Spammers use unicode trick to manipulate file names

Posted by   Virus Bulletin on   May 16, 2011

Windows executable appears to be Word document.

An unicode trick that reverses the order of some characters has been used by spammers to disguise malicious attachments as harmless file types.

Unicode enables the representation on computer screens of a very large and still growing number of alphabets, together with many special characters. Among this latter category are some 'characters' that reverse the order of the the visible characters. While this may be useful for some applications, it has now been abused by spammers to hide the file type of malicious attachments.

Security vendor Norman reports having seen some samples where a Chinese trojan - in this case a Windows executable - was attached to emails, with the .exe extension instead made to appear as .doc, making it look like a Word document. While Word documents may well contain malware themselves, they are less likely to trigger alarms than executables. The spammers must have expected some users to open them, thus infecting themselves with a nasty trojan.

As the special characters only alter the way the file is displayed, filters blocking executables are unlikely to be vulnerable from this kind of attack. Moreover, those Windows versions that are vulnerable (in the default settings only Vista and Windows 7), do display the file type correctly.

Nevertheless, this should be a warning to users that any kind of file has the potential to be malicious. And that basing one's trust on file extensions is a bad idea.

More at Norman's blog here, which also includes an example of how a screensaver can masquerade as a JPEG image.

Posted on 16 May 2011 by Virus Bulletin

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