Inefficient Regular Expression Complexity

Summary

Wagtail is an open-source content management system built on Django. A bug in Wagtail's `parse_query_string` would result in it taking a long time to process suitably crafted inputs. When used to parse sufficiently long strings of characters without a space, `parse_query_string` would take an unexpectedly large amount of time to process, resulting in a denial of service. In an initial Wagtail installation, the vulnerability can be exploited by any Wagtail admin user. It cannot be exploited by end users. If your Wagtail site has a custom search implementation which uses `parse_query_string`, it may be exploitable by other users (e.g. unauthenticated users). This issue affects versions 2.0b1 through 5.2.5, 6.0rc1 through 6.0.5, and 6.1rc1 through 6.1.2.