Summary

A DNS rebinding vulnerability in the web_fetch tool allows an unauthenticated attacker to bypass URL validation and access internal resources on the server, including private IP addresses (e.g., 127.0.0.1, 192.168.x.x). By crafting a malicious domain that resolves to a public IP during validation and subsequently resolves to a private IP during execution, an attacker can access sensitive local services and potentially exfiltrate data.

Details

The vulnerability exists because the web_fetch tool lacks complete DNS pinning. The application performs URL validation only once via validateParams(), but the URL is then passed unchanged to the fetchHTMLContent() function, which eventually reaches fetchWithChromedp(). The headless browser (Chromedp) resolves the hostname independently without DNS pinning, allowing a time-of-check-time-of-use (TOCTOU) attack.

Validation phase (first DNS resolution):

if err := t.validateParams(p); err != nil {
    // Returns error for private IPs
    results[index] = &webFetchItemResult{
        err: err,
        // ...
    }
    return
}

Execution phase (second DNS resolution):
The original URL (not the resolved IP) is passed through the execution chain:

output, data, err := t.executeFetch(ctx, p)
// Calls fetchHTMLContent(ctx, targetURL) where targetURL is the original hostname

Chromedp execution (vulnerable DNS resolution):

func (t *WebFetchTool) fetchWithChromedp(ctx context.Context, targetURL string) (string, error) {
    // targetURL is not DNS-pinned; browser resolves it independently
    err := chromedp.Run(ctx,
        chromedp.Navigate(targetURL),  // Third DNS lookup occurs here
        chromedp.WaitReady("body", chromedp.ByQuery),
        chromedp.OuterHTML("html", &html),
    )
}

The attacker controls a domain that can be configured to return different DNS responses to different queries, enabling them to bypass the initial private IP check and access restricted resources during the actual fetch.

PoC

Setup:

1. Deploy the DNS rebinding server (attached Python file) with the following systemd configuration:

[Unit]
Description=DNS Rebinding Test Server
After=network.target

[Service]
Type=simple
User=root
WorkingDirectory=/root/Repos/dns-rebinding-server
ExecStart=/root/.proto/shims/python -u /root/Repos/dns-rebinding-server/server.py --token aleister1102 --domain aleister.ninja --port 53 --global-tracking --ip1 1.1.1.1 --ip2 0.0.0.0 --first-response-count 1 --reset-time 0
Restart=always
RestartSec=3

[Install]
WantedBy=multi-user.target

This configures the DNS server to:

• Return 1.1.1.1 (a public IP) for the first DNS query
• Return 127.0.0.1 (localhost) for all subsequent queries
• TTL is set to 0 to prevent caching

The sequence can also be reset via reset.domain.com (reset to 1.1.1.1).

Note: We may need to reset the sequence as the TOCTOU attack is not truly reliable and needs to be triggered multiple times.

2. Set up a simple HTTP server on the localhost of the backend service:

python -m http.server 8888

3. Configure the malicious domain to point to the DNS rebinding server

Execution:

1. Enable web search on an agent.
2. Prompt the agent to fetch content from the attacker-controlled domain (e.g., http://attacker.example.com)
3. The sequence of events:
   • First DNS query (validation phase): attacker.example.com → 1.1.1.1 ✓ Passes validation
   • Second DNS query (execution phase): attacker.example.com → 127.0.0.1 ✗ Bypass achieved
   • The web_fetch tool successfully connects to 127.0.0.1:8080 and returns the local server's content

Result:
The attacker gains access to the local HTTP server and can read its content, demonstrating that internal resources are now accessible through the rebinding attack.

PoC video:

Impact

Vulnerability Type: DNS Rebinding / Server-Side Request Forgery (SSRF)

Who is impacted:

• Any user or agent with web search capability can exploit this vulnerability
• The vulnerability grants access to internal services, configuration files, metadata services, and other sensitive resources normally restricted to the internal network
• In cloud environments, this could allow access to metadata endpoints (e.g., AWS IMDSv1) to obtain credentials and secrets\