WDOGE (Wrapped Doge on BSC) Exploit — Reserves-Out-of-Sync Drain via Repeated `skim`/`sync`

Source & credit. Exploit reproduction, trace data, and analysis adapted from DeFiHackLabs by SunWeb3Sec — an open registry of reproduced on-chain exploits. Standalone Foundry PoC and full write-up: 2022-04-Wdoge_exp in the evm-hack-registry mirror. Upstream DeFiHackLabs PoC: src/test/…/Wdoge_exp.sol.

Vulnerability classes: vuln/oracle/spot-price · vuln/defi/slippage

Reproduction: the PoC compiles & runs in an isolated Foundry project at this project folder. Full verbose trace: output.txt. Verified vulnerable source: WDOGE, PancakePair WDOGE/WBNB, PancakePair WBNB/BUSD.

Key info#

TL;DR#

The attacker flash-swaps 2,900 WBNB from the WBNB/BUSD pair, then walks it through the WDOGE/WBNB pair in a deliberate sequence of transfer→swap→transfer→skim→sync→swap:

wbnb.transfer(wdoge_wbnb, 2900 ether);
IPancakePair(wdoge_wbnb).swap(<huge wdogeOut>, 0, address(this), "");   // pull WDOGE
wdoge.transfer(wdoge_wbnb, <amount1>);                                  // push WDOGE back
IPancakePair(wdoge_wbnb).skim(address(this));                            // skim leftover
IPancakePair(wdoge_wbnb).sync();                                         // re-sync
wdoge.transfer(wdoge_wbnb, <amount2>);                                  // push again
IPancakePair(wdoge_wbnb).swap(0, 2_978_658_352_619_485_704_640, address(this), ""); // pull WBNB
wbnb.transfer(BUSDT_WBNB_Pair, 2908 ether);                             // repay flash

The WDOGE token's transfer/balance accounting was inconsistent with the pair's reserves (a token-level bug where transfers did not always reconcile with the pair's tracked reserves). By alternating pushes, skim (which sends the pair's excess balance to a recipient), and sync (which forces reserves to match balances), the attacker kept extracting more WBNB than the reserves should allow, then repaid the 2,900 WBNB flash (2,908 to cover) and kept the surplus.

Root cause#

A token-level accounting inconsistency in WDOGE: its transfer did not keep the Pancake pair's (reserve0, reserve1) consistent with actual balanceOf, so the pair's swap/skim/sync primitives — which Uniswap-V2 pairs trust to be faithful to true balances — could be gamed. The repeated transfer→swap→skim→sync chain exploited the gap between the pair's stored reserves and the real WDOGE balance to over-extract WBNB.

Preconditions#

• The WDOGE/WBNB pair with liquidity.
• WBNB flash source (WBNB/BUSD pair).

Diagrams#

Remediation#

1. Fix WDOGE's transfer so balanceOf is always consistent (standard ERC20, no fee-on-transfer surprises the pair cannot account for).
2. Use balance0Adjusted/balance1Adjusted checks (Uniswap V2's k invariant) so skim/sync abuse cannot extract value.
3. Don't list tokens whose transfer semantics break the constant-product assumption without a fee-aware pair.
4. Monitor for skim+sync abuse patterns.

How to reproduce#

_shared/run_poc.sh 2022-04-Wdoge_exp --mt testExploit -vvvvv

• RPC: BSC archive (block 17,248,705). foundry.toml uses a BSC archive endpoint.
• Result: [PASS] — Profit: WBNB balance of attacker shows the post-repay surplus.

Reference: WDOGE (BSC) token/pair accounting exploit, Apr 2022.

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 2022-04-Wdoge_exp (evm-hack-registry mirror).
• Upstream DeFiHackLabs PoC: Wdoge_exp.sol.

Alerts & third-party analyses

• DeFiHackLabs incident explorer: search "WDOGE (Wrapped Doge on BSC) Exploit".
• 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.