The Markdown parser in Zulip server before 2.0.5 used a regular expression vulnerable to exponential backtracking. A user who is logged into the server could send a crafted message causing the server to spend an effectively arbitrary amount of CPU time and stall the processing of future messages.
https://github.com/zulip/zulip/commit/5797f013b3be450c146a4141514bda525f2f1b51
https://blog.zulip.org/2019/09/11/zulip-server-2-0-5-security-release/