AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
securitycritical PriorityID: ssrf-private-ip-dns-rebinding

Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard

Production network security standard (CWE-918 / OWASP A10:2021). Mandates strict validation of user-supplied URLs before outbound HTTP requests, resolves DNS to target IP, rejects private/internal RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.1, 169.254.169.254), and prevents DNS Rebinding attacks by pinning resolved IPs.

Prohibited Anti-Pattern: AI fetches user-provided URLs with fetch(req.body.url) without IP resolution or private range filtering, allowing attackers to access internal cloud metadata and databases.

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard - Production network security standard (CWE-918 / OWASP A10:2021). Mandates strict validation of user-supplied URLs before outbound HTTP requests, resolves DNS to target IP, rejects private/internal RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.1, 169.254.169.254), and prevents DNS Rebinding attacks by pinning resolved IPs.
globs: **/proxy/**/*.ts, **/fetcher/**/*.py, **/webhook/**/*.go
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/ssrf-private-ip-dns-rebinding"
---

# Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI fetches user-provided URLs with fetch(req.body.url) without IP resolution or private range filtering, allowing attackers to access internal cloud metadata and databases.

## System Constraints
You are an Application Security Engineer specializing in SSRF and Network Hardening. NEVER pass user-provided URLs directly to fetch() or requests.get(). Attackers can supply URLs like http://169.254.169.254/latest/meta-data/ or http://localhost:6379/ to steal cloud credentials or interact with internal Redis caches. You MUST resolve the DNS, verify that the IP is not private/loopback/link-local, and pin the socket connection to the verified IP.

## Strict Guidelines
- MUST resolve DNS hostnames before issuing outbound HTTP requests using dns.promises.lookup().
- MUST reject requests targeting loopback (127.0.0.0/8, ::1), private networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), and cloud metadata (169.254.169.254).
- MUST connect directly to the resolved IP address to eliminate DNS Rebinding (Time-of-Check to Time-of-Use) attacks.
- MUST restrict allowed protocols strictly to ['http:', 'https:'] (reject file://, gopher://, dict://).
- MUST disable automatic redirect following (redirect: 'manual') or re-validate redirect target URLs against the same IP filter.

## Bad vs Good Implementations
### Safe Outbound URL Fetcher with DNS Resolution & Private IP Guard

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Directly fetching arbitrary user URLs enables Server-Side Request Forgery. Attackers can query internal cloud metadata services, local Redis/Memcached instances, and intranet databases.
```typescript
// FATAL: Direct fetch of un-sanitized user URL (Critical SSRF Vulnerability)
app.post('/api/fetch-avatar', async (req, res) => {
  const userUrl = req.body.avatarUrl;
  // ATTACK: userUrl = 'http://169.254.169.254/latest/meta-data/iam/security-credentials/'
  const response = await fetch(userUrl); // Leaks AWS master IAM credentials!
  res.send(await response.text());
});
```

