Glossary · Interoperability

Relayer

Published 23 May 2025 · Updated 21 Oct 2026 · 12 min read

A relayer is an off-chain service that submits someone else's signed transaction or emitted message to a blockchain and pays gas. It carries instructions rather than deciding their validity. When the destination verifies correctly, the relayer can delay or censor delivery but cannot forge the message it carries.

  • Relayers deliver and verifiers decide, so sound destination checks block forgery, not delay.
  • The largest bridge losses came from verifiers or keys, not delivery, like Ronin's 5 of 9 keys in March 2022.
  • The term also covers gas relayers, intent relayers that front funds, and Ethereum's MEV-Boost relays.
The different types of relayersHow a cross-chain relayer worksWho verifies and who deliversGas relayers and sponsored transactionsIntent relayers that front their own fundsWhen bridge security failsMEV-Boost relays in block productionEngineering systems that depend on relayersRelayer FAQ

The different types of relayers

In October 2026, the term describes several jobs with different trust and payment models. Identify what the operator carries, whether it advances money, and who checks its work before grouping them together.

Cross-chain message relayers

These operators carry a cross-chain message and its evidence to another blockchain. They transport an instruction rather than decide its validity.

Gas relayers

These operators submit meta-transactions or sponsored transactions and pay gas for users. A gasless interface still needs someone to fund execution.

Intent relayers

A filler or solver pays recipients from its own funds, then seeks repayment. The relayer accepts capital and settlement risk.

MEV-Boost relays

These services connect Ethereum block builders and validators. Their role sits inside block production, not between source and destination blockchains.

In 2017, 0x relayers hosted an off-chain order book, while trades settled through contracts. In 2020, 0x moved to a liquidity aggregation API. Jito's Solana relayer is a transaction processing proxy, with its repository archived on 5 July 2026. Polkadot's relay chain is a blockchain, not a delivery process. Nostr relays serve a social protocol. Relay is a protocol whose actors are called solvers.

How a cross-chain relayer works

A cross-chain relayer watches one chain for messages, submits them with proof to another, and pays gas. IBC's ICS 18 states: "Relaying should be permissionless, all requisite verification should be performed on-chain." Safety must not depend on relayer behavior. One correct, live relayer is enough for liveness, the ability to keep delivering packets. A bridge between blockchains therefore needs two distinct answers: who carries the data and who accepts its proof.

Number 1

Emit

An app emits a message on the source chain. It describes the action requested from the app on the destination.

Number 2

Attest

A verifier signs the message, or an on-chain light client tracks source state. That evidence supports verification on the destination chain.

Number 3

Carry

A relayer fetches the message and its proof or signatures. It submits a destination transaction and pays the gas needed there.

Number 4

Verify

The destination contract checks the proof or required signatures. It accepts the message only if the protocol's verification rules are satisfied.

Number 5

Execute

The destination app acts on the accepted message. Depending on the protocol, the sender, the app or the relayer itself bears delivery costs.

Figure 1 · Four protocols, five behavioursTest delivery against verificationChoose a protocol and delivery behavior, then compromise its verifier to see which step accepts or stops the message.
Protocol
What happens
Block iconEmitEmittedThe core contract emits a message
Shield iconAttestAttested19 Guardians observe. 13 signatures make a valid VAA
Message packet iconCarryChangedThe Executor or any other relayer
Magnifier iconVerifyRejectedThe destination contract checks the 13 of 19 quorum
Contract iconExecuteNot runThe destination app acts on the VAA
Who verifiesDestination contracts require thirteen signatures from the nineteen Guardians.
Who may deliverAnyone may submit verified messages through the Executor.
What a forgery needsAn attacker needs thirteen Guardian signatures or a verification bypass.
Rejected at verificationChanged message contents fail destination verification. The destination app does not execute the altered request.

These simplified flows follow each protocol's documentation in October 2026, rather than every deployment configuration.

Who verifies and who delivers

Bridges tracked by DefiLlama moved $29.9 billion in the 30 days to 21 October 2026. Permissionless delivery does not imply permissionless verification. IBC Eureka launched in April 2025 with permissioned relayers, and its docs still say permissioning will be removed later.

