Hardening new Roles system

New roles add great freedom when assigning rights to students. The problem might arise when students are assigned permission that allows adding of content that is not cleaned before display - such as editting Resources, adding activities, etc. They could then use any type of XSS attack to gain full administrative access without any restrictions.

Proposed solution 1

Assign trust level to each user and capability. Add trust level checks to has_capability() and require_capability().

Implementation

• define basic trust levels (as integer constants)
  • minimal - not logged-in, guests
  • standard - students and non-editing teachers (must not be able to add HTML with javascript, can not upload files to coursefile area)
  • high - teachers adding active content and handling sensitive information (backups, editing of activities and resources, uploading of course files, creating courses, etc.)
  • absolute - usually administrators and trust level managers only
• add new column trustlevel to user table
• add new column requiredtrustlevel to table capabilities
• fix role management GUI
  • indicate required trust level next to each capability (different color and label or icon)
  • allow filtering of capabilities based on trust level required
• add moodle/site:managetrustlevel with required absolute trust level
• add trust level management GUI
  • predefined trust level for new users
  • changing of trust level (also from user/edit page)
  • request trust level change form (something like new course request)
• add trust level checks to has_capability() and require_capability() (veto when user does not have required trust level)
• assign levels based on legacy capabilities during upgrade
• do security audit of each capability in modules and core; set proper required levels (this is going to be the hardest part)

Benefits

1. Easy to implement, administer and explain to teachers compared to implementation based on capabilities.
2. Trust level manager has full control over potentially dangerous capabilities - it is necessary for large sites (or connected sites in the future).
3. Trust level mechanism can be turned off by assigning high level to all users except admins - needed for small insecure workshop sites.
4. Security audits could concentrate on standard and minimal capabilities.
5. Module authors will be forced to think about security when defining capabilities.

Proposed solution 1a

This proposal is pretty good, in that it is simple, and will work. I just worry that it is just tring to preseve the existing 7 roles:

minimal

guest

standard

student, non-editing teacher

high

editing teacher, course creator

absolute

admin, oringinal admin

Of course our current security model is build round these concepts, so changing it is a risk. Also, it is clearly not future proof to have 4 hard-coded levels.

As I understand it: the new roles system is secure with the default settings, and it remains secure if the roles and permissions settings at a site are only editing by people who fully understand all the consequences of their actions. It is just the the new system is so powerful that it makes it too easy for people to screw up. Therefore, we need to protect them from themselves. Do I understand the problem correctly?

However, the new roles system is meant to make things more flexible, and at a large installation you might want to trust some people with some of the things you classify as high rist, but not others. For example, you might want to trust them to edit or upload uncleaned content, but not to handle backups nor create courses. Under your model, you would have to give these people high trust level, and then it would be possible give them capabilities that we would rather they did not have by mistake.

So can I suggest a slightly more flexible (and hence slightly more complicated solution). Instead of trust levels, have risks. For example:

• Access to sensitive information
• Add uncleaned content
• Destroy the site

Then for each user, list which risks you trust them not to exploit maliciously. For each cabability, list which risks it opens up.

All the checks you want to perform are still possible under my system. Explaining it is still easy, if not easier, because you can explain the possible consequences of trusting someone one risk at a time. Basically, instead of using integer trust levels and >= operator, we are using sets of risks and the superset operator. Very similar to implement, but much more flexible when we discover new classes of risk in the future.

Proposed solution 1b

I like the idea of the risk system but I wonder if adding it to all users right now will cause a new interface overload. The interface for assigning roles could also be a bit scary to think about.

As a first step, a simpler idea might be just be to add a column to the capabilities table called "risk" which defaults to "0" but could be set to higher numbers that are a mask (list of flags) with the kinds of risk this capability includes.

On the capability screens (eg role definition and override screens) any capabilities with a higher-than-zero entry could flags or colours or warnings next to it to help the user know that these capabilities had possible security or privacy issues that they should be aware of (linking off to the docs wiki for more details).

This is exactly what I am proposing in 1a. Just you call them flags, I call them risks. Your proposed capability screen UI is exactly what I had in mind.

Oh, I see, the bit that 1b is missing is Petr's bit where you give each user a trust rating, the the system double-checks and prevents the user being allowed access to a capability that exceeds the user's trust rating.

Proposed solution 2 (independent of above)

1. Have one new capability called "moodle/site:trusttext".
2. Certain roles who you trust to edit text and allow to have Javascript, EMBED etc can have permission for this capability set to "allow" (these people are generally teachers).
3. When saving a text from a user, modules can call a function on the text and insert a special tag (eg ####TRUST#####) if the current user is trusted (and actively REMOVE all such tags if the user is NOT trusted).
4. When displaying the text with format_text(), a new parameter needs to be passed to enable the trust system ($options->usetrust = true).
5. If $options->usetrust is present, each text is checked for ####TRUST#### in the text and output is cleaned appropriately, depending on whether the tag was found. If this new parameter is not present (eg in old modules or 3rd party modules) then all such tags are removed and the text is always fully cleaned.
6. For caching, the text is stored in the final form as it is now.

Example

Storing the text

Here is the original text from the user:

  Elephant says <script>alert('hello')</script>

Here it's converted before storage:

  $text = apply_trust_to_text($text);

For a trusted user, this will return this text for storage:

  ####TRUST####Elephant says <script>alert('hello')</script>

For an untrusted user, this will return:

  Elephant says <script>alert('hello')</script>

Showing the text

If the tag is found AND the code is trust-enabled then cleaning is NOT done, and only that special tag is removed before output.

 Elephant says <script>alert('hello')</script>

If not found then cleaning is done fully (as it is now in Moodle):

 Elephant says