BitPaymer Source Code Fork: Meet DoppelPaymer Ransomware and Dridex 2.0

July 12, 2019

| | Counter Adversary Operations
CrowdStrike® Intelligence has identified a new ransomware variant identifying itself as BitPaymer. This new variant was behind a series of ransomware campaigns beginning in June 2019, including attacks against the City of Edcouch, Texas and the Chilean Ministry of Agriculture.

 

 We have dubbed this new ransomware DoppelPaymer because it shares most of its code with the BitPaymer ransomware operated by INDRIK SPIDER. However, there are a number of differences between DoppelPaymer and BitPaymer, which may signify that one or more members of INDRIK SPIDER have split from the group and forked the source code of both Dridex and BitPaymer to start their own Big Game Hunting ransomware operation.

 

INDRIK SPIDER Origins

INDRIK SPIDER was formed in 2014 by former affiliates of the GameOver Zeus criminal network who internally referred to themselves as “The Business Club.” Shortly after the group’s inception, INDRIK SPIDER developed their own custom malware known as Dridex. Early versions of Dridex were primitive, but over the years the malware became increasingly professional and sophisticated. In fact, Dridex operations were significant throughout 2015 and 2016, making it one of the most prevalent eCrime malware families. At this time, INDRIK SPIDER was primarily conducting wire fraud, resulting in the loss of millions of dollars globally. Over time, INDRIK SPIDER encountered a number of obstacles to their wire fraud operations. First, in 2015 the group had to overcome a takedown operation, which resulted in the arrest of one of its affiliates, who used the alias “Smilex.” This setback was followed by a law enforcement operation in the U.K. designed to break up the money laundering network supporting INDRIK SPIDER's monetization of Dridex campaigns. The dismantling of this network also coincided with the arrest, and subsequent imprisonment, of a U.K. bank employee who helped set up fake accounts. Perhaps as a result of these obstacles, INDRIK SPIDER changed their methods of operation in 2017, conducting smaller Dridex distribution campaigns. In August 2017, the group introduced BitPaymer ransomware and began to focus on leveraging access within a victim organization to demand a high ransom payment.

BitPaymer Origins

 

CrowdStrike Intelligence, has tracked the original BitPaymer since it was first identified in August 2017. In its first iteration, the BitPaymer ransom note included the ransom demand and a URL for a TOR-based payment portal. The payment portal included the title “Bit paymer” along with a reference ID, a Bitcoin (BTC) wallet, and a contact email address. An example of this portal is shown in Figure 1. Within the first month of operation, the ransom amount was dropped from the ransom note. In July 2018, the payment portal URL was also removed. From July 2018 until present, the ransom note has only included two contact emails, which are used to negotiate the ransom.
Figure 1. Original BitPaymer Payment Portal via a TOR Hidden Service

Latest BitPaymer Version

In November 2018, there was a significant update to BitPaymer. The ransom note was updated to include the victim’s name, and the file extension appended to encrypted files was also customized to use a representation of the victim’s name. An example of the new ransom note is shown below in Figure 2.
screenshot of BitPaymer ransom note Figure 2. Latest BitPaymer Ransom Note
In addition to the updated ransom note and encrypted file extension, BitPaymer’s file encryption routine was updated to use 256-bit AES in cipher block chaining (CBC) mode with a randomly generated key and a NULL initialization vector. Previous versions of BitPaymer had used 128-bit RC4. Since AES is a block cipher, the implementation requires padding in cases where the data is not a multiple of the block size. Typically, this is implemented by adding zeros or the number n of padding bytes n times (also known as PKCS#7). However, INDRIK SPIDER chose to generate n bytes randomly for padding. As a result, the malware developer had to preserve the random padding bytes in order to correctly decrypt the last data block of an encrypted file. This is reflected in the BitPaymer ransom note with a new field of TAIL, as shown above in Figure 2, which contains the Base64-encoded TAIL padding and encrypted AES KEY. Interestingly, the BitPaymer developers implemented an encryption initialization function in the ransomware code that selects one of three desired encryption algorithms. The algorithm is chosen by an argument that is passed as an integer parameter to the function. The current values supported are 1, 2, and 3 for 128-bit RC4, 128-bit AES and 256-bit AES, respectively. Newer versions of BitPaymer pass the hard-coded value of 3 for 256-bit AES encryption into the function, as shown in Figure 3.
screenshot of pseudocode Figure 3. Latest BitPaymer Encryption Selection Pseudocode
Along with the updated file encryption routine, the size of the victim-specific RSA public key has also been increased from 1,024-bit to 4,096-bit. This asymmetric key is used to encrypt the generated symmetric file encryption keys. If the ransom is paid, INDRIK SPIDER will provide a decryption tool that contains the corresponding victim’s RSA private key. It is unclear why INDRIK SPIDER moved from RC4 to AES encryption, but it may be due to concerns about the relative weakness of RC4 in comparison to AES. The increase in the RSA key size also greatly augments the cryptographic strength protecting the file encryption keys. However, there is no evidence that BitPaymer’s prior or current encryption has been broken. Since the update in November 2018, INDRIK SPIDER has actively used the latest version of BitPaymer in at least 15 confirmed ransomware attacks. These attacks have continued throughout 2019, with multiple incidents occurring in June and July of 2019 alone.

