Hi all! This is an announcement for a new developer feature in MediaWiki.
If you don’t develop MediaWiki core, extensions or skins, you can stop
reading :)
MediaWiki interface messages are generally “safe” to edit: when they
contain markup, it is either parsed (as wikitext), sanitized, or fully
HTML-escaped. For this reason, administrators are allowed to edit normal
messages on-wiki in the MediaWiki: namespace, while editing JS code (which
is more dangerous) is restricted to interface administrators. (A few
exceptions, messages that are not escaped and which can only be edited by
interface administrators, are configured in $wgRawHtmlMessages
<https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:$wgRawHtmlMessages>.)
Occasionally, a bug in the software means that a message isn’t properly
escaped, which can in theory be abused by administrators to effectively
gain interface administrator powers (by editing a MediaWiki: page for a
message to contain <script> tags, or onclick="" attributes, or whatever).
Such bugs are usually considered low-severity security issues; some of them
are tracked in T2212 <https://phabricator.wikimedia.org/T2212>. (The
general issue is known as cross-site scripting
<https://www.mediawiki.org/wiki/Special:MyLanguage/Cross-site_scripting>
and can be much more severe when it’s not limited to interface messages.)
Previously, checking for these issues as a developer was tedious: if you
suspected that a message was vulnerable to HTML injection, you had to
create a page for it in the MediaWiki: namespace, or edit the corresponding
en.json file on disk (and potentially rebuild the localisation cache). The
recently merged “xss language code” feature simplifies this process. If the
developer setting $wgUseXssLanguage is set to true, then an “x-xss”
language code becomes available and can be selected with *?uselang=x-xss*
in the URL. When using this language code, all messages become “malicious”:
every message is replaced by a snippet of HTML that tries to run alert('
*message-key*'). If everything is implemented correctly, all of those HTML
snippets should be escaped, and no alerts should fire, although the wiki
will look quite strange:
If you see any alert, then that means that a message has not been escaped
correctly; use the message key shown in the alert to hunt down the buggy
code (or add the message key to $wgRawHtmlMessages). This feature is
intended to be especially useful during code review: check out the change,
load a page with ?uselang=x-xss, and see if any alerts come up.
Miscellaneous notes:
- This is a developer-only feature. I strongly recommend against
setting $wgUseXssLanguage
= true; in any production setting. (It will be added to
DevelopmentSettings.php soon.)
- Above, I focused on the possibility to abuse unescaped messages via
the MediaWiki: namespace. You might also be thinking about the potential
for
translatewiki.net contributors to inject malicious HTML into message
translations; however, the translation exports from
translatewiki.net to
the JSON files automatically check for any HTML in translations, and flag
suspicious cases for human review. Therefore, it’s much harder to exploit
an unescaped message via
translatewiki.net than via the MediaWiki:
namespace.
- Finally, I should mention that we already found several
vulnerabilities using this feature, which will be fixed with the upcoming
security release
<https://lists.wikimedia.org/hyperkitty/list/wikitech-l@lists.wikimedia.org/thread/QTRFMDRQAL7QK4RN53URX5YBBV744AWI/>.
If you try out this feature now, and find a vulnerable message, I suggest
you wait until then, and check whether it’s still vulnerable, before
reporting it.
Cheers,
Lucas
--
Lucas Werkmeister (he/er)
Software Engineer
Wikimedia Deutschland e. V. | Tempelhofer Ufer 23-24 | 10963 Berlin
Phone: +49 (0)30-577 11 62-0
https://wikimedia.de
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