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Zulip attempts to validate that the regular expressions that admins enter for linkifiers are well-formatted, and only contain a specific subset of regex grammar. The process of checking these properties (via a regex!) can cause denial-of-service via backtracking. Furthermore, this validation itself does not prevent the creation of linkifiers which themselves cause denial-of-service when they are executed. As the validator accepts literally anything inside of a `(?P<word>...)` block, any quadratic backtracking expression can be hidden therein. Switch user-provided linkifier patterns to be matched in the Markdown processor by the `re2` library, which is guaranteed constant-time. This somewhat limits the possible features of the regular expression (notably, look-head and -behind, and back-references); however, these features had never been advertised as working in the context of linkifiers. A migration removes any existing linkifiers which would not function under re2, after printing them for posterity during the upgrade; they are unlikely to be common, and are impossible to fix automatically. The denial-of-service in the linkifier validator was discovered by @erik-krogh and @yoff, as GHSL-2021-118.
Zulip overview
Zulip is a powerful, open source group chat application that combines the immediacy of real-time chat with the productivity benefits of threaded conversations. Zulip is used by open source projects, Fortune 500 companies, large standards bodies, and others who need a real-time chat system that allows users to easily process hundreds or thousands of messages a day. With over 700 contributors merging over 500 commits a month, Zulip is also the largest and fastest growing open source group chat project.
Getting started
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Contributing code. Check out our guide for new contributors to get started. Zulip prides itself on maintaining a clean and well-tested codebase, and a stock of hundreds of beginner-friendly issues.
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Contributing non-code. Report an issue, translate Zulip into your language, write for the Zulip blog, or give us feedback. We would love to hear from you, even if you're just trying the product out.
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Supporting Zulip. Advocate for your organization to use Zulip, become a sponsor, write a review in the mobile app stores, or upvote Zulip on product comparison sites.
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Checking Zulip out. The best way to see Zulip in action is to drop by the Zulip community server. We also recommend reading Zulip for open source, Zulip for companies, or Zulip for working groups and part time communities.
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Running a Zulip server. Use a preconfigured DigitalOcean droplet, install Zulip directly, or use Zulip's experimental Docker image. Commercial support is available; see https://zulip.com/plans for details.
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Using Zulip without setting up a server. https://zulip.com offers free and commercial hosting, including providing our paid plan for free to fellow open source projects.
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Participating in outreach programs like Google Summer of Code.
You may also be interested in reading our blog or following us on Twitter. Zulip is distributed under the Apache 2.0 license.