Unicode's "right-to-left" override obfuscates malware's filenames

Unicode has a special character, U+202e, that tells computers to display the text that follows it in right-to-left order; this facility is used to write text in Arabic, Hebrew, and other right-to-left scripts. However, this can (and is) also used by malware creeps to disguise the names of the files they attach to their phishing emails. For example, the file "CORP_INVOICE_08.14.2011_Pr.phylexe.doc" is actually "CORP_INVOICE_08.14.2011_Pr.phyldoc.exe" (an executable file!) with a U+202e placed just before "doc."

This is apparently an old attack, but I've never seen it, and it's a really interesting example of the unintended consequences that arise when small, reasonable changes are introduced into complex systems like type-display technology.

Some email applications and services that block executable files from being included in messages also block .exe programs that are obfuscated with this technique, albeit occasionally with interesting results. I copied the program that powers the Windows command prompt (cmd.exe) and successfully renamed it so that it appears as “evilexe.doc” in Windows. When I tried to attach the file to an outgoing Gmail message, Google sent me the usual warning that it doesn’t allow executable files, but the warning message itself was backwards:

“evil ‮”cod.exe is an executable file. For security reasons, Gmail does not allow you to send “this type of file.

Unfortunately, many mail applications don’t or can’t reliably scan archived and zipped documents, and according to Commtouch and others, the malicious files manipulated in this way are indeed being spammed out within zip archives.

(via Command Line)

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