Meet DoppelPaymer

While the first known victims of DoppelPaymer were targeted in June 2019, we were able to

 

recover earlier builds of the malware dating back to April 2019. These earlier builds are missing many of the new features found in later variants, so it is not clear if they were deployed to victims or if they were simply built for testing.

 

 To date, we have identified eight distinct malware builds and three confirmed victims with ransom amounts of 2 BTC, 40 BTC and 100 BTC. Based on the USD to BTC exchange rate at the time of this writing, these ransom amounts vary from approximately $25,000 to over $1,200,000.

 

 The ransom note used by DoppelPaymer is similar to those used by the original BitPaymer in 2018. The note does not include the ransom amount; however, it does contain a URL for a TOR-based payment portal, and instead of using the keyword KEY to identify the encrypted key, the note uses the keyword DATA as shown in Figure 4.
Figure 4. DoppelPaymer Ransom Note
The payment portal for DoppelPaymer is almost identical to the original BitPaymer portal. The “Bit paymer” title is still present on the web page and a unique ID is still used to identify the victim. The portal provides a ransom amount, a countdown timer and a BTC address where the ransom payment can be sent. An example of the DoppelPaymer ransom portal web page is shown below in Figure 5.
Figure 5. DoppelPaymer Ransomware Payment Portal

DoppelPaymer and BitPaymer Encryption Comparison

Although DoppelPaymer and BitPaymer share significant amounts of code, there are some notable encryption differences, which are described in Table 1.

 

DoppelPaymerBitPaymer
Ransom noteEach readme file contains an encrypted 256-bit AES key in a field named DATA.Each readme file contains an encrypted 256-bit AES key in a field named KEY. Older versions contained an encrypted 128-bit RC4 key in the KEY field. Current versions use anonymous email services such as ProtonMail for ransom payment negotiations.
Encryption2048-bit RSA + 256-bit AES

 

4096-bit RSA + 256-bit AES. Older versions used 1024-bit RSA + 128-bit RC4.
Encryption (AES) padding schemeStandard padding (PKCS#7)Random bytes specified in a field named TAIL
Ransom filenameEncrypted files are renamed with a .locked extension.Encrypted files are renamed with the victim name as the extension. Older versions are appended the suffix .locked to the names of encrypted files.
Table 1. Encryption-Related Differences Between DoppelPaymer and BitPaymer There are obvious similarities between the tactics, techniques and procedures (TTPs) used by DoppelPaymer and prior TTPs of BitPaymer, such as the use of TOR for ransom payment and the .locked extension. However, the code overlaps suggest that DoppelPaymer is a more recent fork of the latest version of BitPaymer. For example, in the latest version of BitPaymer, the code for RC4 string obfuscation reverses the bytes prior to encryption, and includes a helper function that provides support for multiple forms of symmetric encryption (i.e., RC4, 128-bit AES, and 256-bit AES), as shown in Figure 3.

