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V4/URI-based HBAC

Name: V4/URI-based HBAC
Ticket: #5030
Author: lhellebr
Incomplete.png Pending review
Last updated: 2016-04-25 by Lhellebr

Overview

A feature allowing Host Based Access Control to decide based not only on triplet (user,service,hostname), but also on URI of the requested resource.

Use Cases

In some services, it may be useful to base authorization decision on what resource within the the service is being accessed.

For example, it is very common in web applications that different parts of the web (with different URIs) are meant to be accessed by different users: there is some public part, some user-only part, some part that should be accessible to administrator only...

Consider a webserver with the following URIs:

http://webserver/application/public-content
http://webserver/application/login-only/all-users
http://webserver/application/login-only/user/user1
http://webserver/application/login-only/admin

Each of these parts needs different access rights; so far, HBAC only supported deciding whether an authorized user is allowed to access the whole non-public part of the web. Obviously, it is possible for the web application to handle access rights of the users. It is, however, a good idea to also handle this on level of service (Apache) for consistency, centralization and security on multiple levels.

This feature is a RFE for ticket #5030.

Design

For this feature, we need to add the URI to HBAC rule in FreeIPA, make SSSD able to determine authorization based on it, and give services some way to communicate with SSSD in terms of URI-based HBAC.

Schema

URI is a part of HBAC rule. It is interpreted as a prefix which decides whether the HBAC rule that would otherwise match (if it wasn't for this feature) matches (the URI attribute is a prefix of resource's URI) or it does not (the URI attribute is not a prefix of resource's URI). FreeIPA itself merely stores this attribute and allows its adding, deletion and editation; it doesn't act based on it in any way. Because there are two parts of URI with different interpretation and meaning (host, schema, port VS path - the rest of URI), we store these two parts separately - the first one as a case-insensitive UTF-8 string and the second as case-sensitive UTF-8 string.

SSSD is a daemon running on service's local machine and deciding based on HBAC rules. It is also possible to use other approaches, for example the application can ask FreeIPA's LDAP and decide itself based on provided data. In case of SSSD, there is already HBAC capability - we need to ensure it becomes URI-aware and it gets important data from both sides - the URI prefix from FreeIPA and the URI to match from the service requesting authorization. We can achieve this using already-existing channels improved to also transfer URI. The prefix is an attribute of HBAC rule and can be retrieved from FreeIPA's LDAP using LDAP provider (URI would be accessible by anyone who has access to the HBAC rule). There are multiple ways of getting the accessed URI from the service.

By using SSSD, it is not required for the service requsting authorization to be able to decide based on authorization data it obtains itself. While there are some minimal changes necessary (in case of Apache, an authorization module must exist), the service merely needs to ask for authorization (providing, user, hostname, service, URI) and receive result.

There are multiple ways of communicating with SSSD, PAM and Infopipe were considered. As there are already some applications using PAM and it seems to be a good interface for this use case, SSSD expects the application to use PAM, setting PAM environment variable to URI of the requested resource. PAM responder uses system bus to communicate with IPA provider in SSSD.

Sssd pam.png

Evaluation

There are multiple possible approaches to comparing the requested resource's URI with URI in HBAC. In all of them, I use only the part of URI after hostname as hostname and service are already matched as part of selecting HBAC rules to evaluate in terms of matching URI.

  • Equal string - only match an HBAC rule if there is the same string (maybe case insensitive comparison?) as the requested URI; that would mean a lot of HBAC rules differing only in specified URI and is probably not a feasible approach
  • Prefix - match HBAC rule <=> HBAC rule URI is a prefix of requested URI. The problem is, in web applications, the longer URI usually means stricter access rules. This means we must do the longest-prefix matching. This is the solution I have chosen: matching any rule with host+scheme+port exactly equal to the requested scheme+host+port and having the URI's path that is the longest prefix of the requested URI's path. (See examples below)
  • Regular expression - the most powerful interpretation of URI in HBAC rule, Perl-compatible regular expression should be able to cover any situation admin should need. They even support lookarounds which is very useful, see example. PCRE library is already used in SSSD so this adds no dependency and is a big boost to usability of the feature.

Example - how it can not work

Consider webapp that has, among others, these pages: http://hostname.net/app/auth/user1 http://hostname.net/app/auth/user42 http://hostname.net/app/auth/admin

We want to set the following access rights:

user’s name except admin.

can access anything with prefix http://hostname.net/app, only admin can access http://hostname.net/app/admin).

As we see, the longer the URI, the stricter access control rules. This leads us to a concept of prefix-matching the URI’s: whenever the URI in rule is a prefix of the requested resource’s URI, the rule matches in terms of URI.

The way HBAC rules are interpreted, however, is currently such that whenever any HBAC rule matches, the access is allowed. It is a correct behavior when only con- sidering (user,service,host), but it causes a problem when trying to include URI as a matching parameter. As shown in the last item of a goal list, we need to have a way to allow every URI with certain prefix A except URI’s with certain prefix B where A is a prefix of B. In other words, we need to exclude a subset from a set of URI’s described by certain HBAC rule. This is not possible when matching any one rule causes access authorization.

How the rules would look (omitted HOST any SERVICE any everywhere):

This does not work ! While the sec ond rule only allows admin to access http://hostname.net/app/auth/admin, the first rule allows any user to access everything with prefix http://hostname.net/app/auth/ , including http://hostname.net/app/auth/admin . We can accidentally allow access to larger set than intended and there is no way to set exceptions from that set. In this example, there is actually no way to set the rules correctly so they achieve the goal, except using every possible prefix other than the intended exception.

