1

I recently began storing image uploads outside the webroot and retrieving them via readfile(). I implemented this and it's working fine, but I was curious if there are any further vulnerabilities that still exist in this method.

If I take the contents of a malicious script from outside the web-root via readfile() and output it on a php script that has an image content-type will this force the browser to always interpret this data as an image or is there some way a user could circumvent this and use it to run a malicious script?

3

Reading the file from outside the webroot with readfile(), it won't be able to run a malicious script on the server.

Providing the proper content-type means that it won't be mishandled as something else in most cases.

However, there are still some cases such as Gifar where a polyglot content is provided in a way that can affect the user (these are actually problems in Java or Flash, but breaking the same-origin), so you should check that the content really is what it to be, and it is not one of certain undesirable formats (for instance, zip-based formats are quite problematic).

1

Storing outside the document root and using readfile() is a great way to protect the server.

Additionally, to protect the client, don't serve HTML files verbatim (always send a Content-Type: text/plain), or else you'll open the door for phishing attacks. (I'm not sure if the security vernacular has a precise term for this; it's close enough to a watering hole attack, but involves phishing emails to a URL on your server that directs the user to the attacker's server when they supply their credentials.)

  • I'm by no means a security expert. Could you elaborate on serving HTML files as text/plain for security? Does that just involve changing a <meta> tag? – DanL Mar 4 '16 at 14:06
  • Look up the header() function. – Scott Arciszewski Mar 4 '16 at 14:27
  • Sorry, perhaps I was unclear. I'm familiar with setting headers and content-types, but I'm asking, is it just as simple as changing that - are there any adverse affects of serving HTML documents as plain-text? Will it cause any abnormal Javascript/CSS/etc mishaps? – DanL Mar 4 '16 at 16:14
  • As I'm searching, I cannot find the security implications of serving HTML as plain-text, it seems to only be brought up as an error or fault. This is why I'm a bit confused, have never heard of this method and just need some more insight please. – DanL Mar 4 '16 at 16:18
  • It's simple: Instead of your browser rendering it as a web page, the user will see the source code as if they were viewing a text file. I don't know if this has been published anywhere but I have a blog post on securing file uploads. – Scott Arciszewski Mar 4 '16 at 16:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.