#### Production Standard (Correct)
> **Standard**: Parses URL protocol, resolves DNS, and blocks private/loopback/cloud metadata IP ranges using ipaddr.js, eliminating SSRF and internal network scanning vectors.
```typescript
import dns from 'node:dns/promises';
import ipaddr from 'ipaddr.js';

export async function validateSafeOutboundUrl(rawUrl: string): Promise<string> {
  const parsed = new URL(rawUrl);
  if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
    throw new Error('Invalid URL protocol: Only HTTP/HTTPS allowed');
  }

  // 1. Resolve DNS to get physical target IP
  const lookupResult = await dns.lookup(parsed.hostname);
  const parsedIp = ipaddr.parse(lookupResult.address);

  // 2. Reject private, loopback, link-local, and reserved ranges
  const range = parsedIp.range();
  const forbiddenRanges = ['loopback', 'private', 'linkLocal', 'carrierGradeNat', 'uniqueLocal'];
  if (forbiddenRanges.includes(range) || lookupResult.address === '169.254.169.254') {
    throw new Error(`Security Violation: Destination IP ${lookupResult.address} is forbidden`);
  }

  return parsed.toString();
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

SSRF & Network Sandbox Auditor MCP
Tests outbound URL fetching endpoints for SSRF and DNS rebinding vulnerabilities.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

askills: SSRF Defense & IP Pinning Guard
Generates hardened HTTP client wrappers with DNS resolution and private IP blocking.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard

Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard defines an industrial-grade behavioral standard for security engineering. Production network security standard (CWE-918 / OWASP A10:2021). Mandates strict validation of user-supplied URLs before outbound HTTP requests, resolves DNS to target IP, rejects private/internal RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.1, 169.254.169.254), and prevents DNS Rebinding attacks by pinning resolved IPs. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI fetches user-provided URLs with fetch(req.body.url) without IP resolution or private range filtering, allowing attackers to access internal cloud metadata and databases. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI fetches user-provided URLs with fetch(req.body.url) without IP resolution or private range filtering, allowing attackers to access internal cloud metadata and databases.Critical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

MUST resolve DNS hostnames before issuing outbound HTTP requests using dns.promises.lookup().

MUST resolve DNS hostnames before issuing outbound HTTP requests using dns.promises.lookup().

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

MUST reject requests targeting loopback (127.0.0.0/8,

MUST reject requests targeting loopback (127.0.0.0/8, ::1), private networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), and cloud metadata (169.254.169.254).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

MUST connect directly to the resolved IP address to eliminate DNS Rebinding (Time-of-Check to Time-of-Use) attacks.

MUST connect directly to the resolved IP address to eliminate DNS Rebinding (Time-of-Check to Time-of-Use) attacks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

MUST restrict allowed protocols strictly to ['http

MUST restrict allowed protocols strictly to ['http:', 'https:'] (reject file://, gopher://, dict://).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

MUST disable automatic redirect following (redirect

MUST disable automatic redirect following (redirect: 'manual') or re-validate redirect target URLs against the same IP filter.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Server-Side Request Forgery (SSRF) & DNS Rebinding Mitigation Standard critical for AI coding assistants?

Production network security standard (CWE-918 / OWASP A10:2021). Mandates strict validation of user-supplied URLs before outbound HTTP requests, resolves DNS to target IP, rejects private/internal RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.1, 169.254.169.254), and prevents DNS Rebinding attacks by pinning resolved IPs. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI fetches user-provided URLs with fetch(req.body.url) without IP resolution or private range filtering, allowing attackers to access internal cloud metadata and databases.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related SECURITY Rules

View all rules →
security
105

OpenFGA Graph Invariants: Cyclic Relation Prevention & Contextual Tuple Hardening

Enforces strict directed acyclic graph (DAG) topology validation during relationship mutation writes and mandates strict server-side schema verification for ephemeral contextual tuples in OpenFGA evaluations. Prohibits persistent storage of ephemeral request attributes and guards against recursive relation loops causing graph expansion denial-of-service.

security
81

WebAuthn Conditional UI Mediation, Nonce Atomicity, and Assertion Cryptographic Verification

Enforces zero-trust cryptographic validation for FIDO2/WebAuthn Passkey authentication with Conditional UI autofill. Requires atomic single-use challenge consumption to eliminate replay attacks, strict RP ID hash and origin binding, authenticatorData flag verification (User Presence and User Verification), and monotonic signature counter enforcement to detect authenticator cloning.

security
111

Enforce Sigstore Keyless OIDC Attestation and Admission Controller Verification

Enforces immutable supply-chain security using Sigstore Cosign keyless signing via ephemeral OIDC tokens issued by Fulcio and verified against the Rekor transparency log. Container workloads must be signed and attested at build time by digest, and admission controllers (such as Kyverno or Sigstore Policy Controller) must strictly validate the OIDC issuer and subject identity before allowing image execution.

1 Rule in Stack

Server-Side