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              Threat Analysis

              Cryzip Ransomware Trojan Analysis

              • Date: March 11, 2006
              • Author: Joe Stewart

              Summary

              In May 2005, a trojan called PGPcoder was discovered in the wild by Websense Security Labs. The trojan's purpose was to encrypt a user's files, then demand a ransom for their decryption. Although this scheme seemed novel, it is actually predated by over 15 years, by a similar scam in 1989. SecureWorks' research team has now discovered a third such scheme involving ransomware which we are calling Cryzip.

              Unlike PGPcoder, which used a custom encryption scheme (which was subsequently reverse-engineered by SecureWorks) Cryzip uses a commercial zip library in order to store files inside a password-protected zip. Although the zip encryption is stronger, a brute-force attack is still possible on the files, especially if one has a copy of the original file inside the zip.

              File Details

              Filename: vcmauth.dll
              Filesize: 1,191,936 bytes
              MD5: 86a48836bced8c4a0b59fca972800890
              SHA1: 0b3a49b3172fc65db607fcb1b8029820ec11c5b6
              Packer: none
              Compiler: Visual C++ 6.0
              Compile Date: Thu Mar 2 18:11:02 2006
              CME Number: none assigned
              Identifying Strings:
              • zippo.dll
              • ZippoCrypt
              • _zippo_crypter_v1.0_

              Analysis

              When run, Cryzip searches the C: drive (except for files in directories named "system" or "system32") for files which it will zip, overwrite with the text "Erased by Zippo! GO OUT!!!", and then delete, leaving only the encrypted zip file with the name original-file-name_CRYPT_.ZIP, where original-file-name is the original file name complete with the file extension.

              Cryzip searches for and zips files with the following extensions:

              • .arh
              • .asm
              • .arj
              • .bas
              • .cdr
              • .cgi
              • .chm
              • .cpp
              • .db1
              • .db2
              • .dbf
              • .dbt
              • .dbx
              • .doc
              • .dpr
              • .dsw
              • .frm
              • .frt
              • .frx
              • .gtd
              • .gzip
              • .jpg
              • .key
              • .kwm
              • .lst
              • .man
              • .mdb
              • .mmf
              • .old
              • .p12
              • .pas
              • .pak
              • .pdf
              • .pgp
              • .pwl
              • .pwm
              • .rar
              • .rtf
              • .safe
              • .tar
              • .txt
              • .xls
              • .xml
              • .zip

              After it has finished processing a directory, Cryzip leaves a text file in the directory named AUTO_ZIP_REPORT.TXT, which contains the following text:

              OUR E-GOLD ACCOUNT: XXXXXXXINSTRUCTIONS HOW TO 
              GET YUOR FILES BACKREAD CAREFULLY. IF YOU DO NOT UNDERSTAND, 
              READ AGAIN.This is automated report generated by auto archiving 
              software.Your computer catched our software while browsing 
              illigal pornpages, all your documents, text files, databases 
              was archivedwith long enought password.You can not guess the 
              password for your archived files - passwordlenght is more then 
              10 symbols that makes all password recoveryprograms fail to 
              bruteforce it (guess password by trying allpossible combinations).
              Do not try to search for a program what encrypted your information - 
              itis simply do not exists in your hard disk anymore.If you really 
              care about documents and information in encrypted filesyou can 
              pay using electonic currency $300.Reporting to police about a 
              case will not help you, they do not knowpassword. 
              Reporting somewhere about our e-gold account will not helpyou to 
              restore files. This is your only way to get yours files back.
              ------------------------------How to pay to get your information 
              back.1. click on this link to open your free e-gold account - 
              the first   screen is the e-gold "terms and conditions" page. 
              You need to   agree to these by clicking on the "I AGREE" button 
              on the bottom   on the page.2. On the next page is the sign up form:    
              1. "Account name" - here is where you name your account - tip:        
              make it easy to remember (as you will be asked for it) and          
              reasonably short, example, "John's e-gold", "My Money e-gold"        
              or perhaps "Felix" (whatever you like, just make it easy for        
              you to remember it).    2. "User Name" - here just repeat the account 
              name (from 1 above).    3. "Point of Contact" - this is where you 
              put our name, address,        phone number and email address 
              (any email address can be used        here but it is recommended 
              you use your ISP address - not a        free hotmail, etc address).        
              It is also recommended your also include a fax number        
              (don't have a fax number? This company offers free fax to email        
              services). Try and make it as easy as possible for e-gold to contactyou.    
              4. "Passphrase" - this is the most important piece of information        
              connected to any e-gold account. We can not stress enough how        
              important it is that your passphrase is kept safe and secure.    
              5. "Turing Number Entry" - type the 6 numbers you see there into theinput        
              box below.    6.  The last step click "Open"On the next page 
              it will tell you that your e-gold account number has beenemailed 
              to you.check your email - you can expect to wait up to 5 minutes 
              for your accountnumberto arrive. If it does not arrive after 
              5 minutes then that means the emailaddressyou supplied was 
              incorrect and you will have to open another new account 
              (gothroughand repeat what you just did above again).To buy e-gold to 
              your account please use official exchange services
              http://www.me-gold.com/http://www.goldex.net/http://usece.com/or try 
              to search own way withhttp://gold-pages.net/e-Gold__1MDC__Pecunix_Wizard_
              Links/Purchase_E-gold/index.htmlhttp://www.google.com/search?hl=en&q=buy+
              e-gold&btnG=Google+SearchFINALLY when you bought e-gold you have to transfer 
              $300 to our e-goldaccount.In next 24 hours you will recieve $1 back 
              to your account. Transfer detailsof this $1 transfer will have a link 
              to software that will automaticallyunzip all your files back to normal 
              state.Next day login to your account https://www.e-gold.com/acct/
              login.html,press History and press submit, you will see LINK TO 
              UNZIP-software.
              ##########################################################################
              Remember you are just $300 away from your files
              ##########################################################################
              
