# CVE-2026-77394: OpenC3 COSMOS: Stored, cross-user XSS via Telemetry screen BUTTON widget

> An authenticated user can inject malicious scripts into shared screens, stealing bearer tokens and taking over other operators' accounts.

- **Severity:** High 7.6 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 23 Sep 2026
- **Updated:** 23 Sep 2026
- **Also known as:** GHSA-gvf2-2rh5-mpgf

## What is CVE-2026-77394?

CVE-2026-77394 is a high-severity cross-site scripting vulnerability in OpenC3 cosmos, published on 23 Sep 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

OpenC3 COSMOS provides the functionality needed to send commands to and receive data from one or more embedded systems. From 5.0.6 until 7.3.0, an authenticated actor with system_set permission can store a shared screen through POST /openc3-api/screen whose BUTTON widget action is evaluated by openc3-cosmos-init/plugins/packages/openc3-vue-common/src/widgets/ButtonWidget.vue in another operator's browser session when the button is activated. The stored script runs in the COSMOS origin and can read localStorage.openc3Token, allowing theft of the victim's bearer token, account takeover, and actions with the victim's privileges. The permissive content security policy contributes to execution but is not the primary root cause. This issue is fixed in version 7.3.0.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-77394?

2 open source packages ship an affected version, including @openc3/vue-common.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@openc3/vue-common` | npm | From 5.0.6 | 7.3.0 |
| `@openc3/vue-common` | npm | From 5.0.6 | 7.3.0 |

## How do I fix CVE-2026-77394?

Upgrade @openc3/vue-common to 7.3.0 or later.

## Is CVE-2026-77394 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-77394 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77394 and fix it if it is affected.

What it is: CVE-2026-77394 is a high-severity cross-site scripting vulnerability in OpenC3 cosmos, published on 23 Sep 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-77394 exists.

Affected packages and fixed versions:
- @openc3/vue-common (npm): affected from 5.0.6; fixed in 7.3.0
- @openc3/vue-common (npm): affected from 5.0.6; fixed in 7.3.0

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-2026-77394/ (Markdown: https://insights.sbomify.com/CVE-2026-77394/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77394
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77394
- OSV: https://osv.dev/vulnerability/CVE-2026-77394

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77394/
Summaries are generated and may contain mistakes; the official description above is the record.