With the release of MediaWiki 1.28, the lifetime of MediaWiki version
1.26.x has come to an end.
Users still using MediaWiki 1.26.x are advised to upgrade to version
1.28.0, the latest stable version.
The parsing team has fixed a security bug in Parsoid .
* Users could send invalid prefixes, formats, or domains and run
* This fix has been applied to the Wikimedia cluster  and also merged
into Parsoid master .
* We have also released a 0.5.3 deb version with this patch applied. 
* We have also released a 0.5.3 npm version of Parsoid. 
* Parsoid is a stateless service and doesn't retain any state between
requests. In private wikis, VisualEditor can be configured to
forward the user cookie to Parsoid to pass along to the MediaWiki API
to parse a page, but this exploit is not exposed through VE.
In addition, Parsoid doesn't receive any user credentials on
* However, if a wiki's Parsoid service is publicly accessible on the
internet *and* is accessible through the wiki's domain, then, this
exploit can be used to leak user cookies for that wiki. For all wikis
that use Parsoid in this fashion, we recommend they patch their
Parsoid installation immediately.
* On the Wikimedia cluster, Parsoid is proxied behind RESTBase and is
not public accessible and as such, this exploit wasn't available for
an exploit to steal user sessions.
Thanks to the reporter of this exploit, Darian Patrick from the
Security Team, Arlo Breault from the Parsing Team, Daniel Zahn and
others from Ops for their assistance handling this bug and preparing
Technical Lead and Manager,