Libra-related Sui blockchain fixes critical bug that put ‘billions’ at risk

The vulnerability was located in a file that translates human-readable code into machine language for storage.

The Sui blockchain network quietly fixed a bug that could have put “billions of dollars” at risk, according to a May 16 announcement from Zellic, the security firm hired to audit the network’s security.

Loss of Funds Bug in Aptos and Sui

Quick spotlight on an unpublished (but fixed) loss-of-funds bug in the move verifier that seems to have been found by @zellic_io.

This would have allowed many types of exploits against Aptos or Sui based protocols.

— Jasper | Neodyme (@JasperCPS) April 11, 2023

The bug was in a dependency of the bytecode verifier, which ensures that the human-readable Move language used to write smart contracts on Sui is correctly transcribed into machine code during deployment. Had the bug not been fixed, it could have “allowed attackers to bypass multiple security properties, leading to potentially significant financial damages,” the announcement said.

According to the announcement, Sui developer Mysten Labs fixed the bug on March 30, in commit 8bddbe65, after Zellic informed them of its existence. The bug may have also been present in other Move-based networks, including Aptos and Starcoin. The Aptos version of the bug was eliminated with a patch on April 10, according to the Zellic team.

In a conversation with Cointelegraph, a representative from the Move-based 0L network stated that the bug does not affect its version of Move. On May 15, 0L added a series of tests to their GitHub, which it says proves the exploit is not possible on the 0L version.

Cointelegraph reached out to Aptos and Starcoin for comment but did not receive a response by publication.

A blockchain network developed by Mysten Labs, Sui was founded by former Meta Platforms engineers. It’s a fork of the open-source Libra project created by Facebook-parent Meta. Libra was shut down in 2019.

Some developers favor Move smart contract language because its security features specifically benefit blockchains. For example, it allows developers to create custom data types, including a “coin” type that cannot be copied or deleted.

Like other blockchain networks, Sui does not store code in the same language it is written in. Instead, it converts this code from the network’s human-readable language to machine-readable bytecode.

In making this translation, Sui runs a series of verifications to ensure the translated code does not violate the security properties of the network. For example, it ensures that coins can’t be deleted or copied.

According to Zellic’s explanatory blog post, it was hired by Mysten Labs to do a security assessment of this verifier program. It did not find a bug in the verifier itself. However, it found a bug in the “Control Flow Graph” or “CFG” file that the verifier uses to accomplish many of its tasks. Because of how it was written, the CFG could allow certain lines of code to be hidden from the verifier, allowing code that violates the network’s security principles to be stored and run without getting caught.

In its explanation, the team stated that the most obvious way this vulnerability could have been exploited is by malicious borrowers taking out flash loans. When flash loans are implemented on Move-based networks, the loan protocol usually sends the borrower an asset that cannot be deleted. If the borrower can delete this asset, they “could successfully take out a flash loan and not repay the borrowed funds,” the team said. Other types of exploits could also have been possible since the vulnerability allowed the basic principles of Move security to be violated. It, therefore, “[placed] potentially billions of dollars at risk,” the security firm stated in its post.

Move-based networks and their apps have been making waves in the fundraising world lately. A Sui-based decentralized exchange called Cetus raised over $6 million in one minute on May 8. The company behind Aptos also raised over $150 million in July 2022.