diff --git a/apps/planner/app/lib/url-validation.server.test.ts b/apps/planner/app/lib/url-validation.server.test.ts
new file mode 100644
index 0000000..ad5dc07
--- /dev/null
+++ b/apps/planner/app/lib/url-validation.server.test.ts
@@ -0,0 +1,82 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import {
+  validateFetchUrl,
+  validateRedirectUrl,
+  getCallbackAllowedHosts,
+} from "./url-validation.server.ts";
+
+describe("validateFetchUrl", () => {
+  it("accepts a plain https URL", () => {
+    expect(validateFetchUrl("https://journal.trails.cool/api/cb").ok).toBe(true);
+  });
+
+  it("rejects javascript: scheme", () => {
+    const r = validateFetchUrl("javascript:alert(1)");
+    expect(r.ok).toBe(false);
+    expect(r.reason).toMatch(/scheme/);
+  });
+
+  it("rejects file: scheme", () => {
+    expect(validateFetchUrl("file:///etc/passwd").ok).toBe(false);
+  });
+
+  it("rejects relative paths (must be absolute)", () => {
+    expect(validateFetchUrl("/foo/bar").ok).toBe(false);
+  });
+
+  it("rejects malformed input", () => {
+    expect(validateFetchUrl("not a url").ok).toBe(false);
+  });
+
+  it("rejects oversized input", () => {
+    expect(validateFetchUrl("https://" + "x".repeat(3000) + ".test").ok).toBe(false);
+  });
+
+  it("enforces the host allowlist when provided", () => {
+    const allowed = ["journal.trails.cool"];
+    expect(validateFetchUrl("https://journal.trails.cool/x", { allowedHosts: allowed }).ok).toBe(true);
+    expect(validateFetchUrl("https://evil.example/x", { allowedHosts: allowed }).ok).toBe(false);
+  });
+
+  it("ignores the allowlist when it's empty/undefined", () => {
+    expect(validateFetchUrl("https://random.example/x").ok).toBe(