New DoppelPaymer Features and the Use of ProcessHacker

In addition to the changes discussed above, numerous modifications were made to the BitPaymer source code to improve and enhance DoppelPaymer’s functionality. For instance, file encryption is now threaded, which can increase the rate at which files are encrypted. The network enumeration code was updated to parse the victim system’s Address Resolution Protocol (ARP) table, retrieved with the command arp.exe -a. The resulting IP addresses of other hosts on the local network are combined with domain resolution results via nslookup.exe. (In a similar approach, previous versions of BitPaymer made use of the command net.exe view to enumerate network shares.)

 

 In addition, DoppelPaymer is designed to run only after a specific command line argument is provided. The malware computes a CRC32 checksum of the first argument passed on the command line and adds it with a constant value that is hard-coded in the binary. The malware then adds the instruction pointer address to this result, which becomes the destination for a jmp used to continue the malware execution. The hard-coded constant value is unique to each build. In the sampled analyzed, this value was 0x672e6eb7, as shown below in Figure 6.
Figure 6. DoppelPaymer Control Flow Obfuscation
If no arguments are provided, or if an incorrect value is provided on the command line, DoppelPaymer will crash. This design was likely intended to hinder automated malware analysis environments. Perhaps the most interesting change that the DoppelPaymer author made is to terminate processes and services that may interfere with file encryption. DoppelPaymer contains several lists of CRC32 checksums of process and service names that are blacklisted. The malware author included CRC32 checksums rather than strings to hinder reverse engineering efforts. However, it is possible to brute-force all of the checksums and recover the respective strings, as shown in Tables 7-11 found in the Appendix.

ProcessHacker

In order to terminate some of these processes and services, DopplePaymer uses an interesting technique that leverages ProcessHacker, a legitimate open-source administrative utility. This application is bundled with a kernel driver that can be used to terminate processes and services. DoppelPaymer is bundled with six portable executable (PE) files that are encrypted and compressed in the malware’s sdata section. These PE files contain 32-bit and 64-bit versions of the following:

 
  • ProcessHacker application
  • ProcessHacker kernel driver
  • A custom stager DLL that is used to exploit ProcessHacker
The modules are extracted by using the first 16 bytes of the sdata section as an RC4 key to decrypt the next 4 bytes of data, which is the size (big endian) of the subsequent encrypted data. The encrypted data that follows also uses the first 16 bytes as an RC4 key to decrypt the remaining data. The format is shown below in Table 2.
16 Bytes4 Bytes16 BytesM Bytes
RC4 keyEncrypted data size (M)RC4 keyEncrypted data
Table 2. Format of Encrypted DoppelPaymer ProcessHacker Related Modules After decryption, the first 4 bytes are the size of the compressed data, and the next 4 bytes are the size of the uncompressed data, followed by the compressed data as shown in Table 3.
4 Bytes4 BytesN Bytes
Compressed size (N bytes)Uncompressed sizeCompressed 32-bit and 64-bit Process Hacker modules
Table 3. Format of Encrypted DoppelPaymer ProcessHacker Related Modules Header and Data The data is decompressed using aPLib, which produces the PE files in a custom structured format, where each PE contains an 8-byte header consisting of a magic 4-byte value, followed by another 4-byte value that specifies the size of the following PE data as shown in Table 4.
4 Bytes4 BytesN Bytes4 Bytes4 BytesN Bytes
Magic value 1Size of Module 1Module 1Magic value 2Size of Module 2Module 2...
Table 4. DoppelPaymer ProcessHacker Packed Module Format Table 5 contains the magic value and SHA256 hash for each ProcessHacker component.

 
Magic ValueSHA256Description
0xf03d938651d8618ec86159327e883615ad8989c7638172cf801f65ab0367e5b2e6af596aDoppelPaymer’s ProcessHacker Stager DLL (32-bit)
0xa68d9640d4a0fe56316a2c45b9ba9ac1005363309a3edc7acf9e4df64d326a0ff273e80fProcessHacker3 (32-bit)
0x53e9cd920f97f6d53fff47914174bc3a05fb016e2c02ed0b43c827e5e5aadba2d244aeccKProcessHacker3 Kernel Driver (32-bit)
0x2fb0f795bfb7e62ba4ad5975e68a1beefb045cb72e056911fd7a8b070a15029dfcbbefe1DoppelPaymer’s ProcessHacker Stager DLL (64-bit)
0x7900f253bd2c2cf0631d881ed382817afcce2b093f4e412ffb170a719e2762f250abfea4ProcessHacker3 (64-bit)
0x8c64a98170211a3f90376bbc61f49c22a63075d1d4ddd53f0aefa976216c46e6ba39a9f4KProcessHacker3 Kernel Driver (64-bit)
Table 5. Encrypted PE Files Embedded in DoppelPaymer

