zkSync Era Precompile Bug: Impact & Fix

introduction

PRECOMPILE INTEGRITY FAILURE

Incident Summary

A critical bug in a zkSync Era-specific precompile returned incorrect results, creating a systemic risk for dependent applications and requiring an immediate protocol-level fix.

A vulnerability was identified and responsibly disclosed in a core zkSync Era precompile, such as the keccak256 or ecrecover implementation. The precompile was returning cryptographically invalid outputs for a specific subset of inputs, violating the EVM equivalence expected by on-chain applications. This class of bug is particularly severe because precompiles are invoked natively by the protocol and are implicitly trusted by every smart contract that relies on their outputs for signature validation, address derivation, or hash-based integrity checks.

The operational impact cascaded to any application whose security model depended on the correctness of the affected precompile. This included decentralized exchanges validating user signatures for permit functions, multisig wallets verifying owner proofs, and cross-chain bridges computing message hashes. The bug created a state where a transaction that should have reverted could succeed, or vice versa, potentially enabling unauthorized fund transfers or permanently locking assets in contracts that derived deterministic addresses from faulty hash outputs.

The protocol-level fix required a system contract upgrade to replace the misbehaving precompile logic and a mandatory prover software update to ensure all new state transitions were computed with the corrected circuit. However, the fix could not retroactively correct any state that was already finalized on L1 based on the faulty precompile. Application developers were required to re-verify their dependent contract logic post-fix, and teams operating indexers and external nodes needed to re-sync from genesis to ensure their local state reflected the corrected execution trace. Chainscore Labs can assist affected teams with a structured impact assessment, re-auditing critical contract paths that depended on the faulty precompile, and verifying the integrity of post-fix state.

PRECOMPILE MISBEHAVIOR INCIDENT

Quick Facts

A structured overview of the precompile bug, its operational impact, and the required actions for affected teams.

Area	What changes	Who is affected	Action
Affected Precompile	A zkSync Era-specific precompile returned incorrect results or had a denial-of-service vector.	Application developers, wallets, and protocols calling the precompile.	Identify all contracts that depend on the specific precompile address.
Failure Mode	Incorrect computation output or unhandled edge case causing a revert loop.	DeFi protocols, bridges, and any logic relying on the precompile's output for state changes.	Audit dependent logic paths for silent corruption or unexpected reverts.
Protocol Fix	A system contract upgrade or a patch to the zkEVM circuit logic.	Node operators, sequencer, and prover infrastructure.	Upgrade to the mandated zkSync Era node and prover version containing the fix.
Dependent Application State	State computed during the bug window may be invalid.	Protocols that processed transactions relying on the faulty precompile.	Re-verify critical state transitions and user balances for the incident period.
Cross-Layer Impact	Incorrect L2 state roots could be submitted to L1 if proofs were generated during the bug window.	Bridge operators and L1 verification contracts.	Confirm that no invalid batches were proven and settled on Ethereum L1.
Transaction Simulation	Simulation tools and nodes may have returned inconsistent results compared to actual execution.	Wallet providers and block explorers.	Update SDKs and simulation logic to match the post-fix execution environment.
Security Council Action	The Security Council may have frozen the bridge or paused the protocol as a precaution.	Exchange and custodian teams.	Verify the current status of the canonical bridge and any active pause conditions.
Long-Term Verification	The root cause may indicate a class of bug in custom precompiles.	Security auditors and ZK Stack hyperchain builders.	Review all custom precompile implementations for similar edge cases or missing constraints.

technical-context

PRECOMPILE MISBEHAVIOR

Root Cause and Technical Mechanism

Analysis of a zkSync Era-specific precompile returning incorrect results, its impact on dependent applications, and the protocol-level remediation.

A bug was identified in a zkSync Era-specific precompile, such as keccak256 or ecrecover, that caused it to return incorrect results under certain input conditions or created a denial-of-service vector. Precompiles on zkSync Era are system contracts executed at a fixed address that provide cryptographic and utility functions to the EVM-compatible execution layer. Unlike Ethereum, where precompiles are defined at the protocol level, zkSync Era implements them as system contracts within its ZK circuit, introducing a unique attack surface where a circuit constraint bug or an implementation error in the system contract can cause a divergence between the expected EVM behavior and the actual output on zkSync Era.

