Directory traversal attacks

Hi Ray, following my client's recent penetration test on their website I think I've now fixed all the vulnerabilities they picked up on, with your help which is much appreciated as always. I only have one outstanding item which I wondered if you could advise on? I use both ordinary download links to files and also download behaviors throughout the site. The pen tests have said this:

"It was possible to download arbitrary files from the web server by manipulating the file details being downloaded by the application. By utilising special characters to move through the directory tree of the web server it was possible to escape the directory being accessed to retrieve other files from the web server. Ensure that the application does not allow for special characters to be interpreted by the backend server when a user attempts to download files via the application."

They sent the attached screenshot to demonstrate. I think they're saying that download links have to be sanitised to remove special characters, however I'm not sure how I can do this and apply to existing download links/behaviors and for the links still to function properly. There's an article here: https://www.invicti.com/blog/web-security/directory-path-traversal-attacks/ but I'm not really any the wiser on how to fix this vulnerability. Can you help?

I'd have to see the download page itself. You probably have to check to see if there is a "../" in the path that would allow them to go to another directory.

I've attached the two pages they highlighted - one uses normal href links, the other uses a download behavior. The entire site uses relative links for the CSS & Javascript includes - so should these be changed to absolute links instead?

The issue is on the eldesk-tech-support.php page.

You are getting the name of the file to download from a hidden form element. That allows a would-be hacker to change the value of the hidden form element to download whatever file they want from your site.

Instead you should remove the hidden form element and get the file name from the recordset directly. So instead of this on line 17:

WA_DFP_DownloadFile("WA_DownloadResult1", "downloads/helpdesk/", "".((isset($_POST["hidden"]))?$_POST["hidden"]:"")  ."", "".((isset($_POST["hiddenname"]))?$_POST["hiddenname"]:"")  ."", false, false, false, "", "", "", "");
}

Instead use:

WA_DFP_DownloadFile("WA_DownloadResult1", "downloads/helpdesk/", "".($rsHelpdesk->getColumnVal('File',false))  ."", "".($rsHelpdesk->getColumnVal('Title'))  ."", false, false, false, "", "", "", "");
}

Then you can delete your two hidden form elements. That way the file name can't be manipulated. Paths in other locations aren't relevant and can be left as they are.

Ah OK, now I see - thank you for this!