Hanji: nominal credit takes 100 from the honest balance

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 55171-incorrect-handling-of-fee-on-transfer-tokens-mixbytes-none-h. Standalone Foundry PoC and full write-up: 55171-incorrect-handling-of-fee-on-transfer-tokens-mixbytes-none-h_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/fee-calculation · vuln/input-validation/missing

Key info#


Protocol	Hanji
Finding	MixBytes, AuditVault 55171
Vulnerable code	`LOB.receiveTokens`
Harm	A 1000 deposit of a 10 percent fee-on-transfer token delivers 900 and is credited as 1000. The attacker withdraws 1000 and is up 100. The book has 900 left against the honest trader's 1000 credit. The fixed path credits 900 and the attacker is flat.
Chain	local synthetic

TL;DR#

receiveTokens does not measure the balance increase after transferFrom. The fee token keeps 100, the book receives 900, and the credit increases by 1000. Withdrawing the credit pulls 100 of the honest trader's tokens.

The vulnerable code#

SOLIDITY

            // @> VULN: credit increases by `amount`, not by the tokens that arrived
            credit[from] += amount;

Root cause#

The credit uses the requested amount. A fee-on-transfer token delivers less than that amount.

Preconditions#

The book already holds 1000 backing an honest credit of 1000. The attacker's token takes a 10 percent fee on transferFrom and pays that fee to the attacker.

Attack walkthrough#

Approve a 1000 pull. See output.txt.
receiveTokens pulls 1000. The book receives 900.
Credit increases by 1000.
The attacker withdraws 1000.
Attacker profit is 100.
The honest credit of 1000 is backed by 900. The fixed path credits 900 and profit is 0.

Diagrams#

flowchart TD A["Pull 1000 of a fee token"] --> B["Book receives 900"] B["Book receives 900"] --> C["Credit increases by 1000"] C["Credit increases by 1000"] --> D["Withdraw 1000"] D["Withdraw 1000"] --> E["Honest balance is short 100"]

Impact#

The honest trader's 1000 credit is backed by 900. The attacker nets 100. Crediting the measured balance delta removes the profit.

Remediation#

Credit balanceAfter - balanceBefore from the transferFrom, and revert if that delta is not the expected amount.

How to reproduce#

BASH

cd 55171-incorrect-handling-of-fee-on-transfer-tokens-mixbytes-none-h_exp
forge test -vv

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 55171-incorrect-handling-of-fee-on-transfer-tokens-mixbytes-none-h_exp (evm-hack-registry mirror).
AuditVault finding: 55171-incorrect-handling-of-fee-on-transfer-tokens-mixbytes-none-h.

Alerts & third-party analyses

Web3Sec X hacked database: search.
Rekt leaderboard: search.
Solodit incident search: search.

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