Several of the Fossil user capabilities form a clear power hierarchy. Mathematically speaking:

Setup > Admin > Moderator > User > Subscriber > Anonymous > Nobody

This document explains the distinction between the first two. For the others, see:

• How Moderation Works

• Users vs Subscribers

• Defense Against Spiders

Philosophical Core

The Setup user "owns" the Fossil repository and may delegate a subset of that power to one or more Admin users.

The Setup user can grant Admin capability and take it away, but Admin users cannot grant themselves Setup capability, either directly via the Admin → Users UI page or via any indirect means. (If you discover indirect means to elevate Admin privilege to Setup, it's a bug, so please report it!)

It is common for the Setup user to have administrative control over the host system running the Fossil repository, whereas it makes no sense for Admin users to have that ability. If an Admin-only user had root access on a Linux box running the Fossil instance they are an Admin on, they could elevate their capability to Setup in several ways. (The fossil admin command, the fossil sql command, editing the repository DB file directly, etc.) Therefore, if you wish to grant someone Setup-like capability on a Fossil repository but you're unwilling to give them full control over the host system, you probably want to grant them Admin capability instead.

Admin power is delegated from Setup. When a Setup user grants Admin capability, it is an expression of trust in that user's judgement.

Admin-only users must not fight against the policies of the Setup user. Such a rift would be just cause for the Setup user to strip the Admin user's capabilities, for the ex-Admin to fork the repository, and for both to go their separate ways.

A useful rule of thumb here is that Admin users should only change things that the Setup user has not changed from the stock configuration. In this way, an Admin-only user can avoid overriding the Setup user's choices.

This rule is not enforced by the Fossil permission system for a couple of reasons:

1. There are too many exceptions to encode in the remaining user capability bits. As of this writing, we've already assigned meaning to all of the lowercase letters, most of the decimal digits, and a few of the uppercase letters. We'd rather not resort to punctuation and Unicode to express future extensions to the policy choices Fossil offers its power users.

2. Even if we had enough suitable printable ASCII characters left to assign one to every imaginable purpose and policy, we want to keep the number of exceptions manageable. Consider the Admin → Settings page, which is currently restricted to Setup users only: you might imagine breaking this up into several subsets so that some subsets are available to non-Setup users, each controlled by a user capability bit. Is that a good idea? Maybe, but it should be done only after due consideration. It would definitely be wrong to assign a user capability bit to each setting on that page.

Let's consider a concrete application of this rule: Admin → Skins. Fossil grants Admin-only users full access to this page so that the Admins can maintain and extend the skin as the repository evolves, not so Admins can switch the entire skin to another without consulting with the Setup user first. If, during a forum discussion one of the mere users notices a problem with the skin, an Admin-only user should feel free to correct this without bothering the Setup user.

Another common case is that the Setup user upgrades Fossil on the server but forgets to merge the upstream skin changes: Admin users are entrusted to do that work on behalf of the Setup user.

Capability Groups

We can break up the set of powers the Admin user capability grants into several groups, then defend each group as a coherent whole.

Security

While establishing the Fossil repository's security policy is a task for the Setup user, maintaining that policy is something that Fossil allows a Setup user to delegate to trustworthy users via the Admin user capability:

• Manage users: The only thing an Admin-only user cannot do on the Admin → Users page is grant Setup capability, either to themselves or to other users. The intent is that Admin users be able to take some of the load of routine user management tasks off the shoulders of the Setup user: delete accounts created by spammers, fix email alert subscriptions, reset passwords, etc.

• Security audit: The Admin → Security-Audit page runs several tests on the Fossil repository's configuration, then reports potential problems it found and offers canned solutions. Those canned solutions do not do anything that an Admin-user could not do via other means, so this page offers the Admin-only user no more power than they otherwise had. For example, this page's "Take it Private" feature can also be done manually via Admin → Users. This page is a convenience, not a grant of new power to the Admin-only user.

• Logging: Admin-only users get to see the various Fossil logs in case they need to use them to understand a problem they're empowered to solve. An obvious example is a spam attack: the Admin might want to find the user's last-used IP, see if they cloned the repository, see if they attempted to brute-force an existing login before self-registering, etc.

Some security-conscious people might be bothered by the fact that Admin-only users have these abilities. Think of a large IT organization: if the CIO hires a tiger team to test the company's internal IT defenses, the line grunts fix the reported problems, not the CIO.

Administrivia

It is perfectly fine for a Fossil repository to only have Setup users, no Admin users. The smaller the repository, the more likely the repository has no Admin-only users. If the Setup user neither needs nor wants to grant Admin power to others, there is no requirement in Fossil to do so. Setup capability is a pure superset of Admin capability.

As the number of users on a Fossil repository grows, the value in delegating administrivia also grows, because the Setup user typically has other time sinks they consider more important.

Admin users can take over the following routine tasks on behalf of the Setup user:

• Shunning: After user management, this is one of the greatest powers of an Admin-only user. Fossil grants access to the Admin → Shunned page to Admin users rather than reserve it to Setup users because one of the primary purposes of the Fossil shunning system is to clean up after a spammer, and that's exactly the sort of administrivia we wish to delegate to Admin users.

  Coupled with the Rebuild button on the same page, an Admin user has the power to delete the repository's entire blockchain! This makes this feature a pretty good razor in deciding whether to grant someone Admin capability: do you trust that user to shun Fossil artifacts responsibly?

