diff --git a/CHANGELOG.md b/CHANGELOG.md
index aab225a..50404aa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Changed
+
+- **Breaking:** Where possible, increase TypeScript strictness around some strings. Only affects TypeScript users. See [#369](https://github.com/helmetjs/helmet/issues/369)
+
 ### Removed
 
 - **Breaking:** Dropped support for Node 12 and 13. Node 14+ is now required
diff --git a/middlewares/cross-origin-embedder-policy/index.ts b/middlewares/cross-origin-embedder-policy/index.ts
index 43a166c..992f6b3 100644
--- a/middlewares/cross-origin-embedder-policy/index.ts
+++ b/middlewares/cross-origin-embedder-policy/index.ts
@@ -1,7 +1,7 @@
 import { IncomingMessage, ServerResponse } from "http";
 
 export interface CrossOriginEmbedderPolicyOptions {
-  policy?: string;
+  policy?: "require-corp" | "credentialless";
 }
 
 const ALLOWED_POLICIES = new Set(["require-corp", "credentialless"]);
diff --git a/middlewares/cross-origin-opener-policy/index.ts b/middlewares/cross-origin-opener-policy/index.ts
index 1200ced..4c6bc03 100644
--- a/middlewares/cross-origin-opener-policy/index.ts
+++ b/middlewares/cross-origin-opener-policy/index.ts
@@ -1,7 +1,7 @@
 import { IncomingMessage, ServerResponse } from "http";
 
 export interface CrossOriginOpenerPolicyOptions {
-  policy?: string;
+  policy?: "same-origin" | "same-origin-allow-popups" | "unsafe-none";
 }
 
 const ALLOWED_POLICIES = new Set([
diff --git a/middlewares/cross-origin-resource-policy/CHANGELOG.md b/middlewares/cross-origin-resource-policy/CHANGELOG.md
index d2cc8ae..cd3f19d 100644
--- a/middlewares/cross-origin-resource-policy/CHANGELOG.md
+++ b/middlewares/cross-origin-resource-policy/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Changed
+
+- **Breaking:** increase TypeScript strictness around arguments. Only affects TypeScript users. See [helmetjs/helmet#369](https://github.com/helmetjs/helmet/issues/369)
+
 ## 0.3.0 - 2021-04-17
 
 ### Added
diff --git a/middlewares/cross-origin-resource-policy/index.ts b/middlewares/cross-origin-resource-policy/index.ts
index 74e6938..a24e91d 100644
--- a/middlewares/cross-origin-resource-policy/index.ts
+++ b/middlewares/cross-origin-resource-policy/index.ts
@@ -1,7 +1,7 @@
 import { IncomingMessage, ServerResponse } from "http";
 
 export interface CrossOriginResourcePolicyOptions {
-  policy?: string;
+  policy?: "same-origin" | "same-site" | "cross-origin";
 }
 
 const ALLOWED_POLICIES = new Set(["same-origin", "same-site", "cross-origin"]);
diff --git a/middlewares/referrer-policy/CHANGELOG.md b/middlewares/referrer-policy/CHANGELOG.md
index 0e4f2a0..9ca3d20 100644
--- a/middlewares/referrer-policy/CHANGELOG.md
+++ b/middlewares/referrer-policy/CHANGELOG.md
@@ -1,10 +1,16 @@
 # Changelog
 
+## Unreleased
+
+### Changed
+
+- **Breaking:** increase TypeScript strictness around arguments. Only affects TypeScript users. See [helmetjs/helmet#369](https://github.com/helmetjs/helmet/issues/369)
+
 ## 2.0.0 - Unreleased
 
 ### Removed
 
-- Dropped support for old Node versions. Node 10+ is now required
+- **Breaking:** Dropped support for old Node versions. Node 10+ is now required
 
 ## 1.2.0 - 2019-05-03
 
diff --git a/middlewares/referrer-policy/index.ts b/middlewares/referrer-policy/index.ts
index 0e6e7e3..c3ca27c 100644
--- a/middlewares/referrer-policy/index.ts
+++ b/middlewares/referrer-policy/index.ts
@@ -1,10 +1,21 @@
 import { IncomingMessage, ServerResponse } from "http";
 
+type ReferrerPolicyToken =
+  | "no-referrer"
+  | "no-referrer-when-downgrade"
+  | "same-origin"
+  | "origin"
+  | "strict-origin"
+  | "origin-when-cross-origin"
+  | "strict-origin-when-cross-origin"
+  | "unsafe-url"
+  | "";
+
 export interface ReferrerPolicyOptions {
-  policy?: string | string[];
+  policy?: ReferrerPolicyToken | ReferrerPolicyToken[];
 }
 
-const ALLOWED_TOKENS = new Set<string>([
+const ALLOWED_TOKENS = new Set<ReferrerPolicyToken>([
   "no-referrer",
   "no-referrer-when-downgrade",
   "same-origin",
@@ -25,7 +36,7 @@ function getHeaderValueFromOptions({
     throw new Error("Referrer-Policy received no policy tokens");
   }
 
