Last Friday, we learned of a [newly disclosed vulnerability](https://secbit.io/blog/en/2024/01/19/trust-wallets-fomo3d-summer-vuln/) in the `Trust Wallet` software which is relevant to Milk Sad. Researchers from [SECBIT Labs](https://secbit.io) 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.<br/>
Open source code shows that [a core component](https://github.com/trustwallet/trezor-crypto-ios/commits/master/) 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](https://secbit.io/blog/en/2024/01/19/trust-wallets-fomo3d-summer-vuln/) 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.
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:
```c
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](https://en.wikipedia.org/wiki/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.
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:
alt: "Histogram of PRNG creation index for discovered wallets<br/>Trust Wallet iOS variant A & B - 128 bit wallets<br/>Yearly X-Axis timestamp markers, graph based on incomplete data"
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.
alt: "Histogram of PRNG creation index for discovered wallets - Trust Wallet iOS<br/>variant A & B, 2018-05 to 2020-01, data is not stacked, graph based on incomplete data"
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.
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 ({{ "cdd9a2aff7cd0707e31023513cc78aceff7ea7e754e3a9bde9c0482b70a9716c" | BtcLinkTxUrlSliced }}) 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.
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.