  Realize that shunning is cooperative in Fossil. As long as there are surviving repository clones, an Admin-only user who deletes the whole blockchain has merely caused a nuisance. An Admin-only user cannot permanently destroy the repository unless the Setup user has been so silly as to have no up-to-date clones.

• Moderation: According to the power hierarchy laid out at the top of this article, Admins are greater than Moderators, so control over what Moderators can do clearly belongs to both Admins and to the Setup user(s).

• Status: Although the Fossil /stat page is visible to every user with Read capability, there are several additional things this page gives access to when a user also has the Admin capability:

  • Email alerts and backoffice status. Admin-only users cannot modify the email alerts setup, but they can see some details about its configuration and current status.

  • The /urllist page, which is a read-only page showing the ways the repository can be accessed and how it has been accessed in the past. Logically, this is an extension to logging, covered above.

  • The Fossil repository SQL schema. This is not particularly sensitive information, since you get more or less the same information when you clone the repository. It's restricted to Admin because it's primarily useful in debugging SQL errors, which happen most often when Fossil itself is in flux and the schema isn't being automatically updated correctly. That puts this squarely into the "administrivia" category.

  • Web cache status, environment, and logging: more administrivia meant to help the Admin debug problems.

• Configure search

Cosmetics

While the Setup user is responsible for setting up the initial "look" of a Fossil repository, the Setup user entrusts Admin users with maintaining that look. An Admin-only user therefore has the following special abilities:

• Modify the repository skin

• Create and modify URL aliases

• Manage the "ad units" feature, if enabled.

• Adjust the /timeline display preferences.

• Change the "logo" element displayed by some skins.

These capabilities allow an Admin-only user to affect the branding and possibly even the back-end finances of a project. This is why we began this document with a philosophical discussion: if you cannot entrust a user with these powers, you should not grant that user Admin capability.

Clones and Backups

Keep in mind that Fossil is a distributed version control system, which means that a user known to Fossil might have Setup capability on one repository but be a mere "user" on one of its clones. The most common case is that when you clone a repository, even anonymously, you gain Setup power over the local clone.

The distinctions above therefore are intransitive: they apply only within a single repository instance.

The exception to this is when the clone is done as a Setup user, since this also copies the user table on the initial clone. A user with Setup capability can subsequently say fossil conf pull all to update that table and everything else not normally synchronized between Fossil repositories. In this way, a Setup user can create multiple interchangeable clones. This is useful not only to guard against rogue Admin-only users, it is a useful element of a load balancing and failover system.

Setup-Only Features

Some features are now and must always be restricted to Setup users only.

• Configuration: The Admin → Configuration page nominally falls under Cosmetics above, but it's such a core part of the Fossil configuration — something every Setup user is expected to fully specify on initial repository setup — that we have trouble justifying any case where an Admin-only user would have good cause to modify any of it. This page is generally set up once and then never touched again.

• Access: The Admin → Access page falls under the Security category above, but like Configuration, it's generally something set up once and never touched, so only Setup users should change it.

• Login-Group: Login groups allow one Fossil repository to delegate user access to another. Since an Admin-only user on one repo might not have such access to another repo on the same host system, this must be a Setup-only task.

• Settings: The repository settings available via Admin → Settings have too wide a range of power to allow modification by Admin-only users:

  • Harmless: Admin-only users on a repository may well have checkin rights on the repository, so the fact that versionable settings like crlf-glob can also be set at the repository level seems like a thing we might want to allow Admin-only users the ability to change. Since Fossil currently has no way to allow only some settings to be changed by Admin-only users and some not, we can't just show these harmless settings to Admin-only users.

  • Low-Risk: The admin-log setting controls whether the Fossil admin log is generated. Since we've already decided that Admin-only users can see this log, it seems fine that the Admin users can choose whether this log gets generated in the first place.

    There's a small risk that a rogue Admin user could disable the log before doing something evil that the log would capture, so ideally, we'd want to restrict changing this setting from 1 to 0 to Setup only while allowing Admin-only users to change it from 0 to 1. Fossil doesn't currently allow that.

  • Risky: The https-login setting falls under the "Security" section above, but it should probably never be adjusted by Admin-only users. Sites that want it on will never want it to be disabled without a very good reason.

    There is also an inverse risk: if the site has a front-end HTTPS proxy that uses HTTP to communicate over localhost to Fossil, enabling this setting will create an infinite redirect loop! (Ask me how I know.)

  • Dangerous: The email-send-command setting could allow a rogue Admin to run arbitrary commands on the host system, unless it's prevented via some kind of host-specific restriction. (chroot, jails, SELinux, VMs, etc.) Since it makes no sense to trust Admin-only users with root level access on the host system, we almost certainly don't want to allow them to change such settings.

• SQL: The Admin → SQL feature allows the Setup user to enter raw SQL queries against the Fossil repository via Fossil UI. This not only allows arbitrary ability to modify the repository blockchain and its backing data tables, it can probably also be used to damage the host such as via PRAGMA temp_store = FILE.

• TH1: The TH1 language is quite restricted relative to the Tcl language it descends from, so this author does not believe there is a way to damage the Fossil repository or its host via the Admin → TH1 feature, which allows execution of arbitrary TH1 code within the repository's execution context. Nevertheless, interpreters are a well-known source of security problems, so it seems best to restrict this feature to Setup-only users as long as we lack a good reason for Admin-only users to have access to it.