ProtocolWho verifiesWho may deliverHow delivery is paid
IBCWho verifiesThe destination's light client checks source chain state.Who may deliverAnyone can deliver, unless optional client allowlists restrict access.How delivery is paidRelayers pay gas. ibc-go removed ICS-29 fees in 2025, and Eureka users pay a relay fee.
WormholeWho verifiesDestination contracts check the Guardian signature quorum.Who may deliverAnyone can deliver verified messages through the permissionless Executor.How delivery is paidThe client pays a relay provider's quoted fee through the Executor. The provider pays destination gas.
LayerZeroWho verifiesThe app's selected DVNs verify each message.Who may deliverExecution is permissionless once the message passes verification.How delivery is paidThe sender pays the Executor a fee in the source chain's native token. The Executor pays destination gas.
HyperlaneWho verifiesDestination Interchain Security Modules verify incoming messages.Who may deliverAnyone can fetch signatures and call Mailbox.process().How delivery is paidThe sender chooses which relayer to pay.
AxelarWho verifiesValidators vote. A threshold of key shares authorizes messages.Who may deliverAnyone can execute the approved payload at any time.How delivery is paidSenders prepay the whole route's gas through the Gas Receiver. The relayer service is free.
Chainlink CCIPWho verifiesCross-Chain Verifiers attest, with a default committee of 16 node operators on every lane.Who may deliverAny party can deliver messages with valid attestations.How delivery is paidThe sender pays fees on the source chain that cover the verifiers and the default executor.
Circle CCTPWho verifiesCircle's Iris attestation service signs each burn message.Who may deliverAnyone can call receiveMessage unless the sender restricts callers.How delivery is paidCircle's optional Forwarding Service charges for destination minting.
AcrossWho verifiesUMA's optimistic oracle backs challenges to repayment bundles.Who may deliverRelayers supply destination funds to fill the user's intent.How delivery is paidRepayment returns the input amount minus the LP fee.

Tooling changes do not remove these trust boundaries. The Go relayer repository was archived on 10 April 2026. Wormhole scheduled Standard Relayer automatic delivery to stop on 1 April 2026. The 14 March 2025 ibc-go release notes record removal of ICS-29 fee middleware. CCTP V1 contracts are scheduled to pause on 1 December 2026. Wormhole has carried about 1.2 billion messages through 21 October 2026.

Gas relayers and sponsored transactions

Under EIP-2771, users sign meta-transactions, a Gas Relay pays transaction gas, and a trusted forwarder checks signatures and nonces. A malicious forwarder can spoof senders. OpenGSN distributes this work among transaction relayers. OpenGSN paymasters fund approved gasless requests through RelayHub. OpenZeppelin closed Defender sign-ups on 30 June 2025 and scheduled retirement for 1 July 2026, pointing users to OpenZeppelin Relayer. In October 2026, Gelato is deprecating its Relay SDK. Its replacement supports sponsorship, not ERC-2771 or SyncFee.

ERC-4337 users sign a UserOperation, which a bundler submits while paying gas. The EntryPoint repays it from the account or a paymaster. EIP-7702, live since Pectra on 7 May 2025, lets ordinary accounts set code and receive sponsorship. By 21 October 2026, BundleBear counted about 1.33 billion UserOperations in total, from about 69.9 million accounts. It also counts about 67.2 million live EIP-7702 smart accounts. ZeroDev found in August 2026 that sponsored UserOperations were roughly 90 to 99% of monthly volume over the prior year.

Figure 2 · EIP-2771, ERC-4337, EIP-7702Follow a sponsored transactionChoose a sponsorship path, then make the relayer change signed data to see where validation stops the altered request.
Path
Relayer
Key iconUser signsSignedThe user signs a UserOperation for their account.
Gas pump iconRelayer submitsSubmittedA bundler packs UserOperations into a transaction and pays gas.
Shield iconCheckPassedThe EntryPoint validates the UserOperation before execution proceeds.
Coins iconRepaymentRepaidThe EntryPoint repays the bundler from the account or paymaster.
Main riskThe bundler pays upfront and depends on EntryPoint reimbursement.
BundleBear, total to 21 October 20261.33 billionUserOperations from about 69.9 million accounts

With sponsorship approved, the call runs as authorized. The sponsor pays gas instead of the user.

