Linea L1 Hash Verification Vulnerability

by Sebastian Müller 41 views

Hey guys, let's dive deep into a critical vulnerability discovered in the Malda ZK-Coprocessor, specifically concerning the validation of Linea blocks. This is super important, so pay close attention!

Summary

The core issue? The validation process accepts Linea blocks without verifying that the specific Linea block hash is included on Layer 1 (L1). The checks in place only confirm an Ethereum block hash, block number, and sequencer signature. This glaring omission opens the door for fraudulent Linea blocks to slip through the cracks, triggering irreversible and potentially devastating state changes. Imagine the chaos!

Root Cause

This vulnerability stems from the fact that the code never verifies the L2 block hash inclusion on L1. The current system uses the block number (ensuring it's less than or equal to the latest) and the sequencer signature as a sort of proxy. However, and this is a HUGE however, this is not sufficient. The actual block hash must be proven on-chain. It's like saying you checked your ID at the door, but you never verified if the picture actually matched the person holding it. Disaster waiting to happen!

Even the comment within the function itself acknowledges this inadequacy:

https://github.com/sherlock-audit/2025-07-malda/blob/798d00b879b8412ca4049ba09dba5ae42464cfe7/malda-zk-coprocessor/malda_utils/src/validators.rs#L679

This makes it crystal clear: deployment should not proceed without proper, robust checks in place. We're talking about serious security implications here, folks.

Impact

The potential impact is significant. Users could have their funds drained due to these invalid proofs. Think about it: if a malicious block can be validated, it can trigger unauthorized withdrawals and updates, leading to irreversible financial losses. This isn't just a theoretical risk; it's a real threat that needs to be addressed immediately. The financial consequences could be catastrophic, making this a top-priority issue.

PoC (Proof of Concept)

Let's walk through how an attacker could exploit this vulnerability:

  1. The attacker crafts a fraudulent Linea block. This block has the same block number as a legitimate block but contains malicious state changes. The attacker also forges a sequencer signature for this fraudulent block.
  2. Malda runs the validation process. The Opstack returns an Ethereum hash. The block number check passes because the attacker used a valid number. The signature check also passes due to the forged signature.
  3. The critical flaw: Because hash inclusion is not checked, Malda mistakenly accepts the fraudulent block and applies its state changes. It's like letting a wolf in sheep's clothing into the henhouse.
  4. Funds are moved based on this fake state. Withdrawals are initiated, balances are updated incorrectly – the damage is done, and it's irreversible. Users' funds are gone.

This scenario highlights the severe risk posed by the missing block hash verification. It's a gaping hole in the security of the system, and attackers can easily exploit it to steal funds. This is a red alert situation.

Mitigation Strategies

So, what can be done to fix this? Here are the key mitigation steps:

  • Require on-chain inclusion of the L2 block hash before accepting a block. This is the most crucial step. Verifying the L2 block hash on the L1 chain ensures the authenticity and integrity of the block. It adds a strong layer of security that prevents fraudulent blocks from being validated.
  • If a hash-proof is unavailable, do not accept Linea validation based on number+signature alone. This is a temporary measure, but it's essential to prevent exploits while a proper solution is implemented. There are two options here:
    • Pause/disable validation: This is the most conservative approach. It prevents any validation from occurring until on-chain proof is available. This eliminates the risk of fraudulent blocks being accepted, but it also means that legitimate transactions cannot be processed.
    • Use a trusted oracle: A trusted oracle can provide a reliable source of truth for the L2 block hash. This allows validation to continue, but it introduces a dependency on the oracle's security and trustworthiness.

The key takeaway is that relying solely on the block number and signature is simply not enough. Robust verification of the block hash on-chain is paramount for the security of the system. Without this, the entire system is vulnerable to attack and user funds are at risk.

In conclusion, guys, this is a serious vulnerability that needs immediate attention. By implementing these mitigation strategies, we can protect the Malda ZK-Coprocessor and ensure the safety of user funds. Stay vigilant, and let's keep building a more secure future for blockchain technology!

Repair Input Keyword

Okay, let's break down the key keywords and what they really mean:

  • Missing Linea L1 Inclusion Block Hash Verification: This is the core of the issue. It means the system isn't properly checking if the Linea block's unique identifier (the hash) has been recorded on the main Ethereum chain (L1). Think of it like not verifying if a document has been officially filed before acting on it.
  • get_validated_block_hash_linea(): This refers to a specific function in the code that's responsible for validating Linea blocks. It's like the gatekeeper function, but it's not doing its job properly right now.
  • Linea blocks: These are blocks of transactions on the Linea network, a Layer 2 scaling solution for Ethereum. Think of them as mini-batches of transactions that eventually get processed on the main Ethereum chain.
  • Ethereum block hash: This is the unique identifier for a block on the Ethereum blockchain. It's like a fingerprint for each block.
  • Block number: This is simply the sequential number assigned to each block in the blockchain. It's like a page number in a ledger.
  • Sequencer signature: This is a digital signature from the entity that sequences transactions on Linea. It's meant to prove that the block was created by a legitimate sequencer.
  • L2 block hash: This is the unique identifier for a block on the Linea network (Layer 2). It's what should be verified on L1.
  • On-chain inclusion: This means that the information (in this case, the L2 block hash) is recorded on the blockchain, making it publicly verifiable and tamper-proof.
  • Fraudulent Linea blocks: These are blocks that have been tampered with or created maliciously to manipulate the state of the system.
  • Irreversible state changes: These are changes to the system's data that cannot be undone. This is a big deal because if a fraudulent block is accepted, the resulting changes could be permanent and damaging.
  • Malda ZK-Coprocessor: This is the system being audited. It's a piece of software that helps process and verify data on the blockchain.
  • Impact (Users can be drained based on invalid proofs): This is the potential consequence of the vulnerability. If fraudulent blocks are accepted, attackers could steal users' funds.
  • Mitigation: These are the steps that can be taken to fix the vulnerability and prevent attacks.
  • PoC (Proof of Concept): This is a step-by-step scenario that demonstrates how an attacker could exploit the vulnerability.
  • Require on-chain inclusion of the L2 block hash: This is the core mitigation. It means the system must verify that the L2 block hash is recorded on the Ethereum blockchain before accepting a Linea block.
  • Trusted oracle: This is a third-party service that can provide reliable information (in this case, the L2 block hash) to the system. However, relying on an oracle introduces a new point of trust.
  • Pause/disable Linea validation: This is a temporary measure that can be taken to prevent attacks while a proper fix is implemented. It involves temporarily stopping the validation of Linea blocks.

In essence, the vulnerability is about the system trusting information without proper verification. It's like accepting a check without verifying the signature or the account balance. By understanding these keywords, we can better grasp the severity of the issue and the importance of the proposed mitigations. Make sense, guys?

Title

Linea L1 Block Hash Verification Vulnerability