update for rename of recentchanges.mdwn to json.tl.ph.mdwn

[git.ikiwiki.info.git] / doc / security.mdwn

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.

If you find a new security vulnerability, please email the maintainers
privately instead of listing it in a public bug tracker, so that we can
arrange for coordinated disclosure when a fix is available. The maintainers
are [[Joey Hess|joey]] (<joey@kitenet.net>),
[[Simon McVittie|smcv]] (<smcv@debian.org>)
and [[Amitai Schleier|schmonz]] (<schmonz-web-ikiwiki@schmonz.com>).

[[!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 [[Json.tl.ph]] like foo committed. One way to avoid
this would be to limit web commits to those done by a certain user.

## other stuff to look at

I 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?

People with direct commit access can upload such files
(and if you wanted to you could block that with a pre-commit hook).

The attachments plugin is not enabled by default. If you choose to
enable it, you should make use of its powerful abilities to filter allowed
types of attachments, and only let trusted users upload.

It is possible to embed an image in a page edited over the web, by using
`img src="data:image/png;"`. Ikiwiki's htmlscrubber only allows `data:`
urls to be used for `image/*` mime types. It's possible that some broken
browser might ignore the mime type and if the data provided is not an
image, instead run it as javascript, or something evil like that. Hopefully
not many browsers are that broken.

## multiple accessors of wiki directory

If multiple people can directly 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 directly write to those directories.

## setup files

Setup files are not safe to keep in the same revision control repository
with the rest of the wiki. Just don't do it.

## page locking can be bypassed via direct commits

A locked page can only be edited on the web by an admin, but anyone who is
allowed to commit directly to the repository can bypass this. This is by
design, although a pre-commit hook could be used to prevent editing of
locked pages, 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...)_

## exploiting 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 program 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(3)` at all, and the only use of backticks is on data supplied by the
wiki admin and untainted filenames. 

Ikiwiki was developed and used for a long time with perl's taint checking
turned on as a second layer of defense against shell and other exploits. Due
to a strange [bug](http://bugs.debian.org/411786) in perl, taint checking
is currently disabled for production builds of ikiwiki.

## 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/", etc. 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 using [[plugins/passwordauth]] 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. Using [[plugins/openid]] is a potentially better option.

## XSS holes in CGI output

ikiwiki has 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, all users can modify templates
like any other part of the wiki. Some trusted users can modify templates
without it too. This assumes that HTML::Template is secure
when used with untrusted/malicious templates. (Note that includes are not
allowed.)

----

# Plugins

The security of [[plugins]] depends on how well they're written and what
external tools they use. The plugins included in ikiwiki are all held to
the same standards as the rest of ikiwiki, but with that said, here are
some security notes for them.

* The [[plugins/img]] plugin assumes that imagemagick/perlmagick are secure
  from malformed image attacks for at least the formats listed in
  `img_allowed_formats`. Imagemagick has had security holes in the
  past. To be able to exploit such a hole, a user would need to be able to
  upload images to the wiki.

----

# 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/git pull/etc 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 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.

## Online editing of existing css and images

A bug in ikiwiki allowed the web-based editor to edit any file that was in
the wiki, not just files that are page sources. So an attacker (or a
genuinely helpful user, which is how the hole came to light) could edit
files like style.css. It is also theoretically possible that an attacker
could have used this hole to edit images or other files in the wiki, with
some difficulty, since all editing would happen in a textarea.

This hole was discovered on 10 Feb 2007 and fixed the same day with the
release of ikiwiki 1.42. A fix was also backported to Debian etch, as
version 1.33.1. I recommend upgrading to one of these versions if your wiki
allows web editing.

## html insertion via title

Missing html escaping of the title contents allowed a web-based editor to
insert arbitrary html inside the title tag of a page. Since that part of
the page is not processed by the htmlscrubber, evil html could be injected.

