Four Attack Classes

Researchers derived four attack vectors from the rule violations :

• Free Shopping – The merchant delivers a service after off-chain verification succeeds, but the on-chain settlement later fails (or an invalid/zero-amount signature is accepted). The buyer gets the product without paying. Researchers directly validated two free-shopping paths and classified another 10 as high-risk.
• Asset Theft – An attacker gains control over facilitator-held assets, such as by tricking the facilitator into approving arbitrary token transfers.
• Service Denial – Payment replay, signature reuse, or malicious payloads cause the facilitator to crash or refuse legitimate transactions.
• Gas Abuse – Attackers force the facilitator to pay for expensive smart-contract deployments or repeated on-chain operations, turning settlement into an unbounded cost sink for the sponsor .

Most Severe Flaw: ERC-6492 Path

The most severe attack path involved ERC-6492, an Ethereum signature standard for smart-contract wallets that may not yet be deployed. Researchers found that malicious metadata could cause a facilitator to fund and submit an arbitrary token-approval transaction instead of the expected payment . The researchers did not move facilitator funds themselves but classified the flaw as a direct path to asset theft because an attacker could use that authority to approve transfers of assets controlled by the facilitator.

Merchant-Side Risk: Coinbase Reference Server Kits

All seven official Coinbase reference server kits examined by researchers lacked explicit mechanisms for reversing actions taken after a successful verification . In versions of Coinbase's Flask kit through 0.2.1, protected resources could be released after verification regardless of whether the subsequent settlement succeeded. This means a merchant using those kits could hand over an irreversible service — e.g., an API response or digital good — while the payment never finalizes on-chain.

Market Concentration and Network Fees

The study analyzed over 119 million x402 transactions on Base and Solana between October 1 and December 26, 2025 . Facilitators collectively spent approximately $202,000 on network fees, including roughly $5,800 on Base transactions that ultimately reverted or failed — demonstrating the economic asymmetry where a facilitator incurs blockchain costs even when a payment never completes . The on-chain measurement further quantified significant facilitator centralization, with Coinbase's Base facilitator dominating the ecosystem in transaction volume during the study period.

Remediation Status (as of February 2026)

The researchers responsibly disclosed all findings to the affected parties . Coinbase, PayAI, and Mogami acknowledged the issues and adopted mitigations, including patching 6 vulnerabilities in February 2026 . The paper notes that Coinbase made specific changes in response to the disclosure. The remediation status of other facilitators was not specified in available sources.

Researchers' Recommendations

• For facilitators: Enforce all eight security rules covering authorization correctness and execution safety; never accept settlement failure as a reason to deliver service; implement signature deduplication (nonce tracking); reject zero-amount payments; generate cryptographically signed receipts linking payment txHash → service endpoint → response hash for dispute resolution .
• For merchants: Never release irreversible services immediately after off-chain verification alone; require on-chain settlement confirmation before delivery; audit any reference server kit to ensure settlement failure triggers a 402 response, not service delivery .
• For the ecosystem: Adopt standardized security invariants, automated black-box testing (the paper introduces X402SCOPE for this purpose), and shared accountability layers to reduce the blast radius of any single facilitator flaw .

Four Attack Classes

Researchers derived four attack vectors from the rule violations :

• Free Shopping – The merchant delivers a service after off-chain verification succeeds, but the on-chain settlement later fails (or an invalid/zero-amount signature is accepted). The buyer gets the product without paying. Researchers directly validated two free-shopping paths and classified another 10 as high-risk.
• Asset Theft – An attacker gains control over facilitator-held assets, such as by tricking the facilitator into approving arbitrary token transfers.
• Service Denial – Payment replay, signature reuse, or malicious payloads cause the facilitator to crash or refuse legitimate transactions.
• Gas Abuse – Attackers force the facilitator to pay for expensive smart-contract deployments or repeated on-chain operations, turning settlement into an unbounded cost sink for the sponsor .

Most Severe Flaw: ERC-6492 Path

The most severe attack path involved ERC-6492, an Ethereum signature standard for smart-contract wallets that may not yet be deployed. Researchers found that malicious metadata could cause a facilitator to fund and submit an arbitrary token-approval transaction instead of the expected payment . The researchers did not move facilitator funds themselves but classified the flaw as a direct path to asset theft because an attacker could use that authority to approve transfers of assets controlled by the facilitator.

Merchant-Side Risk: Coinbase Reference Server Kits

All seven official Coinbase reference server kits examined by researchers lacked explicit mechanisms for reversing actions taken after a successful verification . In versions of Coinbase's Flask kit through 0.2.1, protected resources could be released after verification regardless of whether the subsequent settlement succeeded. This means a merchant using those kits could hand over an irreversible service — e.g., an API response or digital good — while the payment never finalizes on-chain.

Market Concentration and Network Fees

The study analyzed over 119 million x402 transactions on Base and Solana between October 1 and December 26, 2025 . Facilitators collectively spent approximately $202,000 on network fees, including roughly $5,800 on Base transactions that ultimately reverted or failed — demonstrating the economic asymmetry where a facilitator incurs blockchain costs even when a payment never completes . The on-chain measurement further quantified significant facilitator centralization, with Coinbase's Base facilitator dominating the ecosystem in transaction volume during the study period.

Remediation Status (as of February 2026)

The researchers responsibly disclosed all findings to the affected parties . Coinbase, PayAI, and Mogami acknowledged the issues and adopted mitigations, including patching 6 vulnerabilities in February 2026 . The paper notes that Coinbase made specific changes in response to the disclosure. The remediation status of other facilitators was not specified in available sources.

Researchers' Recommendations

• For facilitators: Enforce all eight security rules covering authorization correctness and execution safety; never accept settlement failure as a reason to deliver service; implement signature deduplication (nonce tracking); reject zero-amount payments; generate cryptographically signed receipts linking payment txHash → service endpoint → response hash for dispute resolution .
• For merchants: Never release irreversible services immediately after off-chain verification alone; require on-chain settlement confirmation before delivery; audit any reference server kit to ensure settlement failure triggers a 402 response, not service delivery .
• For the ecosystem: Adopt standardized security invariants, automated black-box testing (the paper introduces X402SCOPE for this purpose), and shared accountability layers to reduce the blast radius of any single facilitator flaw .