To solve the problem of exception from a set of allowed URI’s, we could come up with a concept of DENY rules. The approach would mean allowing access when any ALLOW rule matches and no DENY rule matches. A DENY rule would otherwise be the very same rule as an ALLOW rule. That would not be completely new for FreeIPA – at certain point in time, there actually were both ALLOW and DENY rules.

DENY rules were, however, dropped from FreeIPA. The reason for this is that we believe that access rules should always be described positively – listing all accesses that are allowed, rather than listing what is not allowed and thus risking we forget something or make a mistake that would allow access that should not be allowed. Another reason is that when we, for some reason, don’t evaluate an ALLOW rule, the result is denial of service at worst, while failing to evaluate a DENY rule could allow access that should not be allowed. It seems DENY rules are absolutely not intended to be added again.

Furthermore, merely adding DENY rules would not be sufficient; for example, there would be no easy way to come up with rules for our example. We would need to deny access to http://hostname.net/app/auth/admin to large or infinite number of users as the access would by allowed by first rule. The rules would look something like:

This could be solved by only matching the user-wise most specific rule or giving the rules some order, e.g.:

This would be a fully working solution, allowing exceptions, describing infinite number of cases (both URI- and user- wise) in a relatively small number of rules, and relatively readable. Still, there are drawbacks:

  • It is not easy to determine a rule to compare which one of the rules is more

specific user-wise. It would also be very error-prone.

  • Adding order to rules would mean a significant change in their semantics which

would be hardly accepted

  • DENY rules will probably never be accepted
  • There are better and simpler solutions, described further

Example - how it works

Using the previous notion, we would in many cases create a pair of rules for subsets we wish to exclude some users from – an ALLOW rule allowing access to certain subset of users, and a DENY rule which is the same except it denies any user access to the same location (which is necessary in case there is an ALLOW rule allowing access to some URI which is a prefix of this location’s URI). The more specific or latter of those (depending on which approach we would choose) two rules would be the ALLOW rule and the result would be only allowing access to that URI to certain users.

In previous example, this exactly happens: rule 1 allows access to http://hostname. net/app/auth to anyone and to allow access to http://hostname.net/app/auth/ admin to admin only, we first need to deny everyone access there by rule 2 before allowing it again for admin only by rule 3.

It is easy to understand why the DENY rule could be there implicitly – when admin allows access to some resource to some user, he means that user only and all other users should be denied. However, there is another rule that allows access to anyone - the first one. To solve this problem, we can state that we only want to decide based on the rule with longest prefix match. Even if there are multiple rules matching, we are only interested in the most specific one. This allows us not to use DENY rules at all because when there is no ALLOW rule, access is denied implicitly, and the more general rule allowing access to a superset of the more specific rule would be ignored. We could use the same rules as in the previous example, just ignore the ordering and drop the DENY rule:

The first rule allows anyone access to http://hostname.net/app/auth, except for those URIs which have URI http://hostname/app/auth as a proper prefix. The second rule’s URI is the first rule’s URI’s proper prefix, thus the first rule is ignored for any URI matching URI of the second rule, regardless whether the first rule’s URI matches or not. This serves as implicit deny for everyone if their access does not match rule 2, regardless whether it would match rule 1 or not. Rule 2 then allows admin access to http://hostname.net/app/auth/admin, the implicit DENY making this the exclusive access right for admin.

Compatibility

If there is no URI in PAM request, we match any HBAC rule that would match without this feature. In that case, we presume the application is not interested in URI. This means effectively ignoring URI and matching even rules with non-matching URI when specific URI is not requested.

This is a solution I picked because of backwards compatibility - because we of course can not change behavior of previous versions and these versions, not aware of URI-based HBAC, would allow any rule that matches in terms of other attributes: user, host and service.

This solution might cause problems and is not ideal: it might be seen as problematic that when the service does not ask for any specific URI, the is always granted if there is any rule matching in terms of other attributes, even if it does not match URI-wise. Also, when using old version of pam_sss or SSSD, the same situation happens as if the application didn't include URI in the request.

I haven't however, found any better solution that would be fully backwards-compatible. I'd be glad for suggestions.

Feature Management

UI

There are two new fields in HBAC rule details for adding URI separated into two parts: scheme+host+port and path

CLI

There are subcommands for "ipa" command to list and modify URI, these are generated automatically.

How to Test

There are unit tests in git.

To test manually:

  • Have a working FreeIPA, SSSD registered as FreeIPA client, Apache, some web application, mod_hbacauthz_pam.
  • Set mod_hbacauthz_pam to "require pam-account <service>" in some location in Apache.
  • Use some authentication method, for example Kerberos, in that location, be logged in (let's use example "/application/login")
  • Set HBAC rules so that without this feature, one HBAC rule would match
  • Set URI in that HBAC rule to some prefix matching the page's URI path; connect the page, notice you are authorized (e.g. "/application")
  • Set URI in that HBAC rule to some prefix NOT matching the page's URI path; connect the page, notice you are NOT authorized (e.g. "/whatever")

Test Plan

HBAC rules can be modified properly and authorization works as it should

Questions

  • For backwards compatibility, lack of URI in request means any URI is matched (as described above). Is it a good idea? Any other solution?
  • How about multiple URI's in one HBAC rule? Is it a good idea? How to interpret combinations of host+scheme+port and URI paths in that case?