This hole was discovered on 21 March 2007 and fixed the same day (er, hour) 
with the release of ikiwiki 1.46. A fix was also backported to Debian etch,
as version 1.33.2. I recommend upgrading to one of these versions if your
wiki allows web editing or aggregates feeds.

## javascript insertion via meta tags

It was possible to use the meta plugin's meta tags to insert arbitrary
url contents, which could be used to insert stylesheet information
containing javascript. This was fixed by sanitising meta tags.

This hole was discovered on 21 March 2007 and fixed the same day
with the release of ikiwiki 1.47. A fix was also backported to Debian etch,
as version 1.33.3. I recommend upgrading to one of these versions if your
wiki can be edited by third parties.

## insufficient checking for symlinks in srcdir path

Ikiwiki did not check if path to the srcdir to contained a symlink. If an
attacker had commit access to the directories in the path, they could
change it to a symlink, causing ikiwiki to read and publish files that were
not intended to be published. (But not write to them due to other checks.)

In most configurations, this is not exploitable, because the srcdir is
checked out of revision control, but the directories leading up to it are
not. Or, the srcdir is a single subdirectory of a project in revision
control (ie, `ikiwiki/doc`), and if the subdirectory were a symlink,
ikiwiki would still typically not follow it.

There are at least two configurations where this is exploitable:

* If the srcdir is a deeper subdirectory of a project. For example if it is
  `project/foo/doc`, an an attacker can replace `foo` with a symlink to a
  directory containing a `doc` directory (not a symlink), then ikiwiki
  would follow the symlink.
* If the path to the srcdir in ikiwiki's configuration ended in "/", 
  and the srcdir is a single subdirectory of a project, (ie,
  `ikiwiki/doc/`), the srcdir could be a symlink and ikiwiki would not
  notice.

This security hole was discovered on 26 November 2007 and fixed the same
day with the release of ikiwiki 2.14. I recommend upgrading to this version
if your wiki can be committed to by third parties. Alternatively, don't use
a trailing slash in the srcdir, and avoid the (unusual) configurations that
allow the security hole to be exploited.

## javascript insertion via uris

The htmlscrubber did not block javascript in uris. This was fixed by adding
a whitelist of valid uri types, which does not include javascript. 
([[!debcve CVE-2008-0809]]) Some urls specifyable by the meta plugin could also
theoretically have been used to inject javascript; this was also blocked
([[!debcve CVE-2008-0808]]).

This hole was discovered on 10 February 2008 and fixed the same day
with the release of ikiwiki 2.31.1. (And a few subsequent versions..)
A fix was also backported to Debian etch, as version 1.33.4. I recommend
upgrading to one of these versions if your wiki can be edited by third
parties.

## Cross Site Request Forging

Cross Site Request Forging could be used to constuct a link that would
change a logged-in user's password or other preferences if they clicked on
the link. It could also be used to construct a link that would cause a wiki
page to be modified by a logged-in user. ([[!debcve CVE-2008-0165]])

These holes were discovered on 10 April 2008 and fixed the same day with
the release of ikiwiki 2.42. A fix was also backported to Debian etch, as
version 1.33.5. I recommend upgrading to one of these versions.

## Cleartext passwords

Until version 2.48, ikiwiki stored passwords in cleartext in the `userdb`.
That risks exposing all users' passwords if the file is somehow exposed. To
pre-emtively guard against that, current versions of ikiwiki store password
hashes (using Eksblowfish).

If you use the [[plugins/passwordauth]] plugin, I recommend upgrading to
ikiwiki 2.48, installing the [[!cpan Authen::Passphrase]] perl module, and running
`ikiwiki-transition hashpassword` to replace all existing cleartext passwords
with strong blowfish hashes. 

You might also consider changing to [[plugins/openid]], which does not 
require ikiwiki deal with passwords at all, and does not involve users sending
passwords in cleartext over the net to log in, either.