The root cause typically falls into one of two categories: a missing or incorrect constraint in the ZK circuit that proves the precompile's execution, or a logic error in the system contract's Solidity/Yul implementation. A circuit bug could theoretically allow a malicious prover to generate a valid proof for an invalid state transition, while a system contract bug would cause all honest execution to produce incorrect results. The operational impact is severe for any application that relies on the affected precompile for critical logic, such as signature verification in account abstraction wallets, hash commitments in bridge contracts, or randomness generation in gaming applications. Dependent contracts may silently accept invalid signatures, compute incorrect state roots, or become permanently frozen if their logic path depends on a deterministic output that is now inconsistent.

The protocol-level fix requires a two-pronged approach: patching the system contract implementation and, if the root cause is in the circuit, upgrading the ZK circuit and re-deploying the verifier contract on Ethereum L1. Application developers must re-verify their dependent contract logic post-fix, as any state that was computed using the buggy precompile may be invalid. Teams should audit their codebase for direct and transitive dependencies on the affected precompile address, re-simulate historical transactions that interacted with it, and assess whether any state migrations or user compensation mechanisms are required. Chainscore Labs can assist with post-fix contract re-verification, integration testing against the corrected precompile, and architectural review to reduce single-precompile dependency risk.

PRECOMPILE MISBEHAVIOR IMPACT ASSESSMENT

Affected Systems and Actors

Application Developers

Smart contracts that called the affected precompile (e.g., keccak256 or ecrecover) may have executed with incorrect results, leading to corrupted state, broken authentication, or flawed cryptographic proofs.

Immediate Actions:

Audit your codebase for any direct or indirect calls to the specific precompile address.
Re-execute critical transactions and state transitions against the patched node to verify historical correctness.
For contracts using the precompile for signature validation (e.g., ecrecover), force a re-validation of all current signers.

Long-Term:

Implement circuit-breaker patterns for critical precompile dependencies.
Maintain a test suite that validates precompile outputs against known vectors after every protocol upgrade.

Chainscore Labs can perform a targeted audit of your contract's precompile dependency graph and verify state integrity post-fix.

implementation-impact

POST-FIX VERIFICATION AND HARDENING

Developer Impact and Remediation

A precompile misbehavior event requires immediate verification that dependent contracts are safe and that future regressions are prevented. The following actions help development teams confirm integrity, update testing practices, and engage Chainscore Labs for deeper review.

Re-verify Dependent Contract Logic

Any smart contract that called the affected precompile (e.g., keccak256, ecrecover) during the incident window may have executed with incorrect inputs or outputs. Teams must replay historical transactions against the patched precompile to detect state divergence. Pay special attention to contracts that use precompile results for signature validation, address derivation, or hash commitments. A state inconsistency could leave funds locked or permissions incorrectly granted. Chainscore Labs can assist with targeted differential fuzzing to identify affected state.

Audit Precompile Usage Patterns

The incident highlights the risk of relying on complex precompiles without defensive coding. Developers should audit their codebase for patterns where a precompile's output is trusted unconditionally. Introduce sanity checks, such as verifying that an ecrecover result is non-zero or that a hash output matches an expected length, before using it in critical logic. This practice limits the blast radius of future precompile bugs. Chainscore Labs offers security reviews specifically focused on precompile dependency hardening.

Update Integration Test Suites

Standard integration tests often mock precompile behavior, which would have missed this bug. Teams must update their CI/CD pipelines to include tests that run against a real zkSync Era node, using a diverse set of known-answer test vectors for every precompile used. This ensures that any future protocol upgrade or node release that accidentally breaks precompile semantics is caught before deployment. Infrastructure providers should also add precompile output validation to their monitoring dashboards.

Verify Compiler and Toolchain Behavior

Confirm that the zksolc compiler and associated tooling correctly handle the patched precompile. A mismatch between compiler assumptions and actual node behavior can lead to hard-to-debug failures. Recompile all affected contracts with the latest compiler version and run a full regression suite against the updated node. Pay specific attention to how the compiler optimizes calls to the fixed precompile, as optimization passes might inadvertently mask or alter the expected behavior.

