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Access denied - Are you sure?

Fri, 21/03/2008 - 04:23 — Heine

It is surely not my intend to discuss each and every security announcement, but the recent Live announcement (SA-2008-021) deserves to get some attention as this particular drupal_access_denied & drupal_not_found pitfall affects you too, especially when you are converting "arrow code" to use guard clauses.

// Arrow code.
// This drupal_access_denied / drupal_not_found pattern is common in Drupal modules.
function page_callback() {
  if (condition) {
    if (another_condition) {
      if (yet_another_condition) {
        return do_something();
      }
    }
  }
  drupal_access_denied();
}

// Guard clauses with a security vulnerability ahead.
function page_callback() {
  if (!condition) {
    drupal_access_denied();
  }
  if (!another_condition) {
    drupal_access_denied();
  }
  if (!yet_another_condition) {
    drupal_access_denied();
  }

  return do_something();
}

The pitfall: drupal_access_denied emits the access denied page, but does not end the request. This means do_something() is still executed, with potential security vulnerabilities as a result. When you actually test an access denied scenario and do_something returns a string, you'll notice you get a double page rendered; one part contains the access denied page, the other the output of do_something.

The remedy is easy, use return drupal_access_denied(); instead of drupal_access_denied();

// Guard clauses (these can often be collapsed into one if()).
function page_callback() {
  if (!condition) {
    return drupal_access_denied();
  }
  if (!another_condition) {
    return drupal_access_denied();
  }
  if (!yet_another_condition) {
    return drupal_access_denied();
  }

  return do_something();
}

Average: 5 (3 votes)

Comments

Fri, 21/03/2008 - 09:26 — peach - all drupal themes (not verified)

Hi, I just did a search in a

Hi, I just did a search in a folder with a bunch of drupal modules;

*.module; "return drupal_access_denied" : found 4 files

* .module; " drupal_access_denied" (note 2 spaces before function) : found 21 files

Does that mean all the modules in the second group are insecure? or possibly insecure?

Fri, 21/03/2008 - 13:09 — greggles (not verified)

maybe

It depends what comes on the following lines.

if it just does

  1. if (something) {
  3.   something_else_important();
  4.   return;
  5. }
  6. other_stuff();

Then the other_stuff is protected by the return on the following line.

Sun, 23/03/2008 - 02:06 — xamox (not verified)

I want to know how Heine has

I want to know how Heine has become such a security ninja. He seems to be the master at finding these security bugs. Props to you man.