 

 After decompression, all three binaries are written to the same directory. Both ProcessHacker and the kernel driver are written as random filenames, but the stager DLL filename is chosen to be one of the DLL names imported by ProcessHacker. DoppelPaymer then executes ProcessHacker which loads the stager DLL via DLL search order hijacking. Once loaded, ProcessHacker’s kernel driver is leveraged to kill the blacklisted processes.

DoppelPaymer Links to “Dridex 2.0”

A Dridex loader sample, identified by SHA256 hash 813d8020f32fefe01b66bea0ce63834adef2e725801b4b761f5ea90ac4facd3a, was distributed through the Emotet malware on June 4, 2019. The Dridex sample contained code to decrypt either a 32-bit or a 64-bit core bot module from its sdata section using the exact same encryption, compression, and data format (previously described) that DoppelPaymer uses to extract PEs from its sdata section. This observation ties this Dridex variant directly with DoppelPaymer. The Dridex sample was also unusual; not only because the Dridex loader was bundled with the bot core module (rather than dynamically retrieving it from a C2 server), but also because the bot core module had a version number of 2.0.0.78. We have seen subsequent updates to this new variant of the Dridex bot core module with the latest version being 2.0.0.80 at the time of writing. Of note, prior samples of Dridex had a version number of 4.0.0.87. It’s unclear why the malware author decided to use lower version numbers, but one explanation is that the threat actor views this new creation as “Dridex 2.0.”

Conclusion

Both BitPaymer and DoppelPaymer continue to be operated in parallel and new victims of both ransomware families have been identified in June and July 2019. The parallel operations, coupled with the significant code overlap between BitPaymer and DoppelPaymer, indicate not only a fork of the BitPaymer code base, but an entirely separate operation. This may suggest that the threat actor who is operating DoppelPaymer has splintered from INDRIK SPIDER and is now using the forked code to run their own Big Game Hunting ransomware operations.

 

Additional Resources

Appendix/Indicators

IndicatorDescription
801b04a1504f167c25f568f8d7cbac13bdde6440a609d0dcd64ebe225c197f9bDoppelPaymer SHA256 hash
813d8020f32fefe01b66bea0ce63834adef2e725801b4b761f5ea90ac4facd3aDridex 2.0 SHA256 hash
Table 6. DoppelPaymer and Dridex 2.0 IOCs

 
CRC32StringCRC32StringCRC32String
0xc622a2b1acronisagent0x5c6cd7acmsexchangeum0xe381a459epredline
0xa8e4e8c2backupexecagentaccelerator0xab07d275msexchangeumcr0xe7a6b2c5mozyprobackup
0x6d7d9112backupexecdevicemediaservice0xe3d46892mssqlserver0xdf73ec1cmasvc
0xfef41240backupexecjobengine0xf203a569msdtsserver0xcc5f5bf1macmnsvc
0x6c99d156backupexecmanagementservice0x6d90a649mysql570x467255e4mfemms
0x8ff434f5backupexecrpcservice0x2181c15eosearch150x0f2ae79cpsqlwge
0xc08e25a9backupexecvssprovider0xcdf97a8boracleclientcache800x7e26520aswprv
0x634332ffdfsr0xcaff10b3quickbooksdb250x656c0e35wsbexchange
0xfd7e1ab0epintegrationservice0x00c7b7a9spadminv40xde2373dewinvnc4
0x69e7bca5epprotectedservice0x66a8eeadspsearchhostcontroller

 

 

0x68507185epsecurityservice0x46b607c2sptracev4

 

 

0x5809f6f7epupdateservice0xdb1ac7bbspusercodev4

 

