diff --git a/AGENTS.md b/AGENTS.md
index 3183224..63b53c6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -48,6 +48,7 @@ Both the resource `id` and every modifier are attacker-controlled. Existing prot
 - `ipxHttpStorage`: domain allowlist, `http(s)` only, redirects followed manually one hop at a time and re-validated (SSRF).
 - `ipxFSStorage`: resolved path must stay inside the configured dir (traversal).
 - `maxOutputDimension` (default 8192) clamps `width`/`height`/`resize`/`extend` so a tiny source cannot force a multi-GB allocation.
+- `allowedModifiers` (opt-in) rejects any modifier not listed (aliases follow their modifier) with a `400` before the source is fetched.
 - SVG is always sanitized (`src/svg.ts`), even with optimization disabled.
 - Server sends `content-security-policy: default-src 'none'` and `x-content-type-options: nosniff`.
 - `safeString()` in `server.ts` escapes parser output — custom `parseURL` results are never trusted.
diff --git a/README.md b/README.md
index 3cde43c..54a806e 100644
--- a/README.md
+++ b/README.md
@@ -480,9 +480,12 @@ Every option can also be set universally with an `IPX_*` environment variable, w
 | -------------------- | -------------------------- | ------- | --------------------------------------------------------- |
 | `alias`              | `IPX_ALIAS`                | `{}`    | Map URL prefixes to other prefixes or remote origins.     |
 | `maxOutputDimension` | `IPX_MAX_OUTPUT_DIMENSION` | `8192`  | Maximum width and height (in pixels) of the output image. |
+| `allowedModifiers`   | `IPX_ALLOWED_MODIFIERS`    | (all)   | Modifiers that requests are allowed to use.               |
 
 Requested `width`, `height` and `resize` dimensions are clamped to `maxOutputDimension`, preserving the requested aspect ratio, and `extend` edges are clamped so the extended canvas stays within it. This bounds how much memory a single request can allocate: sharp only limits the _input_ size, so without it `/enlarge,s_20000x20000/image.jpg` (or `/extend_10000_10000_10000_10000/image.jpg`) allocates gigabytes from a small source image. Set to `false` to disable, which is only safe when modifiers come from a trusted source.
 
+`allowedModifiers` restricts a public endpoint to the modifiers a site actually uses, for example `allowedModifiers: ["width", "height", "format", "quality"]` or `IPX_ALLOWED_MODIFIERS=width,height,format,quality`. Aliases follow their modifier (allowing `width` also allows `w`). Any other modifier, including an unknown one, is rejected with a `400` before the source is fetched, so expensive operations (`blur`, `median`, an `avif` re-encode, ...) cannot be triggered and junk modifiers cannot be used to bypass a CDN cache.
+
 ### Filesystem source (`ipxFSStorage`)
 
 Enabled by default with the CLI only.
diff --git a/src/ipx.ts b/src/ipx.ts
index b321def..75a0841 100644
--- a/src/ipx.ts
+++ b/src/ipx.ts
@@ -162,6 +162,22 @@ export type IPXOptions = {
    */
   maxOutputDimension?: number | false;
 
+  /**
+   * Modifiers that requests are allowed to use. Any other modifier (including
+   * an unknown one) is rejected with a `400` before the source is fetched.
+   *
+   * Every modifier is allowed by default. Restricting them to the ones a site
+   * actually uses (for example `["width", "height", "format", "quality"]`)
+   * shrinks the attack surface of the public endpoint: expensive operations
+   * such as `blur`, `median` or an `avif` re-encode cannot be triggered, and
+   * junk modifiers cannot be used to bypass a CDN cache.
+   *
+   * Aliases follow their modifier: allowing `width` also allows `w`.
+   *
+   * @optional
+   */
+  allowedModifiers?: (keyof IPXModifiers)[];
+
   /**
    * A mapping of URL aliases to their corresponding URLs, used to simplify resource identifiers.
    * @optional
@@ -215,6 +231,45 @@ export type IPXOptions = {
 // which bounds what a single request can allocate.
 const DEFAULT_MAX_OUTPUT_DIMENSION = 8192;
 
+// `format` and `animated` are read directly by `createIPX` rather than through
+// a handler, so their aliases are spelled out here.
+const NON_HANDLER_MODIFIERS = new Map([
+  ["format", "format"],
+  ["f", "format"],
+  ["animated", "animated"],
+  ["a", "animated"],
+]);
+
+// Handler aliases share the handler object (`w === width`), so comparing by
+// handler makes an allowed modifier cover all of its aliases.
+function getModifierKey(name: string): unknown {
+  return NON_HANDLER_MODIFIERS.get(name) ?? getHandler(name as HandlerName);
+}
+
+function resolveAllowedModifiers(
+  names: string | string[] | undefined,
+): Set<unknown> | undefined {
+  if (names === undefined) {
+    return undefined;
+  }
+  if (typeof names === "string") {
+    names = names
+      .split(",")
+      .map((s) => s.trim())
+      .filter(Boolean);
+  }
+  return new Set(
+    names.map((name) => {
+      const key = getModifierKey(name);
+      // Fail loudly on a typo rather than rejecting every request using it.
+      if (!key) {
+        throw new Error(`[ipx] Unknown modifier in allowedModifiers: ${name}`);
+      }
+      return key;
+    }),
+  );
+}
+
 // https://sharp.pixelplumbing.com/#formats
 // (gif and svg are not supported as output)
 const SUPPORTED_FORMATS = new Set([
@@ -253,6 +308,11 @@ export function createIPX(userOptions: IPXOptions): IPX {
     } as SharpOptions,
   } satisfies Omit<IPXOptions, "storage">;
 