Simplified flows from the EIP texts. Account counts come from BundleBear's 21 October 2026 data.

Intent relayers that front their own funds

Across fills an intent by having a relayer pay the recipient from its own destination funds. In October 2026, Across reports fills of about two seconds. A dataworker proposes repayment bundles on Ethereum at intervals of at least about 1.5 hours. UMA's optimistic oracle backs the challenge period. One honest actor can dispute an invalid bundle before repayment proceeds.

Figure 3 · Across and Circle CCTPCompare speed and funding costsSet an amount and annual capital cost to compare the funding wait with CCTP Fast Transfer's fee.
$100,000
10% a year
CCTP source chain
Coins iconAcross intent fillTime until the recipient has fundsAbout 2 secondsIllustrative capital cost to the relayer$1.71Illustrative, at 10% a year for 1.5 hoursThe relayer fronts its own funds, then waits about 1.5 hours or more for repayment.
Flame iconCCTP Fast TransferTime until the recipient has funds8 to 20 secondsCircle's fee$10.001 basis point of the amountCircle's Fast Transfer allowance carries the wait. No relayer fronts funds.
Clock iconCCTP Standard TransferTime until the recipient has funds15 to 19 minutesCircle's fee$0No feeFree. The recipient waits for the standard attestation.

The relayer pays first. After settlement it receives the user's input amount minus the LP fee, on a chain it picks.

Circle's allowance carries the wait, not a relayer. In October 2026, Fast Transfer takes about 8 to 20 seconds and costs up to 13 basis points.

Gas spending, tied-up funds, reorganizations, software bugs, and exposed hot keys remain risks while the relayer waits.

Capital cost is illustrative, not Across's fee formula. Timing follows Across and Circle documentation in October 2026.

ERC-7683 remains a draft in October 2026. Proposed by Uniswap Labs and Across, it gives solvers, also called fillers, a common way to read and evaluate orders from intent protocols. This model shifts funding risk onto the operator. Across's relayer fee covers gas, capital opportunity cost, and risk. CCTP Fast Transfer instead pauses when Circle's allowance runs out.

When bridge security fails

A bridge hack does not necessarily mean a dishonest courier. In these incidents, the failed boundary was verification or key control, not delivery. A multisig wallet can still fail when enough signing keys are compromised. Contract bugs and verifier infrastructure can also make false messages appear authorized.

  1. 2 February 2022

    Wormhole signature verification bypass

    A Solana verification bug let an attacker bypass signature checks and mint about 120,000 wETH. The assets were worth over $320 million.

  2. 23 March 2022

    Ronin validator keys compromised

    The attacker controlled five of nine validator keys and drained 173,600 ETH plus 25.5 million USDC. An old allowlist gave access to the fifth key.

  3. 23 June 2022

    Harmony Horizon signing keys

    Compromised private keys enabled the theft of about $100 million. Analysts described the bridge's signing arrangement as essentially a two of five multisig.

  4. 1 August 2022

    Nomad trusted root error

    An upgrade marked a zero hash as a trusted root, accepting messages without proof. More than 300 addresses copied the attack, draining $190 million.

  5. May to July 2023

    Multichain centralized key control

    Transfers stalled in May 2023. Key-holding MPC servers were controlled through the CEO's personal cloud account. Outflows reached $126 million on 6 July 2023 and $103 million on 10 July 2023.

  6. 31 December 2023

    Orbit Bridge signer compromise

    A researcher reported the attacker controlled seven of ten multisig signers, taking $81.5 million. Compromised signer keys were the suspected cause, not a relayer.

  7. 21 February 2026

    IoTeX ioTube owner compromise

    The owner account of the Validator contract on Ethereum was compromised. About $4.4 million was drained through a failure of privileged account security.

  8. 18 April 2026

    Kelp DAO verifier poisoning

    Kelp DAO's LayerZero app relied on one required DVN. Attackers poisoned its RPC nodes, so the DVN signed a message that never existed. The Executor delivered it, releasing 116,500 rsETH, about $292 million.

MEV-Boost relays in block production

Ethereum builders and validators rely on trusted intermediaries in the maximal extractable value market. Builders trust these services for fair routing, while validators trust them for block validity. Running one is permissionless. Relayscan lists seven active relays for 21 October 2026. Ultra Sound and Titan account for about 66% of payloads in that snapshot's preceding 24 hours. Mevwatch records 17.7% of deliveries through OFAC-filtering relays for 19 October 2026, below the 94.8% peak of 26 October 2022.