Monitor for Protocol-Level Regressions

After a protocol fix, subscribe to the official ZKsync security advisories and node release notes. Set up automated alerts that trigger if a new protocol upgrade modifies the gas schedule, address, or ABI of the patched precompile. A silent regression in a minor node release could reintroduce the vulnerability. Chainscore Labs provides ongoing monitoring services that track protocol changes against a database of known exploit patterns and regression risks.

Engage for Formal Verification of Fix

Understanding the root cause is critical for preventing similar issues in other ZK Stack components. Chainscore Labs can perform a formal verification of the patched precompile's circuit implementation against its specification. This process mathematically proves that the fix is complete and that no other edge cases exist in the corrected logic. For teams building high-security applications like bridges or custodial wallets, this level of assurance is a necessary step before fully trusting the remediated protocol.

PRECOMPILE MISBEHAVIOR IMPACT

Risk Assessment Matrix

Evaluates the security and operational impact of a zkSync Era-specific precompile returning incorrect results or exhibiting a denial-of-service vector, and the required actions for affected teams.

Area	Failure Mode	Who is affected	Action
Incorrect Result Return	A precompile (e.g., keccak256, ecrecover) returns a cryptographically invalid or deterministic but incorrect output for a specific input range.	DeFi protocols, wallets, bridges, and any contract relying on the precompile for signature verification or hash commitments.	Re-verify all dependent contract logic against the patched precompile behavior. Audit for state corruption or unauthorized actions that occurred during the bug window.
Denial-of-Service Vector	A specific input causes the precompile to consume excessive gas, panic, or hang the sequencer/prover, preventing block production or proof generation.	Sequencer operators, prover operators, and any user whose transaction interacts with the vulnerable precompile path.	Apply the mandatory node/prover software update. Implement input validation or gas limits in dependent contracts as a defense-in-depth measure.
State Transition Inconsistency	The sequencer and prover evaluate the buggy precompile differently, leading to a state root mismatch and a potential chain halt or inability to generate valid proofs.	All node operators, exchanges, and the canonical bridge, as finality may stall.	Halt deposits and withdrawals until the chain is consistent. Verify that your node's state matches the canonical state root after the fix is applied.
Proof System Soundness	A circuit bug in the precompile's ZK-circuit implementation allows a malicious prover to generate a valid proof for a false execution trace.	The entire L2 state, as a forged proof could finalize an invalid state transition on Ethereum L1.	Verify that the circuit fix has been deployed and that the on-chain verifier contract has been upgraded. Re-validate the integrity of the post-fix state.
Application-Level Exploit	An attacker crafts a transaction that exploits the incorrect precompile output to bypass signature checks, drain a paymaster, or manipulate a DeFi protocol's internal accounting.	Users of the affected protocol, liquidity providers, and the protocol's treasury.	Pause affected contracts immediately. Conduct a post-mortem to identify all exploited transactions. Implement a merkle-tree based claims process for affected users if funds are recoverable.
Ecosystem Integration Breakage	A wallet, indexer, or block explorer simulates transactions using a different precompile implementation than the buggy zkSync Era node, leading to incorrect gas estimates or simulation failures.	Wallet developers, indexer operators, and block explorer teams.	Update simulation engines to match the exact (buggy or patched) behavior of the current canonical node. Clearly communicate any temporary simulation inaccuracies to users.
Cross-Chain Message Corruption	An L2-to-L1 message generated by a contract relying on the buggy precompile contains an incorrect state root or message hash, causing the message to be unprovable or to relay incorrect data.	Bridge operators and any L1 protocol that receives messages from the affected L2 contracts.	Verify the integrity of all pending cross-chain messages. Re-generate and re-submit messages that were created during the bug window after the fix is confirmed.

PRECOMPILE MISBEHAVIOR RESPONSE

Incident Timeline and Remediation Checklist

A structured checklist for teams to verify their exposure to a zkSync Era precompile bug, confirm the protocol-level fix, and ensure dependent application logic is re-validated. This process covers detection, containment, remediation, and postmortem learning for operators, builders, and security engineers.

