A quick read of the SANS Forensics blog, courtesy of Gregory Pendergast, and you'll get a feel for all the positive feedback for Lenny Zeltser's REMnux.
Lenny has dedicated himself to furthering the malware reverse engineering cause, both as a teacher and analyst; his SANS courses are popular for good reason.
September's toolsmith covers REMnux and offers some detail specific to its use.
One area I often use REMnux for is malicious Flash analysis.
Evil Flash, distributed in particular via online advertising platforms, is a constant concern for online providers. Suffice it to say that my team has encountered such problem children more than once. ;-)
As an example, an older sample (MD5: 525445764564B34070CF2F9DCC6C2DAA) makes for a great test case. You can grab the sample for your own testing at OffensiveComputing.net.
Imagine you've grabbed the sample via wget from your REMnux VM, after proxy-based analysis of the malicious URL.
A simple check for interesting results might be the likes of
flasm 525445764564b34070cf2f9dcc6c2daa.swf, which would result in a .flm file named identically for SWF file analyzed. Figure 1 shows the concatenated results.
While flasm is convenient, the preferred method would be
swfdump -Ddu 525445764564b34070cf2f9dcc6c2daa.swf
The -D switch provides full (everything) output, the -d switch prints the hex output, and -u shows the Tag IDs.
Figure 2 offers the results.
Note that that the DEFINEBUTTON2 config for Tag ID 4 grabs an URL then issues the ActionScript FSCommand:exec to execute arquivo.scr (never a good thing).
Tag ID 4 was conveniently named "bot" by its creator; why bother hiding, right?
With a modicum of effort, maliciousness confirmed, you're ready to take action: report the malicious SWF to the provider, or remove it you are the provider.
You'll enjoy REMnux; it's an excellent collection of useful tools gathered in a simple but functional distro.
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