security: authorize and validate presigned upload requests

POST /api/v1/uploads minted an upload key from caller input with no
ownership check, no content-type allowlist, and the raw filename
interpolated into the S3 key.

- Ownership: a caller may only mint upload URLs for a route/activity
  they own, enforced via the branded loadOwnedRoute/loadOwnedActivity
  (404 on miss, no existence leak). Closes writing into another
  user's resource key-space.
- Content type: the request schema now constrains contentType to an
  image/gpx allowlist, so active content (HTML/SVG/JS) that would
  execute if served inline is rejected at the request boundary.
- Filename: sanitizeUploadFilename() reduces the client filename to a
  safe basename (charset-restricted, no path components, no leading
  dots, length-bounded) before it becomes part of the key.

Tests: schema allow/deny + filename sanitization in @trails-cool/api;
handler tests covering owned-success, not-owner 404, disallowed
content-type, and the route-vs-activity ownership branch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/api/src/index.ts

@@ -55,5 +55,6 @@ export {
 // Uploads
 export {
   PresignedUploadRequestSchema, PresignedUploadResponseSchema,
+  ALLOWED_UPLOAD_CONTENT_TYPES, sanitizeUploadFilename,
   type PresignedUploadRequest, type PresignedUploadResponse,
 } from "./uploads.ts";

packages/api/src/uploads.test.ts

@@ -0,0 +1,63 @@
+import { describe, it, expect } from "vitest";
+import {
+  PresignedUploadRequestSchema,
+  sanitizeUploadFilename,
+} from "./uploads.ts";
+
+const ROUTE_ID = "11111111-1111-4111-8111-111111111111";
+
+describe("PresignedUploadRequestSchema", () => {
+  it("accepts an allowed content type", () => {
+    const r = PresignedUploadRequestSchema.safeParse({
+      filename: "photo.jpg",
+      contentType: "image/jpeg",
+      resourceType: "activity",
+      resourceId: ROUTE_ID,
+    });
+    expect(r.success).toBe(true);
+  });
+
+  it("rejects active/disallowed content types", () => {
+    for (const contentType of ["text/html", "image/svg+xml", "application/javascript"]) {
+      const r = PresignedUploadRequestSchema.safeParse({
+        filename: "x", contentType, resourceType: "route", resourceId: ROUTE_ID,
+      });
+      expect(r.success, contentType).toBe(false);
+    }
+  });
+
+  it("rejects a non-uuid resourceId and empty filename", () => {
+    expect(PresignedUploadRequestSchema.safeParse({
+      filename: "a.png", contentType: "image/png", resourceType: "route", resourceId: "nope",
+    }).success).toBe(false);
+    expect(PresignedUploadRequestSchema.safeParse({
+      filename: "", contentType: "image/png", resourceType: "route", resourceId: ROUTE_ID,
+    }).success).toBe(false);
+  });
+});
+
+describe("sanitizeUploadFilename", () => {
+  it("strips path components (no traversal into the key)", () => {
+    expect(sanitizeUploadFilename("../../etc/passwd")).toBe("passwd");
+    expect(sanitizeUploadFilename("/abs/path/img.png")).toBe("img.png");
+    expect(sanitizeUploadFilename("a\\b\\c.jpg")).toBe("c.jpg");
+  });
+
+  it("restricts to a conservative charset and collapses runs", () => {
+    expect(sanitizeUploadFilename("my photo (1)!.jpg")).toBe("my_photo_1_.jpg");
+  });
+
+  it("strips leading dots so dotfiles / .. can't form the segment", () => {
+    expect(sanitizeUploadFilename("...hidden")).toBe("hidden");
+    expect(sanitizeUploadFilename("..")).toBe("upload");
+  });
+
+  it("never returns empty", () => {
+    expect(sanitizeUploadFilename("")).toBe("upload");
+    expect(sanitizeUploadFilename("////")).toBe("upload");
+  });
+
+  it("bounds the length", () => {
+    expect(sanitizeUploadFilename("a".repeat(500)).length).toBeLessThanOrEqual(128);
+  });
+});

packages/api/src/uploads.ts

@@ -1,12 +1,40 @@
 import { z } from "zod";
 
+/**
+ * Content types accepted for uploads. Constrained so a caller can't
+ * stash active content (HTML/SVG/JS) that would execute if the object
+ * were ever served inline. Keep in sync with what the upload UI sends.
+ */
+export const ALLOWED_UPLOAD_CONTENT_TYPES = [
+  "image/jpeg",
+  "image/png",
+  "image/webp",
+  "image/gif",
+  "application/gpx+xml",
+] as const;
+
 export const PresignedUploadRequestSchema = z.object({
-  filename: z.string(),
-  contentType: z.string(),
+  filename: z.string().min(1).max(255),
+  contentType: z.enum(ALLOWED_UPLOAD_CONTENT_TYPES),
   resourceType: z.enum(["route", "activity"]),
   resourceId: z.uuid(),
 });
 
+/**
+ * Reduce a client-supplied filename to a safe S3 object-key segment:
+ * basename only (drop any path), restrict to a conservative charset,
+ * collapse runs, strip leading dots, and bound the length. Never empty.
+ */
+export function sanitizeUploadFilename(filename: string): string {
+  const base = filename.split(/[/\\]/).pop() ?? "";
+  const cleaned = base
+    .replace(/[^A-Za-z0-9._-]/g, "_")
+    .replace(/_{2,}/g, "_")
+    .replace(/^\.+/, "")
+    .slice(0, 128);
+  return cleaned.length > 0 ? cleaned : "upload";
+}
+
 export const PresignedUploadResponseSchema = z.object({
   uploadUrl: z.url(),
   storageKey: z.string(),