+  const allowedModifiers = resolveAllowedModifiers(
+    userOptions.allowedModifiers ??
+      getEnv<string | string[]>("IPX_ALLOWED_MODIFIERS"),
+  );
+
   // Normalize alias to start with leading slash
   options.alias = Object.fromEntries(
     Object.entries(options.alias || {}).map((e) => [
@@ -283,6 +343,19 @@ export function createIPX(userOptions: IPXOptions): IPX {
       });
     }
 
+    // Validate modifiers (before any source is fetched or processed)
+    if (allowedModifiers) {
+      for (const name of Object.keys(modifiers)) {
+        if (!allowedModifiers.has(getModifierKey(name))) {
+          throw new HTTPError({
+            statusCode: 400,
+            statusText: `IPX_FORBIDDEN_MODIFIER`,
+            message: `Modifier is not allowed: ${name}`,
+          });
+        }
+      }
+    }
+
     // Enforce leading slash for non absolute urls
     id = hasProtocol(id) ? id : withLeadingSlash(id);
 
diff --git a/test/index.test.ts b/test/index.test.ts
index 5566697..23e5b83 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -6,6 +6,7 @@ import { imageMeta } from "image-meta";
 
 import {
   type IPX,
+  type IPXOptions,
   type IPXStorage,
   createIPX,
   createIPXFetchHandler,
@@ -411,6 +412,99 @@ describe("ipx", () => {
     });
   });
 
+  describe("allowedModifiers", () => {
+    const createRestrictedIPX = (
+      allowedModifiers?: IPXOptions["allowedModifiers"],
+    ) =>
+      createIPX({
+        storage: ipxFSStorage({ dir: resolve(__dirname, "assets") }),
+        allowedModifiers,
+      });
+
+    const forbidden = { statusCode: 400, statusText: "IPX_FORBIDDEN_MODIFIER" };
+
+    it("allows listed modifiers and their aliases", async () => {
+      const restricted = createRestrictedIPX(["width", "format"]);
+      for (const modifiers of [
+        { width: "100", format: "png" },
+        { w: "100", f: "png" },
+      ]) {
+        const { data, format } = await restricted(
+          "bliss.jpg",
+          modifiers,
+        ).process();
+        expect(format).toBe("png");
+        expect(imageMeta(data as Uint8Array).width).toBe(100);
+      }
+    });
+
+    it("allows a modifier listed by its alias", async () => {
+      const restricted = createRestrictedIPX(["w"]);
+      await expect(
+        restricted("bliss.jpg", { width: "100" }).process(),
+      ).resolves.toBeDefined();
+    });
+
+    it.each<[string, Record<string, any>]>([
+      ["another modifier", { width: "100", blur: "5" }],
+      ["another modifier's alias", { w: "100", h: "100" }],
+      ["`animated` alias", { a: "" }],
+      ["an unknown modifier", { width: "100", cachebust: "1" }],
+    ])("rejects %s", (_name, modifiers) => {
+      const restricted = createRestrictedIPX(["width", "format"]);
+      expect(() => restricted("bliss.jpg", modifiers)).toThrow(
+        expect.objectContaining(forbidden),
+      );
+    });
+
+    // Rejected up front, so the source is never fetched (or found missing).
+    it("rejects before resolving the source", () => {
+      const restricted = createRestrictedIPX(["width"]);
+      expect(() => restricted("missing.jpg", { blur: "5" })).toThrow(
+        expect.objectContaining(forbidden),
+      );
+    });
+
+    it("allows no modifiers when empty", async () => {
+      const restricted = createRestrictedIPX([]);
+      expect(() => restricted("bliss.jpg", { width: "100" })).toThrow(
+        expect.objectContaining(forbidden),
+      );
+      await expect(restricted("bliss.jpg").process()).resolves.toBeDefined();
+    });
+
+    it("throws on an unknown modifier name", () => {
+      expect(() => createRestrictedIPX(["widht" as any])).toThrow(
+        "Unknown modifier in allowedModifiers: widht",
+      );
+    });
+
+    it("can be set with `IPX_ALLOWED_MODIFIERS`", () => {
+      vi.stubEnv("IPX_ALLOWED_MODIFIERS", "width, format");
+      try {
+        const restricted = createRestrictedIPX();
+        expect(() =>
+          restricted("bliss.jpg", { w: "100", f: "png" }),
+        ).not.toThrow();
+        expect(() => restricted("bliss.jpg", { blur: "5" })).toThrow(
+          expect.objectContaining(forbidden),
+        );
+      } finally {
+        vi.unstubAllEnvs();
+      }
+    });
+
+    it("responds with a 400 from the server", async () => {
+      const handler = createIPXFetchHandler(createRestrictedIPX(["width"]));
+      expect((await handler("http://localhost/w_100/bliss.jpg")).status).toBe(
+        200,
+      );
+      expect(
+        (await handler("http://localhost/w_100,blur_5/bliss.jpg")).status,
+      ).toBe(400);
+    });
+  });
+
   describe("maxAge", () => {
     const sourceMaxAge = async (maxAge: unknown) => {
       const storage: IPXStorage = {