As stablecoins become a larger part of onchain activity, payments are becoming more than a secondary use case for onchain systems. Stablecoins now have a combined market capitalization above $300B, while payment companies are increasingly exploring stablecoin settlement, payouts, cards, and merchant-facing infrastructure. The more interesting shift, though, is not only that stablecoins are being used more. It is that infrastructure is now being shaped around the payment flow itself.

A stablecoin payment is simple at the token level: value moves from one address to another. In practice, though, payment products usually need more context around that transfer. The sender may not want to hold a separate gas asset. The recipient may need a memo, invoice reference, or account identifier to reconcile the payment. An application may want to sponsor fees, batch transfers, schedule payments, enforce receive rules, or route between different stablecoins. Institutions may also care about policy controls, privacy boundaries, and deterministic settlement.

This is where stablecoin-payment-first chains become an interesting design category. Across newer ecosystems such as Tempo, Arc, Plasma, and others, a common design direction is starting to appear: payment-specific behavior is moving closer to the infrastructure layer. As of July 2026, Tempo and Plasma are live on mainnet, while Arc remains on public testnet. Across live and planned designs, stablecoin-denominated fees, sponsored transactions, payment metadata, transfer policies, batching, scheduled execution, and custom gas-token mechanisms can become part of the payment path rather than something every application rebuilds around the transfer. These features shape what applications can assume about how a payment is created, executed, observed, and settled.

For builders, integrators, and security researchers, this shift changes the questions worth asking. When gas can be paid in stablecoins, the fee asset becomes part of the payment model. When fees can be sponsored, the sponsor becomes part of the execution path. When transfers carry metadata, reconciliation data may need privacy and integrity guarantees. And when policies or other payment-specific infrastructure sit close to the payment flow, applications may start depending on them without explicitly naming the trust boundary.

This article focuses on that shift: how payment-specific primitives change the assumptions builders, integrators, and security researchers need to reason about. The threat model is written from the perspective of teams building payment flows on this infrastructure, including smart contracts, wallets, transaction services, indexers, and payment backends.

From Token Transfers to Payment Guarantees

At the lowest level, a stablecoin payment may still look familiar: a transaction is included, balances update, and events are emitted. But for a payment product, “the transfer executed” and “the payment succeeded” are not always the same statement. A merchant may only consider the payment complete if the right asset arrived, the amount matches the invoice, the memo can be reconciled, the transfer passed the relevant policy checks, and the transaction reached whatever finality threshold the application relies on.

This is the first place where payment-first infrastructure changes the threat model. The unit of analysis is no longer just the token contract or the transfer function. It is the full payment path. Tempo’s payment model, for example, combines TIP-20 transfers, memos, and configurable transfer policies with consensus-level payment lanes, while Tempo Transactions support atomic call batching, fee sponsorship, and scheduled execution. Arc uses USDC as its native gas asset and provides transaction extensions for memos and batched calls that preserve the original EOA sender. Plasma is building custom gas-token support through a protocol-maintained paymaster so applications can abstract XPL from the user experience and allow users to pay fees in tokens they already hold.

For builders, this means the payment guarantee has to be defined explicitly. Is success measured by transaction inclusion, token receipt, invoice reconciliation, policy approval, final settlement, or some combination of these? If a payment is bundled with other actions, should failure of one action revert the whole flow, or can the application tolerate partial completion? If the sponsored fee path is unavailable, did the payment fail, or only that specific execution route? If a memo is missing or cannot be decoded by the recipient’s backend, should the funds be accepted but left unreconciled, or should the application treat the payment as incomplete?

Defining payment success also clarifies what the threat model is trying to protect. Beyond the stablecoin balance itself, the relevant assets may include the authority to move or sponsor funds, the integrity of payment references and policy outcomes, the availability of execution and reconciliation, and the confidentiality of sensitive payment data. An onchain transfer can execute successfully and still represent a failed payment flow if one of these properties is lost.

Actors and Trust Boundaries in the Payment Flow

With those assets defined, the next step is to identify who can influence them. In a simple transfer, the obvious actors are the sender, the recipient, and the token contract. In a payment-first flow, more components can matter. A wallet or payment app may prepare the transaction, a sponsor may cover fees, a policy layer may decide whether a transfer is allowed, a token issuer or admin may control minting, pausing, or restrictions, and an indexer or backend may be responsible for matching the payment to an invoice or account. Validators may determine ordering and finality, while infrastructure operators can affect transaction submission, indexing, and reconciliation. Some of these components are smart contracts, while others are wallets, backends, operators, or infrastructure services; for a payment product, the security boundary often crosses that line.

