Check if string is valid filename - php

Question

I have a question. I got a PHP script (PHP 5) which is saving a URL-Parameter $_GET['file'] to the variable $file. Is there now a way to check if this variable is a valid filename (for example: hello.txt and not /../otherdir/secret.txt). Because without checking the $file variable a hacker would be able to use the /../ to get to my parent folder.

Answer 1

You may have a look in php's basename function, it will return with filename, see example below:
$file = '../../abc.txt';
echo basename($file); //output: abc.txt
Note: basename gets you the file name from path string irrespective of file physically exists or not. file_exists function can be used to verify that the file physically exists.

Answer 2

POSIX "Fully portable filenames" lists these: A-Z a-z 0-9 . _ -
Use this code to validate the filename against POSIX rules using regex:
/ - forward slash (if you need to validate a path rather than a filename)
\w - equivalent of [0-9a-zA-Z_]
- . - dash dot space
$filename = '../../test.jpg';
if (preg_match('/^[\/\w\-. ]+$/', $filename))
echo 'VALID FILENAME';
else
echo 'INVALID FILENAME';
If you want to ensure it has no path (no forwardslash) then change the regex to '/^[\w\-. ]+$/'.

Answer 3

Instead of checking valid characters why not looking for character you don't want. Also filenames are limited to 255 characters:
function valid_filename(string $filename)
{
if (strlen($filename) > 255) { // no mb_* since we check bytes
return false;
}
$invalidCharacters = '|\'\\?*&<";:>+[]=/';
if (false !== strpbrk($filename, $invalidCharacters)) {
return false;
}
return true;
}
valid_filename('hello'); // true
valid_filename('hello.php'); // true
valid_filename('foo:bar.php'); // false
valid_filename('foo/bar'); // false
Adapt $invalidCharacters according to your needs/OS.
Source: https://www.cyberciti.biz/faq/linuxunix-rules-for-naming-file-and-directory-names/

Answer 4

Would that work?
http://php.net/manual/en/function.file-exists.php
E.g, in your case,
if(file_exists(str_replace("../", "", $file))){
// valid
}
else{
// invalid
}
Files can be in subfolders but not in parent folders.
However, if you're just interested in the filename,
if(file_exists(pathinfo($file, PATHINFO_BASENAME))){
// valid
}
else{
// invalid
}
should work.

Answer 5

i will like to combine kamal pal's and Pancake_M0nster's answers to create simple:
if(file_exists(basename($file))){
// valid
}
else{
// invalid
}