EIP-7732 introduces enshrined proposer-builder separation, which removes the need for trusted middleware when block construction is delegated to a builder. In October 2026, it is in Last Call and part of Glamsterdam. Glamsterdam has been live on Sepolia since 6 October 2026, with Hoodi scheduled for 26 October 2026. The mainnet date remains undecided. A researcher writing on ethresear.ch in August 2026 expects relays to continue in some form by registering as in-protocol builders.

Engineering systems that depend on relayers

A product that depends on relayers needs more than a transaction submitter. It needs an explicit trust boundary and a plan for failed delivery. The PixelPlex blockchain development service covers cross-chain interoperability and bridging. The engineering scope below separates message validity from operating the service that transports it.

PixelPlex case icon

PixelPlex built the Echo layer-2, whose Bitcoin and Ethereum sidechains rely on a validator committee. If the committee fails to withdraw funds in time, users can unblock their ECHO themselves. In July 2026, we joined the first live non-custodial cross-chain atomic swaps on Canton MainNet alongside Arqitech, MPCH and sFOX. Every party kept its own private keys.

  1. Trust and verifiers

    Define who can approve a message, which threshold applies, and whether any one compromised signer or administrator can authorize it.

  2. Verification and replay protection

    Keep proof checks on the destination. For signed requests, specify signature and nonce validation so submission cannot change authorization.

  3. Infrastructure and monitoring

    Track delivery delays and light client updates. IBC clients can expire after prolonged inactivity, stopping their channels until governance replaces them.

  4. Gas and fee economics

    Our decentralized exchange development includes gasless transactions via meta-transactions. Separate user sponsorship from the operator's gas budget and reimbursement exposure.

  5. Audits and incident response

    Our audit service includes a cross-chain security audit. Review verifier checks and privileged keys, then define incident brakes such as pauses and limits.

Where does trust sit in your cross-chain flow?

We can discuss verifier choices, delivery operations, and sponsorship needs against the scope of your product.

Discuss your project

Relayer FAQ

What does a relayer do in a bridge?

A relayer submits a cross-chain message and its proof for destination checks. It can delay or censor delivery, but cannot forge a message when the destination verifies it. In IBC, proof of non-receipt after a packet's deadline gives the sender a timeout.

Can a relayer steal my bridge funds?

Not by delivery alone. Losses can come from the verifier side: compromised signer keys, admin keys, and verifier bugs. After the April 2026 Kelp DAO attack, LayerZero said its own DVN will no longer act as the sole verifier on any channel. Its May 2026 report says Mandiant, CrowdStrike and independent researchers attribute the attack to North Korea's TraderTraitor group.

Who pays relayers for submitting blockchain transactions?

Senders, apps, or sponsors can fund delivery, while operators pay gas upfront. Across separates the relayer fee from the LP fee. Its LP fee is zero for origin-chain repayment, so the repayment location affects costs.

How do relayers differ from validators and oracles?

A relayer carries evidence, Axelar validators authorize messages, and UMA's optimistic oracle handles disputes over Across repayment bundles. Those roles answer different trust questions. Submitting a transaction is separate from approving the action that it requests. An oracle reports data about the world, while a relayer carries a message that already exists.

What is a meta-transaction relayer in EIP-2771?

It pays gas to submit a user's signed request. Recipients read the original sender from the final 20 bytes of call data. OpenZeppelin's 7 December 2023 disclosure warns that combining Multicall and ERC-2771 enables address spoofing.

Is an ERC-4337 bundler also a relayer?

It handles submission and gas payment, but ERC-4337 calls it a bundler, not a relayer. The standard needs no protocol changes. EntryPoint v0.8 added native EIP-7702 authorization support in March 2025, connecting the two designs.

What is an MEV-Boost relay in Ethereum?

It connects block builders and validators, rather than transporting cross-chain messages. Flashbots describes a trusted data-availability layer and communication interface. The separate mev-boost software runs on validators, giving them access to a competitive block-building market.

Related terms

All definitions
Sources (49)

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