Sure, I’ll take that! New ComboJack Malware Alters Clipboards to Steal Cryptocurrency

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Unit 42 researchers have discovered a new currency stealer which targets cryptocurrencies and online wallets. "CryptoJack" functions by replacing clipboard addresses with an attacker-controlled address which sends funds into the attacker's wallet. This technique relies on victims not checking the destination wallet prior to finalizing a transaction. In 2017, CryptoShuffler was the first malware to utilize this tactic. In contrast to that one, which focused on numerous cryptocurrencies, ComboJack targets both a range of cryptocurrencies, as well as digital currencies such as WebMoney and Yandex Money.
Early on the morning of February 25, 2018, Unit 42 and Proofpoint researchers observed an interesting malspam campaign targeting Japanese and American users. This particular campaign tried to entice users by claiming a passport was lost and that the attached PDF contained a scanned copy of the document.

Image 1. Example malspam recieved by users.

Users opening this PDF would find a single line of text which refers to an embedded doc file.

Figure 1 Prompt displayed to the victim when opening the embedded RTF file

Similar to techniques utilized by Dridex and Locky in mid-2017, the PDF contained an embedded RTF file which contains an embedded remote object that attacks CVE-2017-8579 as discussed in this FireEye report.
This embedded remote object is an HTA file which was located at hXXps://a.doko[.]moe/tnejln which contains encoded PowerShell commands.

Image 2. Contents of the HTA file retrieved from hXXps://a.doko[.]moe/tnejln

Decoding the contents of the HTA file yields the following PowerShell command which downloads and executes a file:

The full flow of execution may be visualized as follows:

Figure 2 Flow of execution leading to ComboJack being installed on victim

That leads us to the payload, which we have dubbed ComboJack because of how it attempts to hijack a combination of digital currencies.
The following files were used for this analysis, which are explained below.

Initial File SHA256 9613aefc12880528040812b0ce9d3827d1c25fe66f8598eaef82c169e8ed02da
Second Stage SHA256 cab010b59cf9d649106477df012ca49f939aa537910b56bfadbe1381b0484d88
Final Payload SHA256 05dfde82a9790943df8dfab6b690ec18711ce3558f027dd74504b125d24d6136

The initially downloaded file is a self-extracting executable (SFX) with embedded commands for extracting the second stage. This second stage is a password protected SFX, however, the password is supplied by the first stage. This allows us to easily recover the contents of the second stage. Helpfully, the “setup.txt” from the first stage contains the following:

Image 3. Contents of setup.txt embedded in the first SFX layer of the payload.

Once the second stage is extracted and run, we are presented with the final stage of this attack, which we refer to as ComboJack. Once ComboJack is extracted it begins by copying itself to the following location:

It then uses the built-in Windows tool, attrib.exe (used for setting file attributes), to set both hidden and system attributes to itself. This hides the file from the user and allows it to execute with SYSTEM level privileges.

Finally, the payload sets the following registry key to ensure persistence:

When the above steps are completed, ComboJack enters into an infinite loop. Every half second it checks the contents of the clipboard. The contents of the clipboard are checked for various criteria to determine if the victim has copied wallet information for various digital currencies. In the event a wallet of interest is discovered, ComboJack will replace it with a hardcoded wallet that the attacker presumably owns in an attempt to have the victim accidentally send money to the wrong location. This tactic relies on the fact that wallet addresses are typically long and complex and to prevent errors, most users will opt to copy an exact string in order to prevent potential errors. If any potential currency addresses are found, they are replaced following the criteria in the table below:

Checks for this criteria Replaces with Wallet Type
Length of 42 and starts with a ‘0’ 0xE44598AB74425450692F7b3a9f898119968da8Ad Ethereum
Length of 106 and starts with ‘4’ 4BrL51JCc9NGQ71kWhnYoDRffsDZy7m1HUU7MRU4nUMXAHNFBE Monero. It's important to note that this replacement string is not long enough, as Monero wallet addresses are either 95 or 106 characters in length. This was likely a mistake made by the author.
Length of 34 and starts with ‘1’ 1LGskAycxvcgh6iAoigcvbwTtFjSfdod2x Bitcoin
Length of 34 and starts with ‘L’ LYB56d6TeMg6VmahcgfTZSALAQRcNRQUV Litecoin
Length of 11 and starts with ‘8’ 79965017478 Qiwi
Length of 13 and starts with ‘R’ R064565691369 WebMoney (Rubles)
Length of 13 and starts with ‘Z’ Z152913748562 WebMoney (USD)
Length of 13 and starts with ‘E’ 88888888888888888888888888888888888888888888888888 Unknown
Length of 15 and starts with ‘4100’ 410014474125403 Yandex Money

Table 1. Replacement address lookup table hardcoded into ComboJack.

ComboJack shares some similarities in basic functionality with CryptoShuffler, which is a malware family discovered by Kaspersky in 2017. However whereas CryptoShuffler focused exclusively on cryptocurrencies, ComboJack also targets popular digital payment systems, such as WebMoney (USD, EUR, and RUB), and Yandex Money.
With the proliferation of Cryptomining malware, it is curious to see some actors take a different route to acquiring web-based currency. Cryptoshuffler in 2017 may have been only the beginning of simple, yet effective clipboard stealers like ComboJack. By targeting multiple cryptocurrencies and web based wallets, the author of ComboJack appears to be hedging his or her bets on which currency will boom and which will bust. As the prices of cryptocurrencies continue to rise it is likely we will see more and more malware targeting cryptocurrencies, as it presents the fastest way to the highest profit.
Palo Alto Networks WildFire customers are protected from this threat through the following ways:

  • ComboJack malware is identified as malicious and blocked via the Traps and WildFire products
  • Customers may monitor and track ComboJack through the AutoFocus tag

Lure PDFs:

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