The important point is that not every security-relevant actor needs direct control over user funds. Some actors affect whether a payment can be submitted, included, interpreted, accepted, or acted on. A sponsor that is unavailable can break the gasless path. A policy update can change whether a transfer is accepted. A memo parser can decide whether received funds are matched to the correct invoice.

This is already visible in the chains we are discussing: the payment path can include fee logic, memo handling, policy checks, batching, sponsorship, or gas abstraction alongside the application’s own business logic. For builders and auditors, that means the security review should cover the full payment flow, not only the contract that moves the tokens.

A practical way to do this is to draw the payment path and mark every actor or component that can block, delay, reinterpret, reroute, or finalize the payment. Then mark each interface where value, data, or authority crosses into a different trust domain. Those interfaces are the trust boundaries. Once they are visible, the security review can separate properties enforced onchain from assumptions handled by wallets, backends, sponsors, operators, or policy systems.

Five Security Assumptions Behind the Payment Flow

With the payment flow and its trust boundaries mapped, the next step is to identify the security assumptions the system depends on. These assumptions connect protocol behavior with the guarantees exposed to users, merchants, and payment backends. Making them explicit turns the threat model into a set of concrete properties that builders, integrators, and auditors can verify.

Assumption 1: A Funded Payment Needs a Valid Fee Path

Having enough stablecoins to cover the transfer amount does not necessarily mean the transaction can be submitted. Arc uses USDC as its native gas asset. Tempo allows USD-denominated TIP-20 tokens to be used for fees. Before execution, it verifies that the selected token is eligible, that the fee payer has sufficient balance, and that the Fee AMM has sufficient liquidity when conversion is required. Plasma currently uses the standard EVM gas model and documents protocol-maintained custom gas-token support as a roadmap feature. The integration therefore assumes that an accepted and sufficiently funded fee path exists when the payment is submitted. A fee-path failure should remain distinguishable from insufficient payment funds, policy rejection, or delivery failure.

Assumption 2: Sponsorship Must Be Bound to the Approved Transaction

Sponsored execution separates the sender’s authorization of the transaction calls from the fee payer’s authorization to cover their execution costs. Tempo makes this explicit with separate sender and fee-payer signatures. The fee payer’s signature commits to the calls, sender, fee token, gas parameters, nonce, and execution window, while a separate signing domain prevents that signature from being reused as the sender’s authorization. For an integration, the security assumption is that sponsorship remains scoped to the exact transaction and fee limits the sponsor approved. The sponsor’s balance and signature must remain valid when the transaction is submitted, while the backend should record whether the fully signed transaction was broadcast and whether it executed before attempting a retry.

Assumption 3: Payment Context Must Remain Bound to the Value Transfer

Reconciliation data is useful only when it can be tied to the movement of funds it describes. Tempo’s memo-bearing transfer event includes the sender, recipient, amount, and memo. Arc’s memo extension records the original EOA sender, target contract, a hash of the forwarded call, an application-defined identifier, and the memo data. These fields provide the information needed to reconcile the memo with the corresponding call, while the backend still needs to verify the resulting token movement. A familiar invoice identifier is not proof of payment by itself. Reconciliation processing should also be idempotent so that re-indexing or duplicated references cannot credit the same obligation twice. On public execution paths, memo data is emitted onchain, so raw customer or business information should be treated as public.

Assumption 4: The Integration Must Account for Live Policy State

A valid signature and sufficient balance may not be enough to credit the intended recipient. On Tempo, a token-level policy failure reverts the transfer, while a receiver-level policy block can leave the call successful but redirect the funds to a guard contract that records a recoverable receipt. Arc likewise documents native USDC transactions that can be rejected or reverted when blocklist restrictions apply, including when blocklist state changes between submission and execution. An integration therefore needs to understand the policy mechanism applied to the asset and recipient. Transaction status alone may be insufficient: the backend may need to verify recipient balances, transfer events, or blocked-payment receipts. Because policy checks apply at execution time, scheduled, queued, and retried payments should be evaluated against the current policy state rather than only the state that existed when they were created.

