Update #5 - Digging Into New Trust Wallet Research

Last Friday, we learned of a newly disclosed vulnerability in the Trust Wallet software which is relevant to Milk Sad. Researchers from SECBIT Labs tracked down an older wallet generation weakness in the iOS platform version of Trust Wallet from 2018 and connected it to the large thefts on 2023-07-12 that triggered our Milk Sad research.
Using the newly available information, we managed to reproduce some of their findings, and can give a first look at additional data we collected.

Vulnerability TL;DR

Open source code shows that a core component of the Trust Wallet app for iOS generated new cryptocurrency wallets using unsafe functions in the trezor-crypto library that were not meant for production. As a result, their wallet entropy is based on a weak PRNG with 31-bit initial state that is seeded with easy-to-guess time values. This makes all wallets generated with vulnerable app versions easy to brute force remotely, like the weak bx seed mechanism in Libbitcoin. Both weaknesses were attacked on-chain at the same time in July 2023.

If you want to understand this better, we recommend taking a look at the Trust Wallet’s Fomo3D Summer: Fresh Discovery of Low Entropy Flaw From 2018 disclosure of the SECBIT team.

Credits for this discovery go to p0n1 and outoflegend from SECBIT Labs. We weren’t involved in their disclosure and didn’t know about it beforehand, but they gave us some useful confirmations and hints via email after their disclosure was public. Thank you!

Vulnerability Types

The newly disclosed vulnerability comes in two forms. Together with the previously known Trust Wallet vulnerability, things are getting confusing, so here’s a short overview:

Each weak PRNGs variant creates different ranges of weak wallets. This article is exclusively about the two newly discovered variants, and does not cover the Mersenne Twister based vulnerability.

Early Results

Unlike other vulnerabilities with time-based PRNG seeding that we looked at before, this one calls the PRNG initialization in a way that can be predicted by outside observers, in both variant A and variant B:

srand((unsigned)time(NULL));

time(NULL) returns the seconds since 1970-01-01 00:00:00, representing the “random” base for srand() to pick the first PRNG state setting. Developers may know this time format as Unix time.

For our initial experiments, we assume that the devices running the code had reasonably correct date and clock settings at the time they generated new wallets. The PRNG has a 31-bit state, but we used the known srand() behavior to focus our initial search onto a smaller subset of wallets which correspond to certain creation dates. This optimization speeds up the initial work.

Over 6500 Weak Wallets 🔥

Within the wallet ranges with 12 mnemonic words (128 bits), we found the following:

Based on the current preliminary (incomplete!) data, we’re counting at least 6572 unique wallet mnemonics, and we still expect to find more.

Wallet Creation Date Pattern

We mapped out the discovered BTC and ETH wallets in a histogram plot to better understand the PRNG index range patterns (and therefore potential timestamps) the wallets in variant A and variant B were created with:

Two main observations:

1. The majority of Bitcoin and Ethereum wallets have a PRNG seed that corresponds to wallet creation timestamps from 2018 and 2019.
2. There is a smaller number of wallets with a more “random” pattern.

Point 1) generally fits to the publicly known timeline of the Trust Wallet iOS development and vulnerability (but is not a hard confirmation). Point 2) indicates that there is some other source of weak wallets in this range that is less time based. Alternatively, it’s possible that a subset of app devices had bad clocks for some reasons when creating the wallets.

Since the situation in 2018-2019 is very busy, here is a zoomed-in and annotated version:

A possible interpretation of the graph history:

1. Parameter space “before” the vulnerable Trust Wallet versions
2. Weak wallets of variant A appear with the first weak software version
3. Weak wallets of variant B appear once a differently vulnerable software version gets released
   • -> New wallets in variant A get more rare
4. A patched version get released
   • -> New wallets in variant B get more rare
5. Historically high BTC prices motivate users to create new wallet mnemonics for Bitcoin accounts (?)
6. Some users of both vulnerable software versions keep creating weak wallets months later, likely because they did not get or install the patched app versions yet

It is unclear to us if Trust Wallet on iOS had full support for Bitcoin wallets at the time, which may would be a relevant factor in the interpretation, and a clue towards the involvement of other wallet software.

Bitcoin On-Chain Movements

Here is an initial, incomplete analysis of BTC movements to and from weak wallets based on our initial data:

Notably, there is a visible peak in the graph of outgoing transfers of the month of 2023-07, which is no coincidence. More on this later.

Early estimations for on-chain funds movement volume:

Note: this describes funds moved over the weak wallets that have been at risk at some point, not the amounts stolen by attackers.

Involvement in the 2023-07-12 Theft

Here are the three biggest outgoing theft transactions from the newly disclosed range of weak wallets on 2023-07-12:

As you can see, the newly discovered weak wallets make up a significant portion of some of the theft transactions on that day. At least one of the transactions (cdd9a2af..70a9716c) had not been on our radar so far, since the stolen funds were exclusively coming from previously untracked wallets. We therefore have to adjust our estimates of the overall figure of BTC stolen from victims that day. It now exceeds $1M USD of equivalent value at the time!

Current Status & Advice

At the time of publication, less than $100 USD in total remain on the known BTC wallets in the analyzed range subsets. Unfortunately, there have still been a number of incoming BTC transactions after 2023-07-12, which are very much at risk of getting stolen the second they get announced on the blockchain.

Trust Wallet users who created their mnemonic seed words with an app version downloaded during the time in question should be very careful. If in doubt, move funds to a new wallet mnemonic generated on a well-regarded and up-to-date wallet solution, such as a popular hardware wallet.

Summary & Outlook

In this research update, we’ve followed up on work by other security researchers who discovered two large new ranges of weak cryptocurrency wallets from (most likely) an older vulnerability in Trust Wallet on iOS. We confirmed that these weak wallets exist and that they were involved in the Milk Sad thefts. We also provided new statistics on their distribution and usage.

This is an exciting new piece of the overall Milk Sad puzzle, and we’ll continue to update this blog post with new information as it becomes available.