Server-side request forgery (SSRF): Silent Gateway for Internal Systems

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Server-side request forgery (SSRF) is an important vulnerability that allows the attackers to trick a server in making unauthorized requests on their behalf. These requests often target internal systems that are not publicly accessible, causing internal data exposure, metadata leaks and even severe security implications such as distance code execution in extreme cases.

In this comprehensive blog post, we’ll delve into:

  • The history and evolution of SSRF attacks
  • How SSRF works
  • Real-world examples
  • Detection methods
  • Exploitation techniques
  • Prevention strategies

📜 A brief history of SSRF

While SSRF weaknesses exist from the early days of web services, their importance increased with the development of cloud computing and microsarvis architecture.

Key Events:

  • 2011: SSRF became widely discussed in web security circles after the rise of cloud-hosted services like AWS and GCP.
  • 2014: The Snapchat SSRF bug became popular in bug bounty communities.
  • 2019: Capital One data breach — one of the biggest incidents where an SSRF vulnerability in AWS WAF was used to access sensitive data from AWS S3 buckets.
  • 2020s: With the proliferation of internal microservices, API gateways, and metadata endpoints, SSRF emerged as a top priority vulnerability in OWASP Top 10 under A10:2021 – Server-Side Request Forgery.

💡 What is SSRF?

SSRF occurs when an attacker can control or manipulate the URL or request that a server makes in another system.

Instead of targeting the client-side directly, the SSRF tricks the server to act as a proxy, enables the attackers:

  • Use internal systems
  • Bypass firewall and IP filtering
  • Remove metadata from cloud platforms (eg, AWS EC2 Metadata Service)
  • Port scan on internal network

🔍 How Does SSRF Work?

Let’s consider a scenario:

A web application allows users to provide a URL to fetch a profile image:

http

GET /fetch-image?url=http://example.com/image.jpg

If the backend server doesn’t validate or sanitize the URL, an attacker can change the URL to:

http

GET /fetch-image?url=http://127.0.0.1:8080/admin

Now the server will fetch data from its own internal services, which might contain sensitive information or administrative endpoints.

🔥 Real-World Exploits and Examples

1. AWS EC2 Metadata Leak

URL like http://169.254.169.254/latest/meta-data/ when injected through an SSRF vulnerability can leak access tokens, IAM roles, and EC2 instance credentials.

2. Capital One Data Breach

Attackers exploited SSRF in AWS WAF, gained temporary credentials, and accessed 100+ million customer accounts.

3. GitHub Gist SSRF

At one point, GitHub allowed uploading gists with external references, which attackers manipulated to make internal GitHub infrastructure calls.

🧪 How to Detect SSRF

Detecting SSRF vulnerabilities requires both manual and automated techniques. Here’s what to look for:

✅ Input Validation Failures:

Check if the application uses unvalidated or unsanitized input to construct outbound URLs or API calls.

✅ Behavior Testing:

Use testing tools like Burp Suite, OWASP ZAP, or Postman to inject URLs pointing to:

  • 127.0.0.1
  • localhost
  • Internal IP ranges (192.168.x.x, 10.x.x.x, etc.)
  • Cloud metadata endpoints

✅ Observe Response Differences:

A successful SSRF might return:

  • Internal error messages
  • Delays due to port scanning
  • Unusual HTTP status codes (e.g., 403, 500)

✅ Log Monitoring:

Review server logs for outbound requests to suspicious or unexpected internal addresses.

🧨 How to Exploit SSRF

⚠️ This section is for educational and ethical security research purposes only.

1. Basic SSRF

http
/fetch-image?url=http://localhost:8080

2. Bypass Blacklists with URL Encoding

http

/fetch-image?url=http://127.0.0.1%2Fadmin

3. Access Cloud Metadata

http

/fetch-image?url=http://169.254.169.254/latest/meta-data/

4. Port Scanning via SSRF

Change the port in the URL to scan:

http

/fetch-image?url=http://127.0.0.1:22

By changing ports and observing response times/status, attackers can map internal services.

🛑 Types of SSRF

Type Description
Basic SSRF Direct access to internal URLs
Blind SSRF No visible response but server performs the action
DNS Rebinding SSRF Tricks DNS to point domain to internal IP
Second-Order SSRF SSRF payload stored and triggered later

🛡️ How to Prevent SSRF

✅ 1. Input Validation

  • Whitelist allowed domains/IPs
  • Reject localhost (127.0.0.1, ::1) and internal ranges
  • Disallow URL redirections

✅ 2. Network Segmentation

  • Isolate application servers from internal services
  • Block egress to internal metadata endpoints by default

✅ 3. Disable Unnecessary Metadata

For AWS EC2:

bash

Add --http-put-response-hop-limit=1 or disable IMDSv1

✅ 4. Use SSRF Protection Libraries

  • Implement middleware that filters and inspects outgoing requests.

✅ 5. Monitor Outbound Traffic

  • Use proxies or gateways to track, log, and restrict outgoing server requests.

✅ 6. Security Headers and Patching

  • Apply latest patches to libraries that handle request forwarding (e.g., urllib, requests, etc.)
  • Use web application firewalls (WAFs) with SSRF filters

🧰 Tools to Detect SSRF

Tool Use Case
Burp Suite Manual SSRF testing
SSRFmap Automates SSRF detection & cloud metadata retrieval
Nuclei templates Automated SSRF scanners
Amass/Sublist3r Recon to find SSRF vectors via subdomains
Wireshark Inspect outbound server connections

🧾 OWASP’s SSRF Guidelines

OWASP recommends SSRF protections via:

  • Restricting addressable IPs
  • Using allow-listing vs blocklisting
  • Disabling unnecessary outbound connections
  • Enforcing network-level controls

You can also refer to OWASP’s SSRF Cheat Sheet for complete guidance.

📌 Summary

Aspect Detail
Name Server-Side Request Forgery (SSRF)
OWASP Category A10:2021
First Major Case 2011, then 2019 Capital One breach
Typical Exploits Access metadata, port scan, internal API abuse
Risk High (often leads to pivoting attacks)
Mitigation Input validation, deny internal IPs, network segmentation

✅ Final Thoughts

SSRF is one of the lowest dangerous weaknesses yet in modern web applications.With the growing reliance on internal microservices and cloud metadata services, even a simple misconfigured image fetch endpoint could become a gateway to an organization's most guarded secrets.

Don't wait for a breach. Test, detect, and defend.

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