# CVE-2026-100404: Zulip: Guests can read new messages in non-subscribed public channels

> Guest users can read new messages in public channels they are not subscribed to, exposing sensitive communication data.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **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:** 14 Oct 2026
- **Updated:** 29 Oct 2026
- **Also known as:** GHSA-5r8f-gq2h-fcgp

## What is CVE-2026-100404?

CVE-2026-100404 is a medium-severity missing authorization vulnerability in zulip, published on 14 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Zulip is an open-source team collaboration tool. In Zulip Server versions through 12.1, guest users can receive new message events for public channels outside their subscriptions. A guest can trigger the issue by registering an event queue with parameters that request messages from an unsubscribed public channel, or an existing event queue can remain affected after the guest's access to the channel is removed. The flaw discloses new public-channel messages to the guest. This issue is fixed in version 12.2.

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-100404?


## How do I fix CVE-2026-100404?

No fix has been published for CVE-2026-100404 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-100404 being exploited?

There is no known exploitation of CVE-2026-100404 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-100404 and fix it if it is affected.

What it is: CVE-2026-100404 is a medium-severity missing authorization vulnerability in zulip, published on 14 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-100404 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-2026-100404/ (Markdown: https://insights.sbomify.com/CVE-2026-100404/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-100404
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-100404
- OSV: https://osv.dev/vulnerability/CVE-2026-100404

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-100404/
Summaries are generated and may contain mistakes; the official description above is the record.