# CVE-2021-41115: Regular expression denial-of-service in Zulip

> Malicious organization administrators can cause server denial-of-service by configuring complex regular expressions in linkifiers, requiring an upgrade to fix.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 7 Oct 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-4h36-mqfq-42jg

## What is CVE-2021-41115?

CVE-2021-41115 is a medium-severity denial of service vulnerability in zulip, published on 7 Oct 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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).

Weakness: CWE-400 (Uncontrolled Resource Consumption), CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2021-41115?

It affects 1 product.


Products: Zulip Zulip.

## How do I fix CVE-2021-41115?

No fix has been published for CVE-2021-41115 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-41115 being exploited?

There is no known exploitation of CVE-2021-41115 in the wild and no public exploit code.

## Other vulnerabilities in zulip

- [CVE-2021-43799](https://insights.sbomify.com/CVE-2021-43799/): RabbitMQ exposes ports with weak default secrets in Zulip Server (high 8.6)
- [CVE-2022-35962](https://insights.sbomify.com/CVE-2022-35962/): Crafted link in Zulip message can cause disclosure of credentials (high 8.0)
- [CVE-2016-4427](https://insights.sbomify.com/CVE-2016-4427/): Auth bypass in zulip (high 7.5)
- [CVE-2021-3866](https://insights.sbomify.com/CVE-2021-3866/): Cross-site Scripting (XSS) - Stored in zulip/zulip (medium 6.8)
- [CVE-2023-28623](https://insights.sbomify.com/CVE-2023-28623/): Unauthorized user can register an account in specific configurations in Zulip (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-41115 and fix it if it is affected.

What it is: CVE-2021-41115 is a medium-severity denial of service vulnerability in zulip, published on 7 Oct 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-41115 in the wild and no public exploit code.

Affected product: zulip. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-41115/ (Markdown: https://insights.sbomify.com/CVE-2021-41115/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-41115
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-41115
- OSV: https://osv.dev/vulnerability/CVE-2021-41115

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-41115/
Summaries are generated and may contain mistakes; the official description above is the record.