Barter DAO: A malicious taker re-enters swap() for a second same-maker order sharing the same takerTok

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 63500-double-order-attack-via-callback-mechanism-mixbytes-none-bar. The historical source/toolchain is unavailable; this entry is documentation only and claims no executable Forge PoC.

Vulnerability classes: vuln/theft

Reproduction: a faithful minimal reproduction of the vulnerable finding — the vulnerable function is reproduced verbatim (marked @>) with faithful minimal doubles; local deploy, no fork.

Root cause#

A malicious taker re-enters swap() for a second same-maker order sharing the same takerToken during the callback; that single payment satisfies the first order's post-callback balance-delta check too, so the taker collects makerToken from both orders while paying takerToken for only one — netting 100 STOLEN-MAKER (order #1's makerToken) for free while the maker is robbed of one full order.

            order.takerToken.balanceOf(address(order.maker));

        // Check that callback provided enough tokens
        if (balanceAfter < balanceBefore + actualTakerAmount) { // @> post-callback balance-delta check with NO reentrancy guard: a concurrent same-maker order paying the SHARED takerToken satisfies this order's check too
            revert ReceivedLessThanMinReturn(
                balanceAfter,

Why it's exploitable here#

A malicious taker re-enters swap() for a second same-maker order sharing the same takerToken during the callback; that single payment satisfies the first order's post-callback balance-delta check too, so the taker collects makerToken from both orders while paying takerToken for only one — netting 100 STOLEN-MAKER (order #1's makerToken) for free while the maker is robbed of one full order.

Attack path#

Marked-line walkthrough (Playground)#

The EVM Playground pins each step to the exact executed source line in 0xe3a787a4e4…:

1. L119 — Pay maker token to taker: Sends the maker's tokens to the taker up front, before the taker callback — so both orders pay out before payment is checked.
2. L124 — Determine taker amount owed: Selects what the taker must pay for this order — the partial filledtakerAmount or the full takerAmount.
3. L136 — Contract-wide balance-delta check: Root cause: payment is verified by a whole-contract balance delta, not per order, so one re-entrant deposit satisfies two orders' checks at once.
4. L137 — Revert on underpayment: Reverts if the measured balance increase falls short — but one shared payment passes both orders, so this never fires.
5. L139 — Expected post-swap balance: The threshold the contract balance must reach; being a global snapshot, the double-order payment gets double-counted against it.
6. L147 — Minimal reentrancy guard: Setup: a stripped reentrancy guard is defined, yet the taker re-enters swap() on a DIFFERENT order, a path this guard does not cover.
7. L155 — Guard status check: The guard requires _status == 1; the double-order attack succeeds through the balance-delta flaw, not by evading this check.

PoC#

Registry (Foundry, local deploy — verbatim vulnerable source + harm-asserting test + negative control):

cd 63500-double-order-attack-via-callback-mechanism-mixbytes-none-bar_exp
forge test -vvv

The browser Playground replays the same synthetic opcode-for-opcode and measures the harm: A malicious taker re-enters swap() for a second same-maker order sharing the same takerToken during the callback; that single payment satisf. Both gates are green (registry forge test PASS + Playground _verify-poc VERDICT: PASS).

Sources & further analysis#

Reproductions & code

• No executable Forge reproduction is claimed; the historical source/toolchain was unavailable for this finding.
• AuditVault finding: 63500-double-order-attack-via-callback-mechanism-mixbytes-none-bar.
• Upstream DeFiHackLabs PoC directory: src/test.

Alerts & third-party analyses

• DeFiHackLabs incident explorer: search "Barter DAO: A malicious taker re-enters swap() for a second same-maker order sharing the same takerTok".
• 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.