web commit by joey

[ikiwiki] / doc / security.mdwn

Let's do an ikiwiki security analysis.

If you are using ikiwiki to render pages that only you can edit, do not
generate any wrappers, and do not use the cgi, then there are no more
security issues with this program than with cat(1). If, however, you let
others edit pages in your wiki, then some possible security issues do need
to be kept in mind.

[[toc levels=2]]

----

# Probable holes

_(The list of things to fix.)_

## commit spoofing

Anyone with direct commit access can forge "web commit from foo" and
make it appear on [[RecentChanges]] like foo committed. One way to avoid
this would be to limit web commits to those done by a certian user.

## other stuff to look at

I need to audit the git backend a bit, and have been meaning to
see if any CRLF injection type things can be done in the CGI code.

----

# Potential gotchas

_(Things not to do.)_

## image file etc attacks

If it enounters a file type it does not understand, ikiwiki just copies it
into place. So if you let users add any kind of file they like, they can
upload images, movies, windows executables, css files, etc (though not html
files). If these files exploit security holes in the browser of someone
who's viewing the wiki, that can be a security problem.

Of course nobody else seems to worry about this in other wikis, so should we?

Currently only people with direct svn commit access can upload such files
(and if you wanted to you could block that with a svn pre-commit hook).
Users with only web commit access are limited to editing pages as ikiwiki
doesn't support file uploads from browsers (yet), so they can't exploit
this.

## multiple accessors of wiki directory

If multiple people can write to the source directory ikiwiki is using, or
to the destination directory it writes files to, then one can cause trouble
for the other when they run ikiwiki through symlink attacks.

So it's best if only one person can ever write to those directories.

## setup files

Setup files are not safe to keep in subversion with the rest of the wiki.
Just don't do it. [[ikiwiki.setup]] is *not* used as the setup file for
this wiki, BTW.

## page locking can be bypassed via direct svn commits

A locked page can only be edited on the web by an admin, but
anyone who is allowed to commit direct to svn can bypass this. This is by
design, although a subversion pre-commit hook could be used to prevent
editing of locked pages when using subversion, if you really need to.

## web server attacks

If your web server does any parsing of special sorts of files (for example,
server parsed html files), then if you let anyone else add files to the wiki,
they can try to use this to exploit your web server.

----

# Hopefully non-holes

_(AKA, the assumptions that will be the root of most security holes...)_

## exploting ikiwiki with bad content

Someone could add bad content to the wiki and hope to exploit ikiwiki.
Note that ikiwiki runs with perl taint checks on, so this is unlikely.

One fun thing in ikiwiki is its handling of a PageSpec, which involves
translating it into perl and running the perl. Of course, this is done
*very* carefully to guard against injecting arbitrary perl code.

## publishing cgi scripts

ikiwiki does not allow cgi scripts to be published as part of the wiki. Or
rather, the script is published, but it's not marked executable (except in
the case of "destination directory file replacement" below), so hopefully
your web server will not run it.

## suid wrappers

ikiwiki --wrapper is intended to generate a wrapper program that
runs ikiwiki to update a given wiki. The wrapper can in turn be made suid,
for example to be used in a [[post-commit]] hook by people who cannot write
to the html pages, etc.

If the wrapper script is made suid, then any bugs in this wrapper would be
security holes. The wrapper is written as securely as I know how, is based
on code that has a history of security use long before ikiwiki, and there's
been no problem yet.

## shell exploits

ikiwiki does not expose untrusted data to the shell. In fact it doesn't use
system() at all, and the only use of backticks is on data supplied by the
wiki admin and untainted filenames. And it runs with taint checks on of
course..

## cgi data security

When ikiwiki runs as a cgi to edit a page, it is passed the name of the
page to edit. It has to make sure to sanitise this page, to prevent eg,
editing of ../../../foo, or editing of files that are not part of the wiki,
such as subversion dotfiles. This is done by sanitising the filename
removing unallowed characters, then making sure it doesn't start with "/"
or contain ".." or "/.svn/". Annoyingly ad-hoc, this kind of code is where
security holes breed. It needs a test suite at the very least.

## CGI::Session security

I've audited this module and it is massively insecure by default. ikiwiki
uses it in one of the few secure ways; by forcing it to write to a
directory it controls (and not /tmp) and by setting a umask that makes the
file not be world readable.

## cgi password security

Login to the wiki involves sending a password in cleartext over the net.
Cracking the password only allows editing the wiki as that user though.
If you care, you can use https, I suppose. If you do use https either for
all of the wiki, or just the cgi access, then consider using the sslcookie
option.