-  const tokensSeen = new Set<string>();
+  const tokensSeen = new Set<ReferrerPolicyToken>();
   tokens.forEach((token) => {
     if (!ALLOWED_TOKENS.has(token)) {
       throw new Error(
diff --git a/middlewares/x-frame-options/CHANGELOG.md b/middlewares/x-frame-options/CHANGELOG.md
index 300649f..0a90132 100644
--- a/middlewares/x-frame-options/CHANGELOG.md
+++ b/middlewares/x-frame-options/CHANGELOG.md
@@ -4,7 +4,7 @@
 
 ### Changed
 
-- Add TypeScript editor autocomplete. See [#322](https://github.com/helmetjs/helmet/pull/322)
+- **Breaking:** increase TypeScript strictness around arguments. Only affects TypeScript users. See [helmetjs/helmet#369](https://github.com/helmetjs/helmet/issues/369)
 
 ## 4.0.0 - 2020-12-21
 
diff --git a/middlewares/x-frame-options/index.ts b/middlewares/x-frame-options/index.ts
index cc99a24..6638c41 100644
--- a/middlewares/x-frame-options/index.ts
+++ b/middlewares/x-frame-options/index.ts
@@ -1,12 +1,11 @@
 import { IncomingMessage, ServerResponse } from "http";
 
 export interface XFrameOptionsOptions {
-  // This offers autocomplete while still supporting regular `string`s.
-  action?: "DENY" | "SAMEORIGIN" | (string & { _?: never });
+  action?: "deny" | "sameorigin";
 }
 
 function getHeaderValueFromOptions({
-  action = "SAMEORIGIN",
+  action = "sameorigin",
 }: Readonly<XFrameOptionsOptions>): string {
   const normalizedAction =
     typeof action === "string" ? action.toUpperCase() : action;
diff --git a/middlewares/x-permitted-cross-domain-policies/CHANGELOG.md b/middlewares/x-permitted-cross-domain-policies/CHANGELOG.md
index 94be14c..9ced42b 100644
--- a/middlewares/x-permitted-cross-domain-policies/CHANGELOG.md
+++ b/middlewares/x-permitted-cross-domain-policies/CHANGELOG.md
@@ -2,9 +2,13 @@
 
 ## Unreleased
 
+### Changed
+
+- **Breaking:** increase TypeScript strictness around arguments. Only affects TypeScript users. See [helmetjs/helmet#369](https://github.com/helmetjs/helmet/issues/369)
+
 ### Removed
 
-- Dropped support for old Node versions. Node 10+ is now required
+- Dropped support for old Node versions. Node 14+ is now required
 
 ## 0.5.0 - 2019-09-01
 
diff --git a/middlewares/x-permitted-cross-domain-policies/index.ts b/middlewares/x-permitted-cross-domain-policies/index.ts
index b991daa..96c0c44 100644
--- a/middlewares/x-permitted-cross-domain-policies/index.ts
+++ b/middlewares/x-permitted-cross-domain-policies/index.ts
@@ -1,7 +1,7 @@
 import { IncomingMessage, ServerResponse } from "http";
 
 export interface XPermittedCrossDomainPoliciesOptions {
-  permittedPolicies?: string;
+  permittedPolicies?: "none" | "master-only" | "by-content-type" | "all";
 }
 
 const ALLOWED_PERMITTED_POLICIES = new Set([
diff --git a/test/cross-origin-embedder-policy.test.ts b/test/cross-origin-embedder-policy.test.ts
index 692cb35..7cc52b3 100644
--- a/test/cross-origin-embedder-policy.test.ts
+++ b/test/cross-origin-embedder-policy.test.ts
@@ -18,7 +18,7 @@ describe("Cross-Origin-Embedder-Policy middleware", () => {
     );
   });
 
-  ["require-corp", "credentialless"].forEach((policy) => {
+  (["require-corp", "credentialless"] as const).forEach((policy) => {
     it(`sets "Cross-Origin-Embedder-Policy: ${policy}" when told to`, async () => {
       await check(crossOriginEmbedderPolicy({ policy }), {
         "cross-origin-embedder-policy": policy,
diff --git a/test/cross-origin-opener-policy.test.ts b/test/cross-origin-opener-policy.test.ts
index ff57056..07ff10a 100644
--- a/test/cross-origin-opener-policy.test.ts
+++ b/test/cross-origin-opener-policy.test.ts
@@ -15,7 +15,7 @@ describe("Cross-Origin-Opener-Policy middleware", () => {
     );
   });
 
-  ["same-origin", "same-origin-allow-popups", "unsafe-none"].forEach(
+  (["same-origin", "same-origin-allow-popups", "unsafe-none"] as const).forEach(
     (policy) => {
       it(`sets "Cross-Origin-Opener-Policy: ${policy}" when told to`, async () => {
         await check(crossOriginOpenerPolicy({ policy }), {
diff --git a/test/cross-origin-resource-policy.test.ts b/test/cross-origin-resource-policy.test.ts
index f1eb4b6..f725b23 100644
--- a/test/cross-origin-resource-policy.test.ts
+++ b/test/cross-origin-resource-policy.test.ts
@@ -18,7 +18,7 @@ describe("Cross-Origin-Resource-Policy middleware", () => {
     );
   });
 
