any type used

Vulnerability potential Low
DDoS potential None

any disables TypeScript’s checks for this binding; prefer a precise type, unknown, or a generic

Impact

any is TypeScript’s escape hatch: a value typed any is exempt from all static checking. You can read any property, call it, index it, pass it anywhere and assign anything to it, and the compiler stays silent. The defect is that this silence is contagious — any flows outward. Accessing a property of an any yields any, the return of a function typed to return any is any, so a single any at an API boundary erases the types of everything downstream that touches it. The cost is the loss of exactly the guarantees TypeScript exists to provide: typos, wrong argument counts, calls on undefined, and shape mismatches that would have been compile errors instead surface as runtime TypeErrors in production, and editor autocomplete/refactoring quietly stops working on the affected values.

Vulnerability potential

any is a type-safety defect, not a direct vulnerability, but it has a real if secondary security effect: it removes the compiler’s ability to enforce the shape and type of data, and that often matters most exactly where untrusted input enters.

  1. Unvalidated input flows unchecked. A request body or parsed JSON typed as any can be passed straight into queries, filesystem paths, or HTML without the type system signalling that its shape was never verified, masking missing validation that could enable injection or path traversal.
  2. Eroded invariants. Type confusion introduced by any (a number where a string was expected, a missing field assumed present) can cause logic to take an unintended branch, including in authorization code.

These are indirect, so the rating is Low — any does not create a hole by itself, it removes a guard rail that would have helped catch one.

Technical details

any is the type that is assignable to and from every other type, and which disables property/call/index checking on its values. This bidirectional assignability is what makes it spread: assign an any into a typed variable and the unsoundness travels with it, with no cast and no warning.

any vs unknown

unknown is the type-safe counterpart. It also holds any value, but you can do nothing with an unknown until you narrow it (via typeof, instanceof, a discriminant check, or a type guard). So unknown forces validation where any skips it; prefer unknown for genuinely dynamic data (JSON, catch bindings, external APIs) and narrow before use.

Where any sneaks in

It often arrives implicitly: an untyped function parameter, a JSON.parse result (typed any), catch (e) before TS 4.4 (now unknown under useUnknownInCatchVariables), and untyped third-party modules. These implicit anys are the ones most worth eliminating.

Better alternatives

Use a precise interface/type, a union, or generics to keep a relationship between input and output types (function first<T>(xs: T[]): T) instead of any[] -> any.

Catching the issue

Compiler

Enable strict (which turns on noImplicitAny) so the compiler errors on any parameter or variable whose type would silently become any. Add useUnknownInCatchVariables (on under strict) so catch bindings are unknown, not any.

Linters

@typescript-eslint provides a battery of rules: no-explicit-any (ban the keyword), and the no-unsafe-* family (no-unsafe-assignment, no-unsafe-member-access, no-unsafe-call, no-unsafe-argument, no-unsafe-return) which flag uses of an any value even when it arrived implicitly from an untyped dependency — these catch what noImplicitAny cannot.

Boundaries

At input boundaries, validate with a schema library (Zod, io-ts, valibot) that parses unknown/external data into a precisely typed value, so any never enters. In review, treat each any and each as any cast as needing a justification.

How to reproduce

Observe that the any version compiles cleanly yet throws at run time, while the unknown version forces a check and is caught by the compiler.

function lengthOf(x: any): number {
  return x.length; // no error, even though x may have no `length`
}

lengthOf(42); // compiles; throws nothing but returns undefined-as-number nonsense
// (42).length is undefined -> returned as number, silent garbage

// safe alternative: unknown forces narrowing
function safeLength(x: unknown): number {
  if (typeof x === "string" || Array.isArray(x)) return x.length;
  throw new TypeError("no length");
}
// safeLength(42) -> compiler is happy, runtime throws a clear error