Preventing Directory Traversal in PHP but allowing paths

Question

I have a base path /whatever/foo/

and $_GET['path'] should be relative to it.

However how do I accomplish this (reading the directory), without allowing directory traversal?

eg.

/\.\.|\.\./

Will not filter properly.

Answer 1

Well, one option would be to compare the real paths:

$basepath = '/foo/bar/baz/';
$realBase = realpath($basepath);

$userpath = $basepath . $_GET['path'];
$realUserPath = realpath($userpath);

if ($realUserPath === false || strpos($realUserPath, $realBase) !== 0) {
    //Directory Traversal!
} else {
    //Good path!
}

Basically, realpath() will resolve the provided path to an actual hard physical path (resolving symlinks, .., ., /, //, etc)... So if the real user path does not start with the real base path, it is trying to do a traversal. Note that the output of realpath will not have any "virtual directories" such as . or .....

Answer 2

ircmaxell's answer wasn't fully correct. I've seen that solution in several snippets but it has a bug which is related to the output of realpath(). The realpath() function removes the trailing directory separator, so imagine two contiguous directories such as:

/foo/bar/baz/

/foo/bar/baz_baz/

As realpath() would remove the last directory separator, your method would return "good path" if $_GET['path'] was equal to "../baz_baz" as it would be something like

strpos("/foo/bar/baz_baz", "/foo/bar/baz")

Maybe:

$basepath = '/foo/bar/baz/';
$realBase = realpath($basepath);

$userpath = $basepath . $_GET['path'];
$realUserPath = realpath($userpath);

if ($realUserPath === false || strcmp($realUserPath, $realBase) !== 0 || strpos($realUserPath, $realBase . DIRECTORY_SEPARATOR) !== 0) {
    //Directory Traversal!
} else {
    //Good path!
}

Answer 3

It is not sufficient to check for patterns like ../ or the likes. Take "../" for instance which URI encodes to "%2e%2e%2f". If your pattern check happens before a decode, you would miss this traversal attempt. There are some other tricks hackers can do to circumvent a pattern checker especially when using encoded strings.

I've had the most success stopping these by canonicalizing any path string to its absolute path using something like realpath() as ircmaxwell suggests. Only then do I begin checking for traversal attacks by matching them against a base path I've predefined.

Answer 4

You may be tempted to try and use regex to remove all ../s but there are some nice functions built into PHP that will do a much better job:

$page = basename(realpath($_GET));

basename - strips out all directory information from the path e.g. ../pages/about.php would become about.php

realpath - returns a full path to the file e.g. about.php would become /home/www/pages/about.php, but only if the file exists.

Combined they return just the files name but only if the file exists.

Answer 5

1

put a null index.htm for -Index block

2

filter sQS on start

// Path Traversal Attack
if( strpos($_SERVER["QUERY_STRING"], "../") ){
    exit("P.T.A. B-(");
}

Answer 6

When looking into the creation of new files or folders, I've figured I can use a two stage approach:

First check for traversal attempts using a custom implementation of a realpath() like function, which however works for arbitrary paths, not just existing files. There's a good starting point here. Extend it with urldecode() and whatever else you think may worth checking.

Now using this crude method you can filter out some traversal attempts, but it may be possible that you miss some hackish combination of special characters, symlinks, escaping sequences etc. But since you know for sure the target file does not exist (check using file_exists) noone can overwrite anything. The worst case scenario would be that someone can get your code creating a file or folder somewhere, which may be an acceptable risk in most cases, provided your code does not allow them to write into that file/folder straight away.

Finally so the path now points to an existing location, therefore you can now do the proper check using the methods suggested above utilising realpath(). If at this point it turns out a traversal has happened, you are still safe more or less, as long as you make sure to prevent any attempts writing into the target path. Also right now you can delete the target file/dir and say it was a traversal attempt.

I'm not saying it cannot be hacked, since after all still it may allow illegitimate changes to be done to the FS, but still better than only doing custom checks, that cannot utilise realpath(), and the window for abuse left open by making a temporary and empty file or folder somewhere is lower, than allowing them to make it permanent and even write into it, as it would happen with only a custom check that may miss some edge cases.

Also correct me if I'm wrong pls!

Answer 7

I have written a function to check for traversal:

function isTraversal($basePath, $fileName)
{
    if (strpos(urldecode($fileName), '..') !== false)
        return true;
    $realBase = realpath($basePath);
    $userPath = $basePath.$fileName;
    $realUserPath = realpath($userPath);
    while ($realUserPath === false)
    {
        $userPath = dirname($userPath);
        $realUserPath = realpath($userPath);
    }
    return strpos($realUserPath, $realBase) !== 0;
}

This line alone if (strpos(urldecode($fileName), '..') !== false) should be enough to prevent traversal, however, there are many different ways hackers can traverse directories so its better to make sure the user starts with the real base path.

Just checking the user starts with the real base path is not enough because a hacker could traverse to the current directory and discover the directory structure.

The while allows the code to work when $fileName does not exist.

Answer 8

I assume you mean without allowing users to traverse the directory yes?

If you are trying to stop your own PHP from traversing the directory you should just make the php work properly in the first place.

What you need to stop users is a modified .htaccess file...

Options -Indexes

(This all assumes you are talking about users)