TypeScript Security: Why Type Safety Is Not Enough

TypeScript's type system creates a false sense of security. Developers assume that compile-time type checking prevents security vulnerabilities, but TypeScript types are erased at runtime—they provide zero runtime protection against malicious input.

Here is what TypeScript's type system does not prevent:

• Injection attacks: A string type annotation does not distinguish between safe text and a SQL injection payload. function query(id: string) accepts "1; DROP TABLE users" just as readily as "1".
• XSS vulnerabilities: Typed React components still allow dangerouslySetInnerHTML. Angular's bypassSecurityTrustHtml() accepts any string regardless of types.
• Prototype pollution: TypeScript's Record<string, unknown> type does not prevent __proto__ key injection during object merging.
• Runtime type mismatches: Data from APIs, databases, and user input is not validated against TypeScript interfaces at runtime. A User type annotation on an API response does not guarantee the data actually matches.
• Any and type assertions: as any, @ts-ignore, and type assertions bypass the type system entirely, and codebases under deadline pressure accumulate these escape hatches.

TypeScript adds developer experience and catches certain bug classes, but treating it as a security layer is a misconception that leads to unprotected code in production.

TypeScript inherits all JavaScript vulnerabilities and adds its own patterns that create false confidence.

Type Assertion Abuse

// Dangerous: asserting untrusted API data is safe
const userData = (await fetch('/api/user').then(r => r.json())) as User;
// userData could contain any shape - no runtime validation

// Also dangerous: suppressing type errors
const config = JSON.parse(rawInput) as any;
db.query(config.query); // SQL injection, type system bypassed

Framework-Specific Risks

TypeScript frameworks introduce their own vulnerability patterns:

• Next.js: Server-side rendering with unsanitized user data, exposed API routes with missing authentication, and getServerSideProps returning sensitive data to the client.
• Angular: bypassSecurityTrustHtml(), bypassSecurityTrustUrl(), and template injection through dynamic component rendering.
• NestJS: Missing validation pipes on controller parameters, exposed GraphQL introspection, and insecure JWT configurations.

TypeScript-Specific Anti-Patterns

// Generic Record types hide injection risks
function processData(input: Record<string, string>) {
  db.query(`SELECT * FROM ${input.table} WHERE id = ${input.id}`);
  // Types look correct, but this is SQL injection
}

These vulnerabilities compile without errors. TypeScript's compiler catches type mismatches, not security flaws.

TypeScript's adoption in production systems has grown rapidly, and with it the number of security incidents in TypeScript codebases:

• Next.js middleware bypass (CVE-2025-29927): A vulnerability in Next.js (TypeScript) allowed attackers to bypass middleware-based authentication by sending a crafted header, granting unauthorized access to protected routes. This affected applications relying on middleware for security checks.
• Angular Universal SSTI: Server-side rendered Angular applications have been vulnerable to template injection when user input reaches template compilation. TypeScript types provided no protection because the vulnerability exists at the template engine level.
• Type confusion in API boundaries: Multiple production incidents have occurred when TypeScript applications trusted API response types without runtime validation. Attackers modified API responses (via MITM or compromised endpoints) to inject malicious payloads that TypeScript's compile-time types could not catch.
• NestJS authentication bypass: Misconfigured decorators and missing validation pipes in NestJS applications have exposed sensitive endpoints. TypeScript's decorators look authoritative but do not enforce security without proper guard implementation.

These incidents demonstrate that TypeScript's type system is a developer tool, not a security boundary. Runtime validation, input sanitization, and security-aware static analysis remain essential.

CodeSlick's TypeScript analyzer is uniquely powerful because it integrates directly with the TypeScript Compiler API—going beyond pattern matching to understand type information, control flow, and data dependencies.

• 56 security checks: The most comprehensive TypeScript security analysis available, covering XSS, injection, prototype pollution, insecure deserialization, and framework-specific vulnerabilities.
• Compiler API integration: CodeSlick uses the TypeScript Compiler API to resolve types, track data flow through generics, and detect type assertion abuse that masks security issues.
• 95%+ type error detection: Beyond security, CodeSlick catches type errors that the standard tsc compiler reports, providing a unified analysis experience.
• Framework coverage: Specific checks for Next.js, Angular, NestJS, and Express patterns including middleware misconfigurations, missing validation, and unsafe rendering.
• AI code detection: 150 signals identifying AI-generated TypeScript including hallucinated type definitions, fabricated generic patterns, and insecure AI suggestions that compile but are vulnerable.

All findings include CWE classification, CVSS 3.1 severity scoring, and OWASP Top 10:2025 mapping. The Compiler API integration means CodeSlick catches vulnerabilities hidden behind type assertions and generic types that text-based scanners miss. Scan your TypeScript at /analyze for free.