## Empty password security hole

This hole allowed ikiwiki to accept logins using empty passwords, to openid
accounts that didn't use a password. It was introduced in version 1.34, and
fixed in version 2.48. The [bug](http://bugs.debian.org/483770) was
discovered on 30 May 2008 and fixed the same day. ([[!debcve CVE-2008-0169]])

I recommend upgrading to 2.48 immediatly if your wiki allows both password
and openid logins.

## Malformed UTF-8 DOS

Feeding ikiwiki page sources containing certian forms of malformed UTF-8
can cause it to crash. This can potentially be used for a denial of service
attack.

intrigeri discovered this problem on 12 Nov 2008 and a patch put in place
later that day, in version 2.70. The fix was backported to testing as version
2.53.3, and to stable as version 1.33.7.

## Insufficient blacklisting in teximg plugin

Josh Triplett discovered on 28 Aug 2009 that the teximg plugin's
blacklisting of insecure TeX commands was insufficient; it could be
bypassed and used to read arbitrary files. This was fixed by
enabling TeX configuration options that disallow unsafe TeX commands.
The fix was released on 30 Aug 2009 in version 3.1415926, and was
backported to stable in version 2.53.4. If you use the teximg plugin,
I recommend upgrading. ([[!debcve CVE-2009-2944]])

## javascript insertion via svg uris

Ivan Shmakov pointed out that the htmlscrubber allowed `data:image/*` urls,
including `data:image/svg+xml`. But svg can contain javascript, so that is
unsafe.

This hole was discovered on 12 March 2010 and fixed the same day
with the release of ikiwiki 3.20100312.
A fix was also backported to Debian etch, as version 2.53.5. I recommend
upgrading to one of these versions if your wiki can be edited by third
parties.

## javascript insertion via insufficient htmlscrubbing of comments

Kevin Riggle noticed that it was not possible to configure
`htmlscrubber_skip` to scrub comments while leaving unscubbed the text
of eg, blog posts. Confusingly, setting it to "* and !comment(*)" did not
scrub comments.

Additionally, it was discovered that comments' html was never scrubbed during
preview or moderation of comments with such a configuration.

These problems were discovered on 12 November 2010 and fixed the same
hour with the release of ikiwiki 3.20101112. ([[!debcve CVE-2010-1673]])

## javascript insertion via insufficient checking in comments

Dave B noticed that attempting to comment on an illegal page name could be
used for an XSS attack.

This hole was discovered on 22 Jan 2011 and fixed the same day with
the release of ikiwiki 3.20110122. A fix was backported to Debian squeeze,
as version 3.20100815.5. An upgrade is recommended for sites
with the comments plugin enabled. ([[!debcve CVE-2011-0428]])

## possible javascript insertion via insufficient htmlscrubbing of alternate stylesheets

Giuseppe Bilotta noticed that 'meta stylesheet` directives allowed anyone
who could upload a malicious stylesheet to a site to add it to a
page as an alternate stylesheet, or replacing the default stylesheet.

This hole was discovered on 28 Mar 2011 and fixed the same hour with
the release of ikiwiki 3.20110328. A fix was backported to Debian squeeze,
as version 3.20100815.6. An upgrade is recommended for sites that have
untrusted committers, or have the attachments plugin enabled.
([[!debcve CVE-2011-1401]])

## tty hijacking via ikiwiki-mass-rebuild

Ludwig Nussel discovered a way for users to hijack root's tty when
ikiwiki-mass-rebuild was run. Additionally, there was some potential
for information disclosure via symlinks. ([[!debcve CVE-2011-1408]])

This hole was discovered on 8 June 2011 and fixed the same day with
the release of ikiwiki 3.20110608. Note that the fix is dependant on
a version of su that has a similar hole fixed. Version 4.1.5 of the shadow
package contains the fixed su; [[!debbug 628843]] tracks fixing the hole in
Debian. An upgrade is a must for any sites that have `ikiwiki-update-wikilist`
installed suid (not the default), and whose admins run `ikiwiki-mass-rebuild`.

