I use the Exiv2 command line tool on Linux to edit image metadata like so:
exiv2 -M"set Iptc.Application2.Caption String This is my caption....." modify IMG.jpg
I want to execute this from PHP, using a caption provide by a user. This will work if the user enters no special characters:
exec('/usr/local/bin/exiv2 -M"set Iptc.Application2.Caption String '.$caption.'" modify IMG.jpg');
I need to allow the user special characters such as single and double quotes. I would like to use escapeshellcmd() to prevent malicious data. How can I correctly escape the command and the argument so that it works? I have tried many options but I can’t get it right.
Yes, this is a hard problem, because the command is using non-standard shell arguments (like it’s own little meta-language). ImageMagick has the same issues.
If you simply use escapeshellarg() within a double-quoted string, it is rendered useless. escapeshellcmd() does escape all special characters, and is safe for use in a double-quoted string. So you need to hard code single quotes around it to make it work right.
exec('/usr/local/bin/exiv2 -M"set Iptc.Application2.Caption String \'' . escapeshellcmd($caption) . '\'" modify IMG.jpg');
The reason escapeshellarg() will not work in a single quoted string is:
# for this input:
The smith's "; rm -rf *; echo "went to town
# after escapeshellarg()
'The smith\'s "; rm -rf *; echo "went to town'
# Works fine if left as a top-level argument
/usr/bin/command 'The smith\'s "; rm -rf *; echo "went to town'
# BUT if put in a double-quoted string:
/usr/bin/command "subcommand1 'The smith\'s "; rm -rf *; echo "went to town'"
# it is broken into 3 shell commands:
/usr/bin/command "something and 'The smith\'s ";
rm -rf *;
echo "went to town'"
# bad day...
What about using heredoc?
$str = <<<'EOD'
/usr/local/bin/exiv2 -M "set Iptc.Application2.Caption String $options" modify IMG.jpg
To modify this to use excapeshellcmd():
$options = excapeshellcmd($caption);
$command = <<<'EOD'
/usr/local/bin/exiv2 -M"set Iptc.Application2.Caption String $options" modify IMG.jpg
Because of Exiv2’s non-standard shell arguments, it is not easy to reach a simple and robust solution to handle user-supplied quotes correctly. There is another solution that is likely to be far more reliable and easy to maintain with a small performance penalty.
Write the Exiv2 instructions to a file
cmds.txt, then call:
exiv2 -m cmds.txt IMG.jpg
to read the instructions from the file.
Update: I have implemented this method and it requires no escaping of the user-supplied data. This data is written directly to a text file which is read in by Exiv2. The Exiv2 command file format is very simple and newline-terminated, allow no escaping within values, so all I need to do is prevent newlines from passing through, which I was not allowing anyway.