 

0xc2b55fa6mb3service0xf3c045e4spwriterv4

 

 

0xeca1f89emsexchangees0xd917e4cbsqlbrowser

 

 

0x44dda068msexchangemgmt0x23bc321esqlsafeolrservice

 

 

0xbebe6687msexchangemta0x9626475bsqlserveragent

 

 

0x03803c01msexchangesa0xf76fde75sqltelemetry

 

 

0x0de53e33msexchangesrs0x9626475bsqlserveragent

 

 

0x822dd426msexchangeadtopology0x25a92500sqlwriter

 

 

0xacedcdb8msexchangedelivery0x243d4975syncoveryvssservice

 

 

0x9060bcd4msexchangediagnostics0xc2a56207veeambackupsvc

 

 

0x50f0d551msexchangeedgesync0x8dbf54dbveeamcatalogsvc

 

 

0xa300bbb0msexchangehm0x82d1c632veeamcloudsvc

 

 

0x3040bb72msexchangehmrecovery0xb97407efveeamendpointbackupsvc

 

 

0x4014b792msexchangeis0x0aabacbaveeamenterprisemanagersvc

 

 

0x7e7e47bcmsexchangemailboxreplication0x43d71e6cveeammountsvc

 

 

0x23a626e2msexchangerpc0x0c6574adveeamnfssvc

 

 

0xa323c785msexchangerepl0x2491fd1cveeamrestsvc

 

 

0xbfec4da3msexchangeservicehost0xe076d4a9veeamtransportsvc

 

 

0xbe3d66d5msexchangetransport0xd67d1e60epag
Table 7. DoppelPaymer Email Server, Backup, and Database Software CRC32 Blacklist

 

CRC32StringCRC32StringCRC32String
0xae5a22b4dropbox.exe0xdc40adbaonenote.exe0x306d51a0sidebar.exe
0x6274fa64cis.exe0x4107aa76oracle.exe

 

 

0xf62526b9cistray.exe0xbfdf529epostgres.exe
Table 8. DoppelPaymer Antivirus, Backup, Database, and Windows Tool CRC32 Blacklist

 
CRC32StringCRC32StringCRC32String
0x45a1c197windefend0x0b4fa6cfmsmpsvc0xe067db30mcafeeframework
0x987163e9wdnissvc0x360b9799sentinelagent0xfc95ba9dmcafeeframeworkmcafeeframework
0x34220c33cylancesvc0xde3dabc7ekrn0x360b9799sentinelagent
0x59d2dbbfmbamservice0xb78f9b4ewrsvc0x3b9f1b3esentinelhelperservice
0x27462fffmbendpointagent0x23b07ca0vipre business service0xa6772c96sentinelstaticengine
0x93a7f221sbamsvc0x9a4f7f43mcafeeengineservice

 

 

Table 9. DoppelPaymer Endpoint Security Software CRC32 Blacklist

 

CRC32StringCRC32String
0xf26f12c8zonealarm.exe0xcff1c71efortiwf.exe
0x993f5471a2guard.exe0x64760001nortonsecurity.exe
0xd5345e50a2service.exe0x43c3c112bullguard.exe
0xc459d010a2start.exe0x0d71efa0bullguardbhvscanner.exe
0x0b02ef94avastsvc.exe0xa7dd5f59bullguardscanner.exe
0x21579df3avshadow.exe0x77a2fba9bullguardtray.exe
0x6b68c4c6avastui.exe0x50dbcbdabullguardupdate.exe
0x0108a03efortiesnac.exe0x6e7d6782avira.servicehost.exe
0x830b705afortiproxy.exe0xb8894b22avira.systray.exe
0xca2d58f0fortisslvpndaemon.exe0x40cb21d3avp.exe
0xe2c0fe91fortitray.exe0xb018d47embcloudea.exe
Table 10. DoppelPaymer Security Software CRC32 Blacklist 1
CRC32StringCRC32String
0x1a2124c0msascuil.exe0x895abd73nod32.exe
0x456b109fwrsa.exe0x2fba3706mcshield.exe
Table 11. DoppelPaymer Security Software CRC32 Blacklist 2
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