2500000000000000000 / 1e18 is 2.
3. The payout is 2 * 1e18, which is 2 REEF.
4. 2.5 minus 2 is 0.5 REEF. That value is not included in the transfer.
5. The seller's balance is 2 REEF. The marketplace balance is 0.5 REEF.
6. The control sends 2.5 REEF to the seller and the contract balance is 0.

Diagrams#

Impact#

The entrant paid 2.5 REEF. The seller receives 2 REEF. 0.5 REEF is left in the marketplace and is not part of this payout. The locked amount is the fractional REEF the division removed. The control pays the seller the full 2.5 REEF.

Remediation#

Record and refund msg.value itself. Do not divide by 1e18 on the way in and multiply by 1e18 on the way out.

How to reproduce#

cd 56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy_exp
forge test -vv

test_exploit_fractional_reef_stuck sends 2.5 REEF and asserts a recorded 2, a 2 REEF payout, a 0.5 REEF contract balance, and a seller balance of 2 REEF. test_control_pays_full_value asserts a 2.5 REEF payout and a zero stuck amount. Captured output is output.txt.

Sources#

• AuditVault finding: https://github.com/Auditware/AuditVault/blob/main/findings/56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy.md
• Zokyo Sqwid report: https://github.com/solodit/solodit_content/blob/main/reports/Zokyo/2022-06-16-Sqwid.md

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy_exp (evm-hack-registry mirror).
• AuditVault finding: 56646-not-the-whole-amount-of-reef-is-transferred-due-to-accuracy.

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.