Security Aspects of DWiki

DWiki has a general attitude about security: it really distrusts incoming requests, it somewhat distrusts itself, but it has a rational trust of the people creating DWiki templates and pages. DWiki will try to save people from accidental mistakes, but doesn't bother with things that are just half-hearted attempts to stop people from deliberately sidestepping security restrictions. Moral: don't let people write DWiki pages unless you trust them.

Some knowledge of the ProcessingModel and the ConfigurationFile (and what can be set there) may be helpful for the rest of this discussion.

A Quick Summary

DWiki itself is written in Python (a lot of Python). This means that unless there is a gross implementation error in the Python interpreter, it is secure from simple problems such as buffer overruns. While DWiki uses some components from the standard Python libraries, they too are well-tested and believed to be entirely safe.

Because it is quite careful at multiple levels about how it handles requests, hostile HTTP requests should not be able to trick DWiki into serving anything from outside the page directory (or the comments directory, or the static content directory). InvalidPageNames discusses things it won't serve even inside them.

DWiki doesn't attempt to stop insiders from using DWiki to serve 'bad' content, ultimately because there are so many ways a malicious insider can do that. ChrisSiebenmann feels that it is better to be honest about not making any attempt rather than making an attempt and causing people to put more trust in it than it warrants.

If run as a CGI-BIN, DWiki should not be run with a UID that has any special access to restricted files. But then, no CGI-BIN should be run that way.

DWiki has some degree of optional Authentication, but it is no stronger than the usual run of the mill login and password on other web sites. Really sensitive content is probably best not served from a web server that the public (whatever that means to you) can access.

Pages versus Templates

What people can do with the ability to write DWikiText in DWiki pages is somewhat less powerful than what they can do with the ability to write DWiki templates. Similarly, errors in DWikiText are considered far less fatal than errors in templates; DWikiText errors just result in funny-looking pages, while template errors result in terse web error pages.

Thus: while it's safe to let people write DWiki pages in general, you probably want to restrict (at least somewhat) who can write or modify your templates. Plus, your templates (being, you know, templates) shouldn't need modification all that often. People can create and modify pages all the time.

How DWiki tries to be secure

Cautious processing

Internally, DWiki tries to operate in a relatively 'security conservative' fashion. For example, the frontend rejects clearly invalid things without passing them through to the DWiki core, because the core has a lot more power than the frontend so a mistake has larger ramifications.

DWiki also is deliberately structured so as to give itself as little power as possible.

Errors Abort Processing

DWiki can hit a number of internal problems while processing a request; for example, a template that's called for might be missing. When this happens, DWiki aborts processing the entire request, throwing an error all the way back to the front end, which generates a terse error page about the situation.

This may be abrupt ... but it is safe.

File Access

DWiki reads only a few files: the ConfigurationFile, the global-authseed-file file, the authfile password file, and things under the page, template, RCS, static files, and comments directories (if those are configured on).

Except for the password file, the DWiki core only accesses files through a simple storage layer abstraction, which provides 'storage pools' to the rest of DWiki. Each storage pool confines all file requests to relative paths under the pool's root, explicitly ruling out InvalidPageNames when retrieving files for the rest of DWiki.

The storage layer has no general file writing capabilities. The only interface it has for writing files is specifically designed for comments, using a specific naming and storage scheme. And only the comments directory uses a storage pool that supports this abstraction.

Following Symlinks

Unlike some web servers (eg, Apache), DWiki takes no special care to not follow symbolic links that point outside one of its storage pool directory roots. If you put such a symbolic link into a storage pool area, DWiki assumes that you know what you're doing.

This is deliberate. Attempting to duplicate the kernel's namei() function in user space is inevitably very complicated (and prone to surprising races). Rather than run the risk of making a mistake in the amount of code required, DWiki is honest about the whole situation.

Limitations DWiki imposes on itself

Limited URL scope

DWiki refuses to serve any request that is not under staticurl (if set) or rooturl. Anything under staticurl must be a static request and is served only as such.

Limited static-content serving

In addition to dynamic DWiki pages, DWiki can serve static content via the staticdir ConfigurationFile directive. Since DWiki's goals for serving static content are very modest (CSS files, images, etc), DWiki refuses requests for static directories. As mentioned in ProcessingModel, static content is served by the frontend, thereby keeping the amount of code involved in the process down.

In addition, DWiki rejects any request for static content that is not in the default 'normal' view.

A brief sketch of the Dwiki processing model

The core of DWiki is a template expansion engine and a collection of (text) renderers; DWiki displays pages by figuring out what template to use and then rendering it out.

Renderers generate text based on the current context, such as the page that is being displayed. The most important (and largest) renderer is the wikitext renderer, which takes page content in DWiki's wiki text format and turns it into HTML.