## javascript insertion via meta tags

Raúl Benencia discovered an additional XSS exposure in the meta plugin.
([[!debcve CVE-2012-0220]])

This hole was discovered on 16 May 2012 and fixed the same day with
the release of ikiwiki 3.20120516. A fix was backported to Debian squeeze,
as version 3.20100815.9. An upgrade is recommended for all sites.

## XSS via openid selector

Raghav Bisht discovered this XSS in the openid selector. ([[!debcve CVE-2015-2793]])

The hole was reported on March 24th, a fix was developed on March 27th,
and the fixed version 3.20150329 was released on the 29th. A fix was backported
to Debian jessie as version 3.20141016.2 and to Debian wheezy as version
3.20120629.2. An upgrade is recommended for sites using CGI and openid.

## XSS via error messages

CGI error messages did not escape HTML meta-characters, potentially
allowing an attacker to carry out cross-site scripting by directing a
user to a URL that would result in a crafted ikiwiki error message. This
was discovered on 4 May by the ikiwiki developers, and the fixed version
3.20160506 was released on 6 May. The same fixes were backported to Debian
8 "jessie" in version 3.20141016.3. A backport to Debian 7 "wheezy" is
in progress.

An upgrade is recommended for sites using
the CGI. ([[!debcve CVE-2016-4561]], OVE-20160505-0012)

## ImageMagick CVE-2016–3714 ("ImageTragick")

ikiwiki 3.20160506 and 3.20141016.3 attempt to mitigate
[[!debcve CVE-2016-3714]], and any
future ImageMagick vulnerabilities that resemble it, by restricting the
image formats that the [[ikiwiki/directive/img]] directive is willing to
resize. An upgrade is recommended for sites where an untrusted user is
able to attach images. Upgrading ImageMagick to a version where
CVE-2016-3714 has been fixed is also recommended, but at the time of
writing no such version is available.

## Perl CVE-2016-1238 (current working directory in search path)

ikiwiki 3.20160728 attempts to mitigate [[!debcve CVE-2016-1238]] by
removing `'.'` from the Perl library search path. An attacker with write
access to ikiwiki's current working directory could potentially use this
vulnerability to execute arbitrary Perl code. An upgrade is recommended
for sites where an untrusted user is able to attach files with arbitrary
names and/or run a setuid ikiwiki wrapper with a working directory of
their choice.

## <span id="cve-2016-9645">Editing restriction bypass for git revert</span>

intrigeri discovered that a web or git user could revert a change to a
page they are not allowed to edit, if the change being reverted was made
before the page was moved from a location where that user had permission
to edit it. For example, if a file is moved from `drafts/policy.mdwn`
(editable by less-trusted users) to `policy.mdwn` (only editable
by more-trusted users), a less-trusted user could revert a change
that was made to `drafts/policy.mdwn` prior to that move, and it would
result in `policy.mdwn` being altered.

This affects sites with the `git` VCS and the `recentchanges` plugin,
which are both used in most ikiwiki installations.

This bug was reported on 2016-12-17. A partially fixed version
3.20161219 was released on 2016-12-19, but the solution used in that
version was not effective with git versions older than 2.8.0.
A more complete fix was released on 2016-12-29 in version 3.20161229,
with fixes backported to Debian 8 in version 3.20141016.4.

([[!debcve CVE-2016-10026]] represents the original vulnerability.
[[!debcve CVE-2016-9645]]/OVE-20161226-0002 represents the vulnerability
in 3.20161219 caused by the incomplete fix.)

## <span id="cve-2016-9646">Commit metadata forgery via CGI::FormBuilder context-dependent APIs</span>

When CGI::FormBuilder->field("foo") is called in list context (and
in particular in the arguments to a subroutine that takes named
arguments), it can return zero or more values for foo from the CGI
request, rather than the expected single value. This breaks the usual
Perl parsing convention for named arguments, similar to CVE-2014-1572
in Bugzilla (which was caused by a similar API design issue in CGI.pm).

