OpenProject is open-source, web-based project management software. In versions prior to 11.3.3, the MessagesController class of OpenProject has a quote method that implements the logic behind the Quote button in the discussion forums, and it uses a regex to strip tags from the message being quoted. The (.|\s) part can match a space character in two ways, so an unterminated tag containing n spaces causes Ruby's regex engine to backtrack to try 2 n states in the NFA. This will result in a Regular Expression Denial of Service. The issue is fixed in OpenProject 11.3.3. As a workaround, one may install the patch manually.