Other renderers create things like the navigation 'breadcrumbs' up at the top of this page and and the page tools and last-modified lines at the bottom. Renderers generally create only the essential pieces of that information; surrounding text is created through template expansion. Renderers are hardcoded parts of DWiki and are thus written in Python.

Templates are text files; they get expanded by the template engine through a recursive process of applying template 'macros' to their text. Template macros can insert other (expanded) templates, insert text taken from context variables, and insert the results of renderers. A typical template might look like:

<html><head><title>${|wikititle|wikiname} :: ${page}</title></head>
<body> @{breadcrumbs} <br/>
@{wikitext}
<hr> #{footer.tmpl} #{site-sig.tmpl} </body> </html>

(the actual templates that render this DWiki are somewhat more complicated than that, but this shows the flavour.)

DWiki produces all pages this way. Displaying different types of pages (regular pages versus directories) and different views of the same page (such as the history view) is done by selecting a different starting template; the template (presumably) uses different renderers that the normal view.

Errors are also rendered using templates (if an appropriate template exists). This allows some error pages to reuse renderers as appropriate; for example, the no-such-page error template includes breadcrumbs just as regular pages do, as you can see at NoSuchPage.

Wart: the view source display is not done by a template: it just barfs the content out straight as plain text. One current limitation of renderers and templates is that they can't control the content-type, which is set in the HTML view core.

Wart: the mapping of view + file attributes to templates is currently hard-coded.

The frontend versus the core

DWiki is divided into two components: the front end and the core. The front end receives raw HTTP requests, figures out if they are proper requests, and then passes them to the core to go through the core's processing. If the front end can detect that a HTTP request is not something that the core can handle, it rejects it immediately with a terse error.

Similarly, if the core encounters a processing error it throws an exception up to the front end, which logs it and generates another terse error.

It is the front end that can optionally serve static files; the core is not involved in that process.

DWiki features

DWiki's job is to be a good way to display version controlled wiki-text pages that you write in a real editor.

The important DWiki features:

• simple but reasonably powerful text rendering (based on WikiText).
• natural support for arbitrarily-named links: you don't have to follow some magic page naming standard that doesn't fit well with the natural names for things.
• pages are normal, simple files, and you edit them directly in Unix.
• support for putting pages in RCS, with strong disincentives to hand-edit files without checking them out (they stop displaying).
• directories can display like changelogs: pages inline, most recent first.
• can generate Atom syndication feeds for recently changed things.

The inevitable feature list:

In no particular order:

• simple WikiText-like text rendering. (Chris wrote pages in GNU Emacs and relentlessly smushed anything that got in the way of how GNU Emacs wanted to autoformat things.)
• The text rendering choices are designed to make it easy to write about Unix systems.
• full support for directly editing wiki pages.
• does not force a flat page namespace; uses straightforward Unix files and directories to organize the DWiki page space. (Thereby keeping the Unix view of DWiki's pages simple.)
• supports a blog-like view of a directory that inlines pages there, most recent first.
• in-filesystem page redirects make it trivial to support plurals, moved/renamed pages, etc.
• text-based page templates control how all pages appear, making it easy to control various bits of a DWiki's appearance.
• pages can be put in RCS for version control and multi-person editing access. RCS files can live in either the page directory hierarchy (for simplicity) or another parallel directory tree (for neatness).
• forces people not to edit RCS-controlled files without locking them by refusing to display inconsistent unlocked files.
• generates Atom syndication feeds for recently changed pages and recent comments, for the entire DWiki or any subtree thereof.
• written in Python.
• simple-ish yet powerful enough (I hope) user authentication system, with an equally simple yet powerful way of restricting who can read DWiki pages.
• supports the option of letting people (possibly including the world) comment on some or all of the pages.
• takes some pride in properly generating and handling Last-Modified: and ETag: headers in HTTP responses.
• wikitext to HTML generates fully HTML 4.01 Transitional compliant HTML provided only that you don't jump multiple indent levels in at once in lists (thus Formatting doesn't validate).
• can run as a CGI-BIN or standalone, and support for additional environments (SCGI, WSGI, whatever) should be easy to add if it is needed. Disclaimer: standalone does not use a production-quality webserver implementation; it uses Python's BaseHTTPServer with a hack to use threading.

Missing DWiki features

Also in no particular order:

• you can't edit DWiki pages from the web, but see WhyNotWebEditing.
• no user authentication.
• therefor, no access restrictions on who can read what.
• searching is primitive at best.

A necessary acknowledgement:

A number of DWiki's features and design decisions are shamelessly inspired by C.J. Silverio's as yet (22 May 2005) unfinished Snippy. Note that Snippy is much more powerful than DWiki probably ever will be, plus if it had been finished when I was writing DWiki I probably wouldn't have.