Version lifecycle

MediaWiki End of Life Policy
Starting with the release of version 1.6.0, MediaWiki introduced a "continuous integration" development model with quarterly snapshot releases. Since the introduction of this development model, release branches continue to receive security updates for about a year from first release.

However, due to time constraints, the developers of MediaWiki are unable to provide support and security fixes for all releases ever made. Outdated and legacy releases, meaning release branches older than a year, are no longer supported and security patches may (and will likely) not be provided. Brion Vibber, CTO of the Wikimedia Foundation, says 'MediaWiki 1.5.x and below are no longer supported. No one should use them, particularly on a public-facing server, as they contain unpatched security vulnerabilities.'

As such, besides of profiting from newly introduced features, increased performance, better scalability and support of new techniques, keeping your systems secure is the main reason to regularly upgrade your MediaWiki installation.

So please make sure you use a version of MediaWiki which is supported. The easiest way to do so, besides of checking the release date of the .0 version of the branch you are using on the mediawiki-announce mailing list or on Sourceforge, is to check the main page which lists the latest versions from all release branches which are currently supported.