critical CVSS 10 EPSS <0.1%

CVE-2026-33054·Python vulnerability

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.

Severity
critical
Software
Python
Fixed in
1.2.3
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 6% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-33054 scores 10 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 1.2.3

Fixed in: 1.2.3

How to fix this CVE

Upgrade Mesop to version 1.2.3 or later to patch a critical path traversal vulnerability in the FileStateSessionBackend. This vulnerability allows attackers to manipulate state_token values to access arbitrary files on disk, potentially causing denial of service or data corruption. Immediately update your Mesop installation and restart all affected applications.

sudo dnf update python3-mesop

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST requests to Mesop UI endpoints containing state_token parameters with path traversal patterns such as: state_token=.*(\.\./|\.\.\\|%2e%2e%2f|%2e%2e%5c).*

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Block or sanitize any state_token parameter values containing path traversal sequences (../, .., encoded variants %2e%2e). Implement strict input validation to reject state_token values that don't match expected base64 or hash format patterns for your Mesop application.

How to check if you are affected

  1. Step 1: Check installed Mesop version with: python3 -c "import mesop; print(mesop.__version__)"
  2. Step 2: Verify if FileStateSessionBackend is configured by searching your application config files for 'FileStateSessionBackend' or checking environment variables with: env | grep -i mesop
  3. Step 3: Review application logs for suspicious state_token patterns in UI stream payloads, looking for entries containing path traversal sequences (../, ..\, or encoded variants) in request parameters
  4. Step 4: Confirm the patch was applied by re-running the version check command and verifying output is 1.2.3 or higher

FAQ

What is CVE-2026-33054?

CVE-2026-33054 is a critical path traversal vulnerability in Mesop (Python UI framework) versions 1.2.2 and below that allows attackers to manipulate the state_token parameter to read, write, or delete arbitrary files on the server when using FileStateSessionBackend.

Is CVE-2026-33054 being actively exploited?

No, CVE-2026-33054 is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are currently available, though the vulnerability is easily exploitable and should be treated with urgency.

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

The vulnerability affects Mesop versions 1.2.2 and below. All versions of Python 3 running vulnerable Mesop versions are at risk, particularly those configured with FileStateSessionBackend for session state management.

How do I check if my server is vulnerable to CVE-2026-33054?

Run: python3 -c "import mesop; print(mesop.__version__)" and confirm the version is 1.2.3 or higher. Also verify your application config uses FileStateSessionBackend by checking configuration files or running: grep -r "FileStateSessionBackend" /path/to/app/

Does Defensia detect CVE-2026-33054?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Mesop is installed on a monitored server, CVE-2026-33054 will appear in your dashboard with remediation steps.

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References

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