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/