X-Git-Url: http://git.vanrenterghem.biz/git.ikiwiki.info.git/blobdiff_plain/0340c45ea16b8bb9dc2a747da45e8c958b491b20..48f9d393393709d8e998a9eb8d4095773ab185a1:/doc/security.mdwn?ds=inline diff --git a/doc/security.mdwn b/doc/security.mdwn index ce4c6f718..e72b3fe2b 100644 --- a/doc/security.mdwn +++ b/doc/security.mdwn @@ -1,4 +1,4 @@ -Let's do an ikiwiki security analysis.. +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 @@ -6,62 +6,52 @@ 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. -# Probable holes +---- -## html attacks +# Probable holes -ikiwiki does not attempt to do any santization of the html on the wiki. -[[MarkDown]] allows embedding of arbitrary html into a markdown document. If -you let anyone else edit files on the wiki, then anyone can have fun exploiting -the web browser bug of the day. This type of attack is typically referred -to as an XSS attack ([google](http://www.google.com/search?q=xss+attack)). +_(The list of things to fix.)_ -## image files etc attacks +## commit spoofing -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. If these files exploit security holes in the browser of someone who's viewing the wiki, that can be a security problem. +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. -Of course nobody else seems to worry about this in other wikis, so should we? +## other stuff to look at -## web server attacks +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. -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. +---- -## symlink attacks +# Potential gotchas -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. +_(Things not to do.)_ -It would certianly be possible to start out with a directory, let ikiwiki -run and find a file in there, then replace it with a symlink, and ikiwiki -would then go ahead and follow the symlink when it went to open that file -to read it. If it was some private file and was running suid, that could be -bad. +## image file etc attacks -TODO: seems that locking to prevent more than one ikiwiki run at a time -would both fix this and is a good idea in general. With locking, an -attacker couldn't get ikiwiki to svn up while another instance was running. +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. -## multiple accessors of wiki source directory +Of course nobody else seems to worry about this in other wikis, so should we? -If multiple people can write to the source directory ikiwiki is using, then -one can cause trouble for the other when they run ikiwiki through symlink -attacks. +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. -So it's best if only one person can ever write to the checkout that ikiwiki -compiles the wiki from. +## multiple accessors of wiki directory -## webserver symlink attacks +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. -If someone checks in a symlink to /etc/passwd, ikiwiki would publish that. -To aoid this, ikiwiki will need to avoid reading files that are symlinks. -TODO and note discussion of races above. +So it's best if only one person can ever write to those directories. ## setup files @@ -69,19 +59,24 @@ 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. -## svn commit logs +## page locking can be bypassed via direct svn commits -Anyone with svn commit access can forge "web commit from foo" and make it appeat on [[RecentChanges]] like foo committed. One way to avoid this would be to limit web commits to those done by a certian user. +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. -It's actually possible to force a whole series of svn commits to appear to have come just before yours, by forging svn log output. This could be guarded against somewhat by revision number scanning, since the forged revisions would duplicate the numbers of unforged ones. Or subversion could fix svn log to indent commit messages, which would make such forgery impossible.. +## web server attacks -ikiwiki escapes any html in svn commit logs to prevent other mischief. +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...) +_(AKA, the assumptions that will be the root of most security holes...)_ ## exploting ikiwiki with bad content @@ -111,20 +106,8 @@ been no problem yet. 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 it runs with taint checks on of course.. - -## 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. +wiki admin and untainted filenames. And it runs with taint checks on of +course.. ## cgi data security @@ -136,15 +119,118 @@ 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. -## CGI::Session security +## XSS holes in CGI output -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. +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. + +---- + +# 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.