              

              At the top of the AUTO_ZIP_REPORT.TXT file, the number of an E-Gold account is inserted. This number is picked at random from a list embedded in the DLL. By operating many accounts simultaneously, the trojan author is betting that even if E-Gold shuts down some of the accounts, he/she will still receive payment on some of the others. The complete list of E-Gold accounts is:

                293436329175012917
                50529175102934
                36929343762934380
                29343822934383293
                4389293439229343942
                93439629344042934409
                2934419293442129344
                252934427289
                7227293443028971912
                89719329344352897209
                289721229344412
                897232293444629
                3444828972432897
                2582934452289702
                12917497293435429
                343562917500289726
                329344552934459293
                44662934469293447729
                344912934501293450
                6293451029345152934474
                29347822934788
                29347992934
                80629348142934
                81629348202
                9348252934829
                293483229348
                37293484129348
                49293485329
                34860293486229
                34866293487
                22934869293488
                52934880293
                48912934895293
                48982934903
                293492529
                349292934938
                29349482934
                9532934956
                29349642934
                4802934487
                29347752934
                80229348112
                93486429352
                772935274293
                526829352642
                935260293525
                2293524429352
                3529352322935
                2292935223

              The text of the AUTO_ZIP_REPORT.TXT file is encrypted inside the Cryzip DLL, using simple XOR (0x13) encoding. The password used to zip the files is also embedded inside the DLL but it is not encrypted - instead, the author decided to hide the password in plain sight, so to speak. The password is:

              • C:\Program Files\Microsoft Visual Studio\VC98

              Because this string often appears inside projects compiled with Visual C++ 6, the author likely figured anyone who found the infecting DLL and examined its strings looking for the password would simply overlook it.

              Conclusion

              At this time the infection vector is unknown. Infection reports are not widespread, so it is not believed this is a mass threat by any means. Malware of this nature is actually more successful when it is delivered in low volumes, as it is less likely that anti-virus vendors will have detection for it, and more attention means the likely closing of the accounts used for the anonymous money transfer. As such, most users will probably not have to worry about this threat - keep in mind however that the two incidents in the last 10 months indicate the possible start of a trend of this type of malware, and future incidents may affect a wider swath of users. However, in most cases, simply having and using proper backup software would mitigate the risk from ransomware.

              Update - March 22, 2006:

              The infection vector has been established to be recent Bagle virus runs. Starting in February and into March, a group of Bagle variants was seen which do not resemble other modern Bagles. Based on our research, we have determined that the author of Cryzip took the circa-2004 source code of Bagle, and adapted it to his/her own purposes. Part of the variants were used to spread a variant of Sality, among other files, downloaded primarily from invis1lblearm3333.com. The other variants directly downloaded an executable which installs Cryzip on the system, downloaded primarily from egozda.com. The author uses a specific User-Agent string in the download requests which is checked by the script serving the executable - therefore attempts to manually download the Cryzip dropper were unsuccessful unless the requestor used the proper User-Agent.

              Further evidence in the binaries indicates that Cryzip and Sality are probably written by the same individual. In one example of this, the same psuedo-random number generator (PRNG) code was used in both Cryzip and Sality. The PRNG was apparently cribbed from the Mydoom source code, which would put it in wider use than just Cryzip and Sality - however, both Cryzip and Sality have minor variations in the binary code of the PRNG caused by a certain non-default compiler optimization setting which is uncommon even in Mydoom or Mytob variants which use the same PRNG. Based on other code similarities and the fact both were spread by open-source Bagle variants around the same time which were packed with the same unique executable packer, gives strong credence to the idea that they share the same author.

              Update - May 22, 2006:

              A second Cryzip variant has been released. This time, instead of storing the password in the trojan, the author uses a list of passwords which are served by a PHP script on a remote site. The trojan downloads the password dynamically and uses it to encrypt the files. Currently the website which served the passwords during the initial infection phase simply redirects to a porn site.

              Due to the ephemeral nature of the password retrieval, users who are infected with this variant of Cryzip will probably not be able to find the password. If they do not wish to pay the ransom, the only other option is to brute-force the password. This is unlikely to work unless the user has a backup copy of one of the files inside one of the password-protected zip files. If so, it may be possible to do a "known plain-text" attack against the zip encryption for that file, using a tool such as Elcomsoft's Advanced ZIP Password Recovery tool. Note that this tool is commercial software, and costs $30 for a Personal license, and $60 for a Business license.

              If a user does not have an original copy of one of the files inside the zips, it is unlikely they will be able to break the password encryption unless a) they pay the ransom or b) the trojan author is arrested and forced to divulge the passwords. While the latter option is not very likely, affected users are still encouraged to report the crime so that the authorities may be more likely to open a case.

              You can report Internet crime online at http://www.ic3.gov/

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