Assumption 5: Workflow Completion Must Be Defined Separately From Finality

Finality establishes that the recorded onchain outcome will not be reorganized; it does not decide whether that outcome satisfies the application’s payment workflow. Arc provides deterministic finality, but its batching extension can be configured either to revert the whole batch when a subcall fails or to return that failure for the application to handle. Tempo’s native batches execute atomically, while scheduled transactions can execute only within their defined time window. An Arc transaction can therefore be final even when an allowed subcall has failed, while a scheduled Tempo transaction can expire without executing. The application must define which actions are mandatory, which failures are acceptable, and which verified onchain result authorizes an offchain effect such as releasing goods, crediting an account, or marking an invoice as paid.

Applying the Threat Model: A Checklist for Builders and Auditors

A threat model is useful only if it can be translated into checks against the implementation and the systems around it. For stablecoin-native payment applications, those checks should cover contracts, wallets, transaction services, indexers, and operational backends.

Fees and Sponsored Execution

• Document every fee path enabled by the integration, including the accepted fee assets, required balances, conversion or liquidity dependencies, and any sponsor or relay dependency.
• Keep payment funding separate from execution funding in both the UI and the error model. An unsupported fee token or unavailable sponsor should not appear as an insufficient payment balance.
• For sponsored transactions, verify exactly which calls, sender, fee token, gas limits, maximum fee fields, and execution window the sponsor authorizes.
• Record whether the fully signed transaction was broadcast and check its onchain status before retrying. A timeout from a sponsor or relay does not prove that the transaction was never submitted.

Delivery, Policies, and Reconciliation

• Before crediting an invoice or account, verify the successful onchain result, the expected token, recipient, and amount, as well as the payer when the flow requires a specific sender. A recognizable memo identifier is supporting context, not proof of payment.
• Make payment crediting idempotent so that event reprocessing, duplicated references, or backend retries cannot credit the same obligation twice.
• Treat memo data emitted on public execution paths as public unless the system provides a documented confidentiality guarantee.
• Identify every token or policy mechanism that can pause, reject, block, or redirect a transfer. Where a protocol can complete the call without crediting the intended recipient, do not rely on transaction success alone.
• For scheduled, queued, or retried payments, do not assume that an earlier policy check remains valid. Delivery should be interpreted according to the policy state enforced when the transaction actually executes.

Completion and Recovery

• For every batch or multi-step payment, document the execution semantics provided by the selected primitive. Identify which operations must succeed, which failures may be tolerated, and whether failure of one operation reverts the entire flow.
• Define the verified onchain result required before triggering an offchain action such as releasing goods, crediting an account, or marking an invoice as paid. Depending on the flow, this may include transaction finality, successful mandatory subcalls, the expected transfer events, and confirmation that funds reached the intended destination.
• Treat transaction finality, batch success, and payment completion as separate states.
• Test the failure modes used by the integration, including sponsor rejection or unavailability, scheduled-transaction expiry, receive-policy redirection, and missed or replayed indexer events. Persist transaction identifiers and processing state, and make retries and recovery idempotent so that uncertainty cannot create duplicate credits or repeated offchain actions.

Making Payment Guarantees Explicit

Stablecoin-native chains are not simply places where stablecoins can move. They expose payment-oriented primitives that bring fee handling, sponsorship, reconciliation, policy enforcement, batching, and settlement closer to the transaction flow. For builders and integrators, the important question is not only whether those primitives work as designed, but what guarantees the application derives from them.

A useful threat model makes those guarantees precise. It identifies the assets that must remain protected, maps the actors and trust boundaries around the payment, records the assumptions the integration relies on, and defines how each failure should be handled. The resulting payment flow should answer concrete questions: when can a transaction be submitted, when have funds actually reached the intended recipient, how is the payment matched to its offchain context, and which verified outcome allows the wider system to act?

The goal is not to eliminate every dependency from the payment path. It is to ensure that each dependency is visible, its failure semantics are understood, and the application never promises a stronger payment guarantee than the underlying system can provide.

If you are building a payment application or integration on Tempo, Arc, Plasma, or another stablecoin-native chain, Certora can help review and secure the payment flow. This includes smart contract audits and formal verification of the onchain components, as well as threat modeling the assumptions connecting transaction logic, wallets, sponsors, indexers, policy systems, and payment backends. Reach out to discuss an architecture review, audit engagement, or longer-term security support.