Furry Tin Puma: Arbitrum Spec Mismatch Vulnerability

Aug 14, 2025 by Chloe Fitzgerald 53 views

Furry Tin Puma: Incorrect Ethereum Spec Use with Arbitrum

Introduction

Hey guys! Today, we're diving deep into a critical vulnerability discovered in the Furry Tin Puma project. This issue, categorized as a medium severity, revolves around the incorrect use of the Ethereum chain specification instead of the Arbitrum specification. This mix-up can lead to serious security implications, potentially allowing invalid state data to be accepted as correct. Let's break down what happened, why it matters, and how it can be fixed.

Summary: The Core Issue

The heart of the problem lies within the sort_and_verify_relevant_params() function. This function is crucial for validating proofs on various chains, including Arbitrum. However, it erroneously uses the Ethereum chain specification (ETH_MAINNET_CHAIN_SPEC) for Arbitrum chains. This is a major no-no because Arbitrum isn't just another OP Stack chain; it's built on the unique Arbitrum Nitro architecture. Nitro has its own distinct proof format and serialization process, which are entirely different from Ethereum's. By using the wrong specification, the system fails to properly validate Arbitrum proofs against their intended rules, opening the door for potential exploits.

Root Cause: Why the Mix-Up?

To understand how this happened, let's peek into the code. Specifically, the issue is located in validators.rs::sort_and_verify_relevant_params(). The function's logic doesn't correctly recognize Arbitrum chain IDs. When it encounters an Arbitrum chain ID, it defaults to using Ethereum's specification.

pub fn sort_and_verify_relevant_params(
 chain_id: u64,
 env_input_for_viewcall: Option<EthEvmInput>,
 linking_blocks: &Vec<RlpHeader<Header>>,
 env_input_eth_for_l1_inclusion: &Option<EthEvmInput>,
 env_input_opstack_for_viewcall_with_l1_inclusion: Option<OpEvmInput>,
) -> (
 EvmEnv<StateDb, EthEvmFactory, Commitment>,
 RlpHeader<Header>,
 B256,
 Header,
 Option<EvmEnv<StateDb, OpEvmFactory, Commitment>>,
 Option<Commitment>,
 u64,
 bool,
) {
 let validate_l1_inclusion = env_input_eth_for_l1_inclusion.is_some();

 // Determine which environment and parameters to use based on chain type and inclusion requirements.
 let (
 env_for_viewcall,
 op_env_for_viewcall_with_l1_inclusion,
 op_env_commitment,
 chain_id_for_length_validation,
 ) = if (chain_id == OPTIMISM_CHAIN_ID
 || chain_id == BASE_CHAIN_ID
 || chain_id == OPTIMISM_SEPOLIA_CHAIN_ID
 || chain_id == BASE_SEPOLIA_CHAIN_ID)
 && validate_l1_inclusion
 {
 // For OpStack L2s with L1 inclusion, use the L1 environment and OpStack environment for inclusion.
 let env_for_viewcall = env_input_eth_for_l1_inclusion
 .as_ref()
 .expect("env_eth_input is None")
 .clone()
 .into_env(&ETH_MAINNET_CHAIN_SPEC);
 let op_env_for_viewcall_with_l1_inclusion =
 env_input_opstack_for_viewcall_with_l1_inclusion
 .expect("op_evm_input is None")
 .into_env(&OP_MAINNET_CHAIN_SPEC);
 let op_env_commitment = op_env_for_viewcall_with_l1_inclusion.commitment().clone();
 let chain_id_for_length_validation = match chain_id {
 OPTIMISM_CHAIN_ID | BASE_CHAIN_ID => ETHEREUM_CHAIN_ID,
 OPTIMISM_SEPOLIA_CHAIN_ID | BASE_SEPOLIA_CHAIN_ID => ETHEREUM_SEPOLIA_CHAIN_ID,
 _ => panic!("invalid chain id"),
 };
 (
 env_for_viewcall,
 Some(op_env_for_viewcall_with_l1_inclusion),
 Some(op_env_commitment),
 chain_id_for_length_validation,
 )
 } else {
 // For L1 or Linea chains, use the provided environment input.
 let chain_spec = match chain_id {
 LINEA_CHAIN_ID => &LINEA_MAINNET_CHAIN_SPEC,
 LINEA_SEPOLIA_CHAIN_ID => &LINEA_MAINNET_CHAIN_SPEC,
 L236: _ => &ETH_MAINNET_CHAIN_SPEC, <@
 };
...

