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securitycritical PriorityID: jwt-algorithm-none-attack-defense

JWT alg: 'none' & Key Confusion Attack Defense Standard

Cryptographic authentication standard for JSON Web Tokens (CVE-2015-9235). Mandates explicit algorithm whitelisting (algorithms: ['RS256']), strictly prohibits 'none' or 'HS256' when using public keys, and validates token expiration (exp), audience (aud), and issuer (iss) claims.

Prohibited Anti-Pattern: AI verifies JWTs without specifying explicit algorithms, allowing attackers to forge arbitrary tokens by setting `alg: 'none'` or executing HMAC key confusion attacks.

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---
description: JWT alg: 'none' & Key Confusion Attack Defense Standard - Cryptographic authentication standard for JSON Web Tokens (CVE-2015-9235). Mandates explicit algorithm whitelisting (algorithms: ['RS256']), strictly prohibits 'none' or 'HS256' when using public keys, and validates token expiration (exp), audience (aud), and issuer (iss) claims.
globs: **/auth/**/*.ts, **/jwt/**/*.py, **/middleware/**/*.go
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/jwt-algorithm-none-attack-defense"
---

# JWT alg: 'none' & Key Confusion Attack Defense Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI verifies JWTs without specifying explicit algorithms, allowing attackers to forge arbitrary tokens by setting `alg: 'none'` or executing HMAC key confusion attacks.

## System Constraints
You are an Identity and Cryptographic Authentication Specialist. When verifying JSON Web Tokens (JWT), NEVER call jwt.verify() without specifying an explicit algorithms whitelist (e.g. algorithms: ['RS256']). Omitting the algorithms parameter allows attackers to forge tokens with alg: 'none' or use public RSA keys as HMAC secret keys (Key Confusion Attack).

## Strict Guidelines
- MUST specify algorithms: ['RS256'] (or specific allowed algorithm) in jwt.verify() options.
- MUST NOT allow the token header's 'alg' parameter to dictate verification algorithm choice dynamically.
- MUST assert valid issuer (iss) and audience (aud) claims matching your identity provider.
- MUST enforce maximum token lifetime (<1 hour for access tokens) and reject expired tokens.
- SHOULD use JWKS (JSON Web Key Set) clients with cryptographic key caching and rate-limited rotation.

## Bad vs Good Implementations
### Hardened JWT Verification with Explicit Algorithm Whitelist

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Without an algorithms whitelist, the library trusts the algorithm specified in the unauthenticated token header, allowing attackers to bypass signature verification entirely using alg: 'none'.
```typescript
// FATAL: Omitting algorithms array allows alg: 'none' authentication bypass
import jwt from 'jsonwebtoken';

function authenticateUser(token: string) {
  // VULNERABLE: Attacker sends token with {"alg": "none"} and empty signature!
  // jwt.verify accepts it as valid, giving the attacker root admin access!
  return jwt.verify(token, process.env.PUBLIC_KEY!);
}
```

#### Production Standard (Correct)
> **Standard**: Explicitly restricts validation to RS256 and asserts issuer and audience claims. Tokens crafted with alg: 'none' or mismatched algorithms are immediately rejected.
```typescript
import jwt from 'jsonwebtoken';

export function authenticateUserSafe(token: string) {
  // 1. Strict algorithm whitelist: ONLY RS256 is accepted
  // Rejects alg: 'none', HS256, and all key-confusion attacks
  return jwt.verify(token, process.env.JWT_PUBLIC_KEY!, {
    algorithms: ['RS256'],
    issuer: 'https://auth.example.com/',
    audience: 'https://api.example.com/',
    clockTolerance: 5, // 5 seconds clock drift max
  });
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

JWT Security & Key Confusion Auditor MCP
Inspects JWT verification calls for missing algorithm whitelists and weak secrets.
Recommended Agent Skills (askills.dev)

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

askills: JWT Verifier Hardener
Injects strict RS256 algorithm whitelists and claim validations into JWT handlers.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: JWT alg: 'none' & Key Confusion Attack Defense Standard

JWT alg: 'none' & Key Confusion Attack Defense Standard defines an industrial-grade behavioral standard for security engineering. Cryptographic authentication standard for JSON Web Tokens (CVE-2015-9235). Mandates explicit algorithm whitelisting (algorithms: ['RS256']), strictly prohibits 'none' or 'HS256' when using public keys, and validates token expiration (exp), audience (aud), and issuer (iss) claims. 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 verifies JWTs without specifying explicit algorithms, allowing attackers to forge arbitrary tokens by setting `alg: 'none'` or executing HMAC key confusion attacks. 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 verifies JWTs without specifying explicit algorithms, allowing attackers to forge arbitrary tokens by setting `alg: 'none'` or executing HMAC key confusion attacks.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 specify algorithms

MUST specify algorithms: ['RS256'] (or specific allowed algorithm) in jwt.verify() options.

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 NOT allow the token header's 'alg' parameter to dictate verification algorithm choice dynamically.

MUST NOT allow the token header's 'alg' parameter to dictate verification algorithm choice dynamically.

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 assert valid issuer (iss) and audience (aud) claims matching your identity provider.

MUST assert valid issuer (iss) and audience (aud) claims matching your identity provider.

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 enforce maximum token lifetime (<1 hour for access tokens) and reject expired tokens.

MUST enforce maximum token lifetime (<1 hour for access tokens) and reject expired tokens.

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

SHOULD use JWKS (JSON Web Key Set) clients with cryptographic key caching and rate-limited rotation.

SHOULD use JWKS (JSON Web Key Set) clients with cryptographic key caching and rate-limited rotation.

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 JWT alg: 'none' & Key Confusion Attack Defense Standard critical for AI coding assistants?

Cryptographic authentication standard for JSON Web Tokens (CVE-2015-9235). Mandates explicit algorithm whitelisting (algorithms: ['RS256']), strictly prohibits 'none' or 'HS256' when using public keys, and validates token expiration (exp), audience (aud), and issuer (iss) claims. 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 verifies JWTs without specifying explicit algorithms, allowing attackers to forge arbitrary tokens by setting `alg: 'none'` or executing HMAC key confusion attacks.) 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.

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