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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.7%, percentile 54: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 54% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes mesop (PyPI) <= 1.2.2, fixed in 1.2.3. Check your lockfile, including transitive dependencies.

What it is

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 statetoken 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.

From the GitHub advisory

Summary A Path Traversal vulnerability allows any user (or attacker) supplying an untrusted statetoken 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 statetoken actively dictates where the runtime session state is physically saved or queried natively on disk. In mesop/server/server.py, specifically the uistream endpoint, the event.statetoken is collected directly from the untrusted incoming protobuf message struct: mesop.protos.uipb2.UserEvent. Because this is unconditionally passed to FileStateSessionBackend.makefilepath(self, token), it evaluates standard path operators (e.g. ../../../).

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

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

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.

Weakness type: CWE-22.

Product: mesop (PyPI) · Assigned by: GitHub_M · Published: 2026-03-20

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.73% probability of exploitation activity in the next 30 days; percentile 54.

Timeline

• 2026-03-18 — GitHub-reviewed advisory GHSA-8qvf-mr4w-9x2c published (critical)
• 2026-03-20 — CVE record last updated
• 2026-03-20 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-8qvf-mr4w-9x2c github.com
2. CVE.org record CVE-2026-33054 cve.org
3. NVD entry CVE-2026-33054 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

Critical PyPI advisories · Today’s Security Digest