See that _ => &ETH_MAINNET_CHAIN_SPEC? That's the culprit! When the function doesn't explicitly recognize a chain ID (like Arbitrum's), it defaults to Ethereum's spec. This bypasses the critical Nitro-specific validation steps. It’s like trying to fit a square peg in a round hole – it just doesn't work, and in this case, it compromises security.

Attack Path: How Can This Be Exploited?

So, how can an attacker take advantage of this? Imagine this: An attacker crafts an Arbitrum proof that is intentionally invalid according to Nitro's rules. However, because the system is using Ethereum's generic spec checks, the invalid proof might just slip through the cracks. The sort_and_verify_relevant_params() function, applying the wrong spec, becomes the weakest link in the chain. Since it's not using the Arbitrum-specific parsing and verification, the invalid proof is mistakenly accepted as valid.

This is a serious problem because it undermines the integrity of the entire system. It means that incorrect state data can be treated as correct, potentially leading to fund losses, incorrect transaction processing, and other nasty consequences. Nobody wants that, right?

Impact: What's the Real-World Consequence?

The impact of this vulnerability is significant. By validating Arbitrum proofs under the Ethereum spec, the protocol fails to enforce the crucial Nitro-specific proof rules. This is a fundamental flaw that can have far-reaching effects. Imagine a scenario where someone manipulates the state of an Arbitrum-based application, and the system, due to this vulnerability, incorrectly validates the fraudulent state change. This could lead to:

Financial losses: Funds could be drained or stolen due to incorrect validation of transactions.
Data corruption: The state of applications on Arbitrum could become corrupted, leading to unpredictable behavior.
Loss of trust: Users could lose faith in the system if they realize that its security is compromised.

In short, this vulnerability undermines the core security guarantees of the Arbitrum chain within the Furry Tin Puma project. It's like having a faulty lock on your front door – it creates a significant risk of unauthorized access and damage.

Mitigation: The Path to a Fix

Alright, so we know what's broken and why. Now, how do we fix it? The solution is relatively straightforward: We need to introduce a dedicated Arbitrum chain specification. This means creating constants like ARBITRUM_MAINNET_CHAIN_SPEC and ARBITRUM_SEPOLIA_CHAIN_SPEC that are specifically aligned with Nitro's proof format. Think of it as creating the right key for the right lock.

Next, we need to update the sort_and_verify_relevant_params function to correctly detect Arbitrum chain IDs. When an Arbitrum chain ID is detected, the function should route the proofs to the appropriate Arbitrum-specific specification for validation. This ensures that Arbitrum proofs are always validated against the rules they were designed for. It's like having a gatekeeper who knows exactly which keys open which doors – only valid proofs get through.

By implementing these changes, we can ensure that the system correctly validates Arbitrum proofs, preventing the acceptance of invalid state data and restoring the intended security guarantees.

Proof of Concept (PoC)

Currently, there's no provided Proof of Concept (PoC) in the original report. A PoC would demonstrate exactly how an attacker could exploit this vulnerability.

Conclusion

The incorrect use of the Ethereum chain specification for Arbitrum proofs in Furry Tin Puma is a significant vulnerability. It highlights the importance of precise chain specification handling in cross-chain validation systems. By failing to properly validate Arbitrum's Nitro-specific proofs, the system risked accepting invalid state data, potentially leading to serious consequences.

The proposed mitigation, which involves introducing dedicated Arbitrum chain specifications and updating the validation logic, is a crucial step towards securing the system. It's a reminder that in the world of blockchain security, attention to detail and adherence to the correct specifications are paramount.

So, there you have it, folks! A deep dive into the Furry Tin Puma vulnerability. Stay safe out there, and always double-check your chain specs!