#38030 [BC-Insight] Coordinator can be crashed by signers on DKG

Submitted on Dec 22nd 2024 at 18:10:47 UTC by @n4nika for Attackathon | Stacks

• Report ID: #38030

• Report Type: Blockchain/DLT

• Report severity: Insight

• Target: https://github.com/stacks-network/sbtc/tree/immunefi_attackaton_0.9/signer

• Impacts:

  • Shutdown of greater than 10% or equal to but less than 30% of network processing nodes without brute force actions, but does not shut down the network

Description

Summary

The wsts library uses insecure array accesses, making it possible for a malicious signer to send certain packets to a coordinator during a DKG which cause the coordinator to panic.

Finding Description

I will put multiple instances of this in one issue since they all are only triggerable on DKG (which will not be triggered in the near future but this was explicitly mentioned by the team to still be desireable).

Instances:

1. wsts::fire.rs#L580

2. wsts::fire.rs#L626

3. wsts::fire.rs#L628

Further description:

1. Here we index dkg_public_shares[bad_signer_id]. If a signer manages to get a malicious bad_signer_id to this point, we will crash. This is the least likely case of the here mentioned cases but if a signer manages to have bad_signer_id not in dkg_public_shares, it is possible. While this is not easy, I wanted to mention it anyways for completeness

2. Here we index dkg_public_shares[src_party_id] where we get src_party_id by iterating over dkg_private_shares.shares of which the integrity is NOT checked so this is directly exploitable by the attack described later

3. Here we index key_shares[key_id] where key_shares is also taken from dkg_private_shares.shares making it a direct target for exploitation

Exploiting 2) and 3)

In order to exploit this, a malicous signer needs to send a BadPrivateShare to the coordinator which contains malformed shares and adheres to the following restrictions:

• tuple_proof is valid

Mitigation

Replace all array accesses in the whole wsts library using [] with .get or equivalent methods and handle errors accordingly.

Proof of Concept

PoC

@djordon mentioned on discord that I don't need to provide a PoC for these since this report is supposed to be marked as Insight anyways.