Worked example · 2021-07

ChainSwap bridge unauthorised-mint exploit — multi-chain (Ethereum, BSC) — 2021-07-02 and 2021-07-11

Loss: two separate events in July 2021: an earlier incident of about $800K (July 2), and a second exploit on July 10-11 in which the attacker's wallet held tokens valued at about $4.4M [rektchainswap]. A cumulative figure of about $8M is reported by some outlets [investingchainswap]; Rekt's own figures are $800K then $4.4M [rektchainswap]. The earlier "SlowMist / Peckshield / CertiK triangulation" claim was unsourced and is removed.
OAK Techniques observed: OAK-T10.002 (Message Verification Bypass — the bridge's Ethereum-side receive function accepted a call with a weak authentication check, so the attacker could mint bridged-token supply to addresses of their choosing [rektchainswap]). OAK-T9.004 (Access-Control Misconfiguration — the weak authorisation check on the mint path). OAK-T7.001 (Mixer-Routed Hop) — Rekt notes the attacker's wallet was funded from Tornado Cash and that about 456 ETH was cashed out via 1inch [rektchainswap]; routing of proceeds through Tornado is not established in the cited sources.
Attribution: pseudonymous — no named-individual attribution. Rekt records ChainSwap's claim to have received an email from the attacker and questions it [rektchainswap].
Corrected 2026-10-11: The mechanism was rewritten: sources describe a weak-authentication mint flaw on existing bridged tokens (20 tokens on BSC), not deployment of a malicious token contract; loss figures and the Tornado Cash role were narrowed to what sources state; unsourced remediation and comparison claims removed.
Key teaching point: Bridge mint authorisation must be bound to a verified message, not to attacker-supplied parameters. Per Rekt, the attacker passed a new address as the "signature" on each call, bypassing the weak auth check and minting tokens directly to chosen addresses [rektchainswap]. A second incident within days of the first indicates that the first response did not close the root cause class.

Summary

ChainSwap was a cross-chain bridge linking Ethereum, BSC, Polygon and Huobi Eco Chain [forklogchainswap]. In July 2021 it was exploited twice:

July 2, 2021: an earlier incident with losses of about $800K [rektchainswap]. Sources consulted did not describe its mechanism.
July 10-11, 2021: the attacker called the receive function of the per-token proxy Factory contracts on Ethereum, minting tokens to addresses of their choosing; the auth check was weak (a new address was supplied as the signature each time) and each call paid a 0.005 ETH fee. On BSC the exploit targeted 20 tokens; for Wilder World's WILD, 40 repeated transactions each minted 500,000 WILD (20M total), sold for about 650 WBNB on PancakeSwap [rektchainswap]. Affected projects included Wilder World, Antimatter, Optionroom and Umbrellabank [halbornchainswap]. Date is given as July 10 by Halborn and July 11 by Rekt's linked post-mortem.

Whether the second exploit used a flaw related to the first patch is not established by the sources consulted; the earlier claim that it did, and that ChainSwap commissioned a full audit and a token-registry gate, was unsourced and is removed. The earlier comparison with Poly Network and Thorchain sub-classes is OAK-internal framing rather than sourced fact.

Timeline (UTC)

When	Event	OAK ref
2021-07-02	First incident, about $800K `[rektchainswap]`	(mechanism not sourced)
2021-07-10/11	Second exploit: unauthorised mint via weak auth on `receive`; 20 BSC tokens targeted `[rektchainswap]` `[halbornchainswap]`	T10.002 + T9.004
2021-07-10/11	Attacker cashes out ~456 ETH via 1inch; wallet had been funded from Tornado Cash `[rektchainswap]`	T7.001 (funding side)

What defenders observed

Mint paths must verify message provenance. The exploited receive path accepted an attacker-supplied value in place of a real signature [rektchainswap].
Thin DEX pools amplify mint exploits. Minted WILD was sold into a WILD/WBNB pool, depleting it, and several affected tokens lost most of their value [rektchainswap] [halbornchainswap].

What this example tells contributors writing future Technique pages

Record whether an incident involved one or several exploit events. ChainSwap had two events in about nine days.
The mechanism here is an unauthorised mint on existing bridged tokens, not token deployment. Contributors should not reuse the earlier "cross-chain token-deployment authorisation" sub-pattern label for this case.

Public references

[rektchainswap] — Rekt, "ChainSwap - REKT" (2021-07-13): https://rekt.news/chainswap-rekt
[halbornchainswap] — Halborn, "Explained: The ChainSwap Hack (July 2021)": https://halborn.com/explained-the-chainswap-hack-july-2021
[forklogchainswap] — Forklog, "Hackers drain more than $4M from DeFi projects due to ChainSwap vulnerability": https://forklog.com/en/hackers-drain-more-than-4m-from-defi-projects-due-to-chainswap-vulnerability/
[investingchainswap] — Investing.com, "$8 Million Lost In Major ChainSwap Exploit": https://investing.com/analysis/8-million-lost-in-major-chainswap-exploit-200590819

Discussion

ChainSwap July 2021 is a bridge unauthorised-mint case: weak authentication on a mint path allowed minting of bridged tokens to arbitrary addresses. The earlier text of this file framed it as the canonical "cross-chain token-deployment authorisation" sub-pattern; the sources consulted do not support that framing. Mechanism details beyond the weak auth check (Rekt's account) are not confirmed by an operator post-mortem seen here. Technique mapping (T10.002 vs a minting-authority T9.004 case) may merit review.