What to check: Identify the exact precompile address (e.g., keccak256, ecrecover) and the zkSync Era protocol version or system contract upgrade that introduced the bug. Cross-reference the official security advisory or ZIP.

Why it matters: Not all precompiles are affected. A misidentified scope leads to unnecessary remediation work or, worse, a false sense of security. The bug may only exist in a narrow window of gateway upgrades.

Readiness signal: You have pinned the exact precompile address, the faulty system contract hash, and the block range or protocol version where the misbehavior was active.

resource-links

CANONICAL REFERENCES

Source Resources

Use these sources to verify zkSync Era precompile behavior, track protocol fixes, and validate whether application-level assumptions still hold after a precompile-related security advisory.

ZKsync Era Protocol Repository

Review the ZKsync Era implementation source when assessing a precompile misbehavior advisory. Security teams should inspect the code path that routes calls to Era-specific precompiles, the bootloader interaction, gas accounting, and any test coverage added with the fix. For dependent applications, compare the patched behavior against assumptions in signature checks, hash commitments, allowlists, bridges, paymasters, and account abstraction validation logic.

EXPLORE

Official ZKsync Documentation

Use the official documentation as the first reference for supported execution semantics, Ethereum compatibility differences, system contracts, account abstraction behavior, and developer-facing migration notes. After a precompile fix, builders should verify whether the documented behavior changed, whether the change is backward-compatible, and whether transaction simulation, contract verification, or deployment tooling needs to be updated.

EXPLORE

ZKsync Status and Operational Notices

Monitor the official status page for network incidents that may coincide with a precompile bug, denial-of-service vector, sequencer mitigation, degraded RPC service, or delayed L1 settlement. Exchanges, custodians, bridges, and market operators should correlate application anomalies with status updates before deciding whether to pause deposits, widen confirmation thresholds, or delay withdrawals.

EXPLORE

Ethereum Execution Baseline

For compatibility-sensitive precompiles such as ecrecover and hashing-related behavior, compare zkSync Era output against Ethereum execution-layer semantics. This helps identify whether the issue is a zkSync-specific implementation defect, a documented compatibility difference, or an application bug caused by assuming byte-for-byte L1 behavior. Teams should build regression tests that run the same vectors on Ethereum and zkSync Era where applicable.

EXPLORE

Application Regression Test Corpus

Maintain an internal corpus of vectors that exercise the affected precompile: valid and invalid signatures, malformed calldata, edge-case hashes, high-gas and low-gas execution, repeated calls, and adversarial inputs. After a protocol fix, re-run these vectors against local tooling, testnet deployments, RPC simulations, and production monitoring. This is especially important for AA wallets, paymasters, bridges, or protocols whose authorization logic depends on precompile output.

Upgrade and Risk Review Workflow

Treat a precompile security advisory as both a protocol event and an integration risk event. Operators should verify node and RPC behavior after the fix; application teams should re-check affected contracts; risk teams should review paused functions, user exposure, and replay or invalid-verification scenarios. Chainscore Labs can support impact assessment, regression planning, smart contract review, and post-fix monitoring design for zkSync Era integrations.

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PRECOMPILE INCIDENT FAQ

Frequently Asked Questions

Operational and security questions for teams assessing the impact of a zkSync Era precompile misbehavior incident. Covers detection, verification, and remediation steps for dependent applications.

Review your contract's interaction with the specific precompile address during the incident window. Check for:

Direct calls: Any STATICCALL, DELEGATECALL, or CALL to the precompile's address.
Indirect dependencies: Libraries or external contracts that internally call the precompile (e.g., an ERC-4337 account using ecrecover for signature validation).
State impact: Trace transactions during the incident period to see if incorrect precompile outputs led to invalid state changes, such as unauthorized balance transfers or corrupted access control.

Use block explorers with trace functionality or replay transactions against a patched node to compare outputs. The canonical fix will restore correct behavior, so any state that was mutated based on a faulty return value will remain corrupted unless explicitly remediated.

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Precompile Misbehavior and Security Impact

Incident Summary

Quick Facts

Root Cause and Technical Mechanism

Affected Systems and Actors

Application Developers