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http://phptester.net/ yields the warning

WARNING base64_decode() has been disabled for security reasons

Why?

Besides the obvious vulnerabilities that have nothing to do with base64_decode (treating it as encryption, as a hash, evaluating base64_decoded data, etc) why would simply executing it be a security vulnerability?

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    Probably because phptester has some major vulnerabilities, and one of the exploits required in base64_decode, so they decided to disable it. Based on that theory, I wouldn't be surprised if their security was something like: if(strstr($code, "eval") !== false || strstr($code, "readfile") !== false || ...) {echo "Security fail!"; exit;} Jan 28, 2017 at 0:43

1 Answer 1

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why would simply executing it be a security vulnerability?

It's not.

base64_decode does exactly what you would expect: It decodes a string.

There are also no know vulnerabilities in it, nor have there been in the past (there was an integer overflow issue in base64_encode - CVE-2003-0861 - but PHP does not consider it a security issue).

I would assume that http://phptester.net/ bans it because it can be used to obfuscate data. They may have some filters in place to prevent execution of dangerous code and may worry that base64_decode could be used to bypassed these filters. I don't think that this is necessary or useful, but that might be their train of thought.

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    Arguably the dumbest part of that line of reasoning is that a malicious user could probably just write their own decoder.
    – jpmc26
    Jan 27, 2017 at 22:09
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    @jpmc26 Someone actually wrote most of PHP in PHP, so yeah... The work is already done! :P github.com/ircmaxell/PHPPHP
    – zenware
    Jan 27, 2017 at 22:59
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    phptester.net does not allow eval anyways, so it's not like you can stuff the output of base64_decode into eval. Jan 28, 2017 at 0:12

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