Syntetika — mint ignores decimals and the BTC price

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 62198-minter-doesnt-account-for-depegs-exchange-rates-and-decimal. Standalone Foundry PoC and full write-up: 62198-minter-doesnt-account-for-depegs-exchange-rates-and-decimal_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/arithmetic/decimal-mismatch

Reproduction: a self-contained Foundry PoC that compiles and runs offline with only forge-std — no fork and no RPC. Trace: output.txt. PoC: test/62198-minter-doesnt-account-for-depegs-exchange-rates-and-decimal_exp.sol.

Key info#


Impact	HIGH — 1 USDC mints 1,000,000 raw hBTC instead of the fair 1,000
Protocol	Syntetika `Minter`
Vulnerable code	`mint` calls `hilBTCToken.mint(to, amount)` with the raw base deposit
Bug class	Decimal and exchange-rate scale dropped on mint
Finding	Cyfrin (Dacian) — Syntetika, 1 August 2025 · id 62198
Report	2025-08-01-cyfrin-syntetika-v2.0.md
Source	AuditVault finding
Status	Team matched decimals in the constructor and added a pause. Reproduced here as a local synthetic.
Compiler	solc 0.8.27, cancun, via IR

TL;DR#

baseAsset can be USDC (6 decimals). hilBTCToken is hBTC (8 decimals) and is redeemable as BTC. mint pulls amount of the base asset and mints amount hBTC.

At $100,000 per BTC, 1 USDC is 1,000 raw hBTC:

1_000_000 * 1e8 / (100_000 * 1e6) = 1_000

The call mints 1,000,000 raw hBTC instead. That is 0.01 hBTC, a thousand times the fair 0.00001 hBTC. The same 1:1 copy is what the report describes for a depegged 8-decimal wrapper: the minter never reads a price.

The vulnerable code#

SOLIDITY

function mint(address to, uint256 amount) external onlyWhitelisted(msg.sender) onlyWhitelisted(to) nonReentrant {
    baseAsset.safeTransferFrom(msg.sender, address(this), amount);
    hilBTCToken.mint(to, amount); // @> VULN: raw base amount, no decimals and no price
    totalDeposits += amount;
    emit Minted(to, amount);
}

The fixed mint replaces that amount with amount * 1e8 / (100_000 * 1e6).

Root cause#

The mint output is the input integer. Nothing rescales 6-decimal base units into 8-decimal hBTC, and nothing divides by the BTC price. A depegged base asset is accepted the same way.

Preconditions#

The caller and the recipient are whitelisted. That is the normal mint path, not the bug.
The base asset is 6-decimal USDC and hBTC is 8 decimals.
BTC is priced at $100,000, so 1 USDC is 1,000 raw hBTC.

Attack walkthrough#

Mint 1,000,000 raw USDC to the caller and approve the minter.
Whitelist the caller.
mint(caller, 1_000_000) pulls the USDC and mints 1,000,000 raw hBTC.
The fair amount is 1,000. The caller holds 1,000 times that.

Control: the same deposit mints 1,000 raw hBTC. The minter still holds the 1,000,000 raw USDC.

Diagrams#

flowchart TD A["Deposit 1000000 raw USDC, which is 1 dollar"] --> B["mint copies that integer into hBTC"] B --> C["Caller receives 1000000 raw hBTC"] C --> D["8 decimals makes that 0.01 hBTC"] D --> E["Fair output at 100000 dollars per BTC is 1000 raw hBTC"] E --> F["Caller was credited 1000 times the fair amount"]

Impact#

For a 1 USDC deposit the caller is credited 1,000,000 raw hBTC instead of 1,000. The extra 999,000 raw hBTC is unbacked by the deposit. The report's depeg case is the same missing scale: a cheap base token is still minted 1:1 into hBTC.

Remediation#

Scale the minted amount by the decimal gap and the base-asset price before calling hilBTCToken.mint. Reject, or pause, when the base asset has depegged from the value hBTC redeems for.

Sources#

Taxonomy#

severity/high
vuln/arithmetic/decimal-mismatch

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 62198-minter-doesnt-account-for-depegs-exchange-rates-and-decimal_exp (evm-hack-registry mirror).
AuditVault finding: 62198-minter-doesnt-account-for-depegs-exchange-rates-and-decimal.

Alerts & third-party analyses

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

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