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64 Can a virus "hide" in a GIF or JPEG file?




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This article is from the Computer Viruses FAQ, by Nick FitzGerald n.fitzgerald@csc.canterbury.ac.nz with numerous contributions by others.

64 Can a virus "hide" in a GIF or JPEG file?

The simple answer is "no". The complete answer is more complex.

GIF and JPEG (.JPG) files contain compressed graphical information.
Every now and then, rumors arise that is possible to infect those files
with a virus in such a way, that it will spread when you display one of
these images. This is technically impossible--no part of the GIF or
JPEG format contains code that is executed by the viewer program.

It *is* possible to use the least significant bit of the color
information for each pixel in GIF files to store additional information,
without visibly altering the quality of the picture contained in the
file. This is called "steganography" and is sometimes used to transmit
secretly encrypted messages. Since a virus is nothing more than
information, it is possible to "encode" it into a GIF file and transmit
it this way. However, the recipients must be aware that the GIF file
contains such hidden information and take some deliberate steps to
extract it--it cannot happen against their will.

 

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