ERC-4337 Bundler Integration Vulnerabilities Affecting Safe

Safe's integration with ERC-4337 account abstraction shifts transaction validation and execution through an external bundler layer, creating security boundaries that do not exist in the canonical multi-sig flow. When a Safe account operates via EntryPoint, the bundler is responsible for simulating UserOperation validity, enforcing paymaster data integrity, and ensuring that validation results are not spoofed. A malicious or compromised bundler can exploit gaps in this trust model to bypass Safe's signature checks, drain gas deposits, or replay operations across chains—even when the underlying Safe singleton and module logic remains sound.

The core vulnerability class stems from the separation between the validateUserOp execution context and the bundler's off-chain simulation environment. Bundlers are expected to enforce rules that the on-chain EntryPoint cannot fully verify, including paymaster sponsorship honesty, fee parameter consistency, and the prohibition of banned opcodes during validation. When a bundler fails to enforce these rules—or actively colludes with a paymaster—Safe accounts can approve UserOperations that appear legitimate in simulation but execute with altered state, drained deposit accounts, or replayed signatures. This is not a flaw in Safe's contract logic but a systemic trust assumption that integrators must explicitly model.

For teams deploying Safe-based account abstraction, bundler selection becomes a security-critical decision equivalent to choosing a sequencer or relayer. Integrators must assess whether their bundler enforces the full ERC-4337 validation ruleset, whether it has been audited for simulation-spoofing vectors, and whether their Safe deployment includes additional guard or module checks that constrain UserOperation execution independently of bundler behavior. Chainscore Labs provides bundler security assessments, Safe module review for ERC-4337 compatibility, and integration audits that map the exact trust boundaries introduced by each bundler architecture.

Safe accounts operating through ERC-4337 EntryPoint contracts delegate transaction validation to a fundamentally different flow than native Safe executions. In the standard Safe model, execTransaction validates signatures against the owner set and threshold directly within the Safe proxy. Under ERC-4337, the EntryPoint calls validateUserOp on the Safe account, which must return a validation result and cannot perform state changes that persist after the call. This separation between the validation phase and execution phase creates a class of vulnerabilities where a malicious bundler can manipulate the context between these two phases—altering paymasterAndData, callData, or gas parameters after validation succeeds but before execution begins.

The primary attack vectors specific to Safe's ERC-4337 integration include: validation logic bypass where a bundler modifies the UserOperation after validateUserOp returns successfully, causing execution to proceed with parameters the Safe's owners did not approve; paymaster data spoofing where a bundler substitutes a malicious paymaster that drains gas deposits or manipulates execution context; and UserOperation replay across chains or EntryPoint versions when the Safe's validation logic does not bind the operation to a specific chainId and entryPoint address. These vectors are distinct from native Safe replay attacks because the UserOperation hash is computed off-chain and its binding to chain state depends on the EntryPoint's replay protection, not Safe's internal nonce and domain separator logic.

The trust model shift is operationally significant: Safe's native security assumes only that n of m owners sign a specific payload. Under ERC-4337, the Safe account must additionally trust that the bundler will faithfully submit the exact UserOperation that was validated, that the paymaster will not revert or drain funds, and that the EntryPoint contract itself has no vulnerabilities. For teams deploying Safe-based account abstraction, this means bundler selection is a security decision, not just an infrastructure choice. Chainscore Labs recommends that integrators implement independent UserOperation simulation against the target EntryPoint before signing, verify that validation logic binds to chainId and entryPoint, and conduct a bundler trust assessment that models the worst-case behavior of a malicious bundler in their specific deployment architecture.