## XSS holes in CGI output

ikiwiki has not yet been audited to ensure that all cgi script input/output
is sanitised to prevent XSS attacks. For example, a user can't register
with a username containing html code (anymore).

It's difficult to know for sure if all such avenues have really been
closed though.

## HTML::Template security

If the [[plugins/template]] plugin is enabled, users can modify templates
like any other part of the wiki. This assumes that HTML::Template is secure
when used with untrusted/malicious templates. (Note that includes are not
allowed, so that's not a problem.)

----

# Fixed holes

_(Unless otherwise noted, these were discovered and immediately fixed by the
ikiwiki developers.)_

## destination directory file replacement

Any file in the destination directory that is a valid page filename can be
replaced, even if it was not originally rendered from a page. For example,
ikiwiki.cgi could be edited in the wiki, and it would write out a
replacement. File permission is preseved. Yipes!

This was fixed by making ikiwiki check if the file it's writing to exists;
if it does then it has to be a file that it's aware of creating before, or
it will refuse to create it.

Still, this sort of attack is something to keep in mind.

## symlink attacks

Could a committer trick ikiwiki into following a symlink and operating on
some other tree that it shouldn't? svn supports symlinks, so one can get
into the repo. ikiwiki uses File::Find to traverse the repo, and does not
tell it to follow symlinks, but it might be possible to race replacing a
directory with a symlink and trick it into following the link.

Also, if someone checks in a symlink to /etc/passwd, ikiwiki would read and
publish that, which could be used to expose files a committer otherwise
wouldn't see.

To avoid this, ikiwiki will skip over symlinks when scanning for pages, and
uses locking to prevent more than one instance running at a time. The lock
prevents one ikiwiki from running a svn up at the wrong time to race
another ikiwiki. So only attackers who can write to the working copy on
their own can race it.

## symlink + cgi attacks

Similarly, a svn commit of a symlink could be made, ikiwiki ignores it
because of the above, but the symlink is still there, and then you edit the
page from the web, which follows the symlink when reading the page
(exposing the content), and again when saving the changed page (changing
the content).

This was fixed for page saving by making ikiwiki refuse to write to files
that are symlinks, or that are in subdirectories that are symlinks,
combined with the above locking.

For page editing, it's fixed by ikiwiki checking to make sure that it
already has found a page by scanning the tree, before loading it for
editing, which as described above, also is done in a way that avoids
symlink attacks.

## underlaydir override attacks

ikiwiki also scans an underlaydir for pages, this is used to provide stock
pages to all wikis w/o needing to copy them into the wiki. Since ikiwiki
internally stores only the base filename from the underlaydir or srcdir,
and searches for a file in either directory when reading a page source,
there is the potential for ikiwiki's scanner to reject a file from the
srcdir for some reason (such as it being contained in a directory that is
symlinked in), find a valid copy of the file in the underlaydir, and then
when loading the file, mistakenly load the bad file from the srcdir.

This attack is avoided by making ikiwiki refuse to add any files from the
underlaydir if a file also exists in the srcdir with the same name.

## multiple page source issues

Note that I previously worried that underlay override attacks could also be
accomplished if ikiwiki were extended to support other page markup
languages besides markdown. However, a closer look indicates that this is
not a problem: ikiwiki does preserve the file extension when storing the
source filename of a page, so a file with another extension that renders to
the same page name can't bypass the check. Ie, ikiwiki won't skip foo.rst
in the srcdir, find foo.mdwn in the underlay, decide to render page foo and
then read the bad foo.mdwn. Instead it will remember the .rst extension and
only render a file with that extension.

## XSS attacks in page content

ikiwiki supports protecting users from their own broken browsers via the
[[plugins/htmlscrubber]] plugin, which is enabled by default.

## svn commit logs

It's was possible to force a whole series of svn commits to appear to
have come just before yours, by forging svn log output. This was
guarded against by using svn log --xml.

ikiwiki escapes any html in svn commit logs to prevent other mischief.

## XML::Parser

XML::Parser is used by the aggregation plugin, and has some security holes. 
Bug #[378411](http://bugs.debian.org/378411) does not
seem to affect our use, since the data is not encoded as utf-8 at that
point. #[378412](http://bugs.debian.org/378412) could affect us, although it
doesn't seem very exploitable. It has a simple fix, and has been fixed in
Debian unstable.

## include loops

Various directives that cause one page to be included into another could
be exploited to DOS the wiki, by causing a loop. Ikiwiki has always guarded
against this one way or another; the current solution should detect all
types of loops involving preprocessor directives.