Didier Stevens

Monday 29 February 2016

More Obfuscated MIME Type Files

Filed under: maldoc,Malware,My Software,Update — Didier Stevens @ 20:57

I’m providing a 2-day training at Brucon Spring Training 2016: “Analysing Malicious Documents“. Use promo-code SPRING16 for a 10% discount.

I received a maldoc sample (MD5 FAF75220C0423F94658618C9169B3568):


You can see it’s a MIME Type file, and that it is obfuscated. The second line is a very long line of seemingly random letters and digits. This throws of Python’s MIME parser used by my emldump tool:


emldump just detects this as a text file, and not as a multipart MIME Type file.

If we remove that second line, for example with findstr /v (or grep -v), emldump recognizes the different parts:


Since obfuscated MIME Type files are becoming more and more prevalent, I’m adding a filter option to emldump to filter out lines that obfuscate the MIME Type files. For the moment, option -f throws out lines longer than 100 characters and header lines that are not fields (just like option -H).

This new version of emldump.py detects some (simple) types of obfuscation:


And with option -f you can filter out these obfuscating lines:



emldump_V0_0_7.zip (https)
MD5: 819D4AF55F556B2AF08DCFB3F7A8C878
SHA256: D5C7C2A1DD3744CB0F50EEDFA727FF0487A32330FF5B7498349E4CB96E4AB284


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