-  ["same-origin", "same-site", "cross-origin"].forEach((policy) => {
+  (["same-origin", "same-site", "cross-origin"] as const).forEach((policy) => {
     it(`sets "Cross-Origin-Resource-Policy: ${policy}" when told to`, async () => {
       await check(crossOriginResourcePolicy({ policy }), {
         "cross-origin-resource-policy": policy,
diff --git a/test/referrer-policy.test.ts b/test/referrer-policy.test.ts
index 0b300fc..fb07ca4 100644
--- a/test/referrer-policy.test.ts
+++ b/test/referrer-policy.test.ts
@@ -17,17 +17,19 @@ describe("Referrer-Policy middleware", () => {
     });
   });
 
-  [
-    "no-referrer",
-    "no-referrer-when-downgrade",
-    "same-origin",
-    "origin",
-    "strict-origin",
-    "origin-when-cross-origin",
-    "strict-origin-when-cross-origin",
-    "unsafe-url",
-    "",
-  ].forEach((policy) => {
+  (
+    [
+      "no-referrer",
+      "no-referrer-when-downgrade",
+      "same-origin",
+      "origin",
+      "strict-origin",
+      "origin-when-cross-origin",
+      "strict-origin-when-cross-origin",
+      "unsafe-url",
+      "",
+    ] as const
+  ).forEach((policy) => {
     it(`can set the header to "${policy}" by specifying it as a string`, async () => {
       await check(referrerPolicy({ policy }), {
         "referrer-policy": policy,
diff --git a/test/x-frame-options.test.ts b/test/x-frame-options.test.ts
index fc09583..7277ffc 100644
--- a/test/x-frame-options.test.ts
+++ b/test/x-frame-options.test.ts
@@ -18,38 +18,42 @@ describe("X-Frame-Options middleware", () => {
   });
 
   it('can set "X-Frame-Options: DENY"', async () => {
-    await check(xFrameOptions({ action: "DENY" }), {
+    await check(xFrameOptions({ action: "deny" }), {
       "x-frame-options": "DENY",
     });
-    await check(xFrameOptions({ action: "deny" }), {
+
+    // These are not allowed by the types, but are supported.
+    await check(xFrameOptions({ action: "DENY" as any }), {
       "x-frame-options": "DENY",
     });
-    await check(xFrameOptions({ action: "deNY" }), {
+    await check(xFrameOptions({ action: "deNY" as any }), {
       "x-frame-options": "DENY",
     });
   });
 
   it('can set "X-Frame-Options: SAMEORIGIN" when specified', async () => {
-    await check(xFrameOptions({ action: "SAMEORIGIN" }), {
+    await check(xFrameOptions({ action: "sameorigin" }), {
       "x-frame-options": "SAMEORIGIN",
     });
-    await check(xFrameOptions({ action: "sameorigin" }), {
+
+    // These are not allowed by the types, but are supported.
+    await check(xFrameOptions({ action: "SAMEORIGIN" as any }), {
       "x-frame-options": "SAMEORIGIN",
     });
-    await check(xFrameOptions({ action: "sameORIGIN" }), {
+    await check(xFrameOptions({ action: "sameORIGIN" as any }), {
       "x-frame-options": "SAMEORIGIN",
     });
-    await check(xFrameOptions({ action: "SAME-ORIGIN" }), {
+    await check(xFrameOptions({ action: "SAME-ORIGIN" as any }), {
       "x-frame-options": "SAMEORIGIN",
     });
-    await check(xFrameOptions({ action: "same-origin" }), {
+    await check(xFrameOptions({ action: "same-origin" as any }), {
       "x-frame-options": "SAMEORIGIN",
     });
   });
 
   it("throws when passed invalid actions", () => {
     for (const action of ["allow-from", "ALLOW-FROM"]) {
-      expect(() => xFrameOptions({ action })).toThrow(
+      expect(() => xFrameOptions({ action: action as any })).toThrow(
         /^X-Frame-Options no longer supports `ALLOW-FROM` due to poor browser support. See <https:\/\/github.com\/helmetjs\/helmet\/wiki\/How-to-use-X%E2%80%93Frame%E2%80%93Options's-%60ALLOW%E2%80%93FROM%60-directive> for more info.$/
       );
     }
diff --git a/test/x-permitted-cross-domain-policies.test.ts b/test/x-permitted-cross-domain-policies.test.ts
index 13c49d0..ac74fa6 100644
--- a/test/x-permitted-cross-domain-policies.test.ts
+++ b/test/x-permitted-cross-domain-policies.test.ts
@@ -18,7 +18,7 @@ describe("X-Permitted-Cross-Domain-Policies middleware", () => {
     );
   });
 
-  ["none", "master-only", "by-content-type", "all"].forEach(
+  (["none", "master-only", "by-content-type", "all"] as const).forEach(
     (permittedPolicies) => {
       it(`sets "X-Permitted-Cross-Domain-Policies: ${permittedPolicies}" when told to`, async () => {
         await check(xPermittedCrossDomainPolicies({ permittedPolicies }), {