In ikiwiki, this appears to have been exploitable in two places, both
of them relatively minor:

* in the comments plugin, an attacker who was able to post a comment
  could give it a user-specified author and author-URL even if the wiki
  configuration did not allow for that, by crafting multiple values
  for other fields
* in the editpage plugin, an attacker who was able to edit a page
  could potentially forge commit authorship (attribute their edit to
  someone else) by crafting multiple values for the rcsinfo field

This was fixed in ikiwiki 3.20161229, with fixes backported to Debian 8
in version 3.20141016.4.

([[!debcve CVE-2016-9646]]/OVE-20161226-0001)

## <span id="cve-2017-0356">Authentication bypass via repeated parameters</span>

The ikiwiki maintainers discovered further flaws similar to CVE-2016-9646
in the passwordauth plugin's use of CGI::FormBuilder, with a more
serious impact:

* An attacker who can log in to a site with a password can log in
  as a different and potentially more privileged user.
* An attacker who can create a new account can set arbitrary fields
  in the user database for that account.

This was fixed in ikiwiki 3.20170111, with fixes backported to Debian 8
in version 3.20141016.4.

([[!debcve CVE-2017-0356]]/OVE-20170111-0001)

## <span id="cve-2019-9187">Server-side request forgery via aggregate plugin</span>

The ikiwiki maintainers discovered that the [[plugins/aggregate]] plugin
did not use [[!cpan LWPx::ParanoidAgent]]. On sites where the
aggregate plugin is enabled, authorized wiki editors could tell ikiwiki
to fetch potentially undesired URIs even if LWPx::ParanoidAgent was
installed:

* local files via `file:` URIs
* other URI schemes that might be misused by attackers, such as `gopher:`
* hosts that resolve to loopback IP addresses (127.x.x.x)
* hosts that resolve to RFC 1918 IP addresses (192.168.x.x etc.)

This could be used by an attacker to publish information that should not have
been accessible, cause denial of service by requesting "tarpit" URIs that are
slow to respond, or cause undesired side-effects if local web servers implement
["unsafe"](https://tools.ietf.org/html/rfc7231#section-4.2.1) GET requests.
([[!debcve CVE-2019-9187]])

Additionally, if the LWPx::ParanoidAgent module was not installed, the
[[plugins/blogspam]], [[plugins/openid]] and [[plugins/pinger]] plugins
would fall back to [[!cpan LWP]], which is susceptible to similar attacks.
This is unlikely to be a practical problem for the blogspam plugin because
the URL it requests is under the control of the wiki administrator, but
the openid plugin can request URLs controlled by unauthenticated remote
users, and the pinger plugin can request URLs controlled by authorized
wiki editors.

This is addressed in ikiwiki 3.20190228 as follows, with the same fixes
backported to Debian 9 in version 3.20170111.1:

* URI schemes other than `http:` and `https:` are not accepted, preventing
  access to `file:`, `gopher:`, etc.

* If a proxy is [[configured in the ikiwiki setup file|tips/using_a_proxy]],
  it is used for all outgoing `http:` and `https:` requests. In this case
  the proxy is responsible for blocking any requests that are undesired,
  including loopback or RFC 1918 addresses.

* If a proxy is not configured, and LWPx::ParanoidAgent is installed,
  it will be used. This prevents loopback and RFC 1918 IP addresses, and
  sets a timeout to avoid denial of service via "tarpit" URIs.

* Otherwise, the ordinary LWP user-agent will be used. This allows requests
  to loopback and RFC 1918 IP addresses, and has less robust timeout
  behaviour. We are not treating this as a vulnerability: if this
  behaviour is not acceptable for your site, please make sure to install
  LWPx::ParanoidAgent or disable the affected plugins.