BigONE Supply Chain Backend

On July 16, 2025, the cryptocurrency exchange BigONE lost approximately $27 million from its hot wallet — but in an unusual technical pattern, no private keys were exposed. Instead, attackers compromised the exchange's backend infrastructure and rewrote the risk-control logic so that the exchange's own systems automatically approved any withdrawal request the attackers issued.

What happened

Traditional exchange compromises follow one of two patterns:

1. Private-key theft — attacker obtains signing authority over wallets and drains them.
2. API/credential theft — attacker uses authorised programmatic interfaces with full or partial signing rights.

BigONE was neither. The attackers gained access to the exchange's backend infrastructure (likely through a supply-chain compromise of a dependency, build system, or CI/CD pipeline). From inside the backend, they modified the code that governed BigONE's account-validation and risk-control logic — replacing the legitimate withdrawal-approval checks with code that automatically approved any withdrawal request matching attacker-controlled criteria.

The exchange's signing systems then processed the malicious withdrawals through the normal signing path, using the legitimate private keys — because, from the signing system's perspective, the withdrawals had been duly authorised by the (compromised) risk-control logic.

Total drained: ~$27M across multiple chains, before BigONE detected the anomaly and shut down withdrawals.

Aftermath

• BigONE covered all customer losses from its insurance reserve fund.
• Bounty program announced: up to $8 million in rewards for help identifying the attackers or recovering funds.
• The exchange paused withdrawals while restoring backend integrity and re-auditing infrastructure.
• No public attribution to a specific threat actor; the supply-chain pattern matches several recent operations targeting exchange infrastructure providers.

Why it matters

BigONE's incident is one of the cleanest cases for why backend code is part of the custody trust boundary. An exchange's signing keys can be in cold storage; its HSMs can be perfectly configured; its API keys can have strict velocity limits — and all of those protections can be bypassed by rewriting the code that decides which signing actions to authorise.

The structural lesson, increasingly central to exchange security in 2025:

• Code-integrity controls (signed deployments, cryptographic verification of running binaries, immutable infrastructure) are as important as key-management controls.
• Privileged backend code changes must be subject to the same approval discipline as wallet operations — multi-party review, off-band verification, audit trails.
• Supply-chain risk extends beyond the explicit vendor relationships to every dependency, build tool, and developer environment touching production code.

BigONE's response — insurance fund + bounty + customer-whole policy — is what the industry has settled on as the minimum credible response to a major exchange compromise. The asset-class-level lesson: trust in an exchange is, in 2025, a function of how it responds to incidents like this, more than whether it has them.