Zulip is an open source team chat server. In affected versions Zulip allows organization administrators on a server to configure “linkifiers” that automatically create links from messages that users send, detected via arbitrary regular expressions. Malicious organization administrators could subject the server to a denial-of-service via regular expression complexity attacks; most simply, by configuring a quadratic-time regular expression in a linkifier, and sending messages that exploited it. A regular expression attempted to parse the user-provided regexes to verify that they were safe from ReDoS — this was both insufficient, as well as _itself_ subject to ReDoS if the organization administrator entered a sufficiently complex invalid regex. Affected users should [upgrade to the just-released Zulip 4.7](https:\/\/zulip.readthedocs.io\/en\/latest\/production\/upgrade-or-modify.html#upgrading-to-a-release), or [`main`](https:\/\/zulip.readthedocs.io\/en\/latest\/production\/upgrade-or-modify.html#upgrading-from-a-git-repository).<\/p>\n