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I tried to send someone an email with a PNG format image attachment but it was stopped by their email filtering software (messagelabs).

They said I could send them the image in a JPG or ZIP format file.

I'm assuming they are blocking this format for security reasons. But, what about the PNG format is potentially unsafe?

Are there any known security vulnerabilites in it? (I've only found something very old.)

Is it possible to include executable or other malicious code in the format?

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Pretty much any file format can be used to breach security. –  Daniel Beck Dec 9 '10 at 17:36
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up vote 4 down vote accepted

It's more likely that they have a whitelist allowing certain extensions, as opposed to a blacklist blocking certain extensions. It's easier to maintain for them and lowers the vector of attack for them.

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you could have upset their image/content control setting. –  mic84 Dec 9 '10 at 17:59
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+1 as this is typically what I would expect to see messagelabs do on request. They will have a list of the file types their client wants, and block everything else. –  Rory Alsop Dec 9 '10 at 18:01
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At least one vulnerability was actually announced for this a few years ago: US-CERT: Microsoft Internet Explorer buffer overflow in PNG image rendering component. HTML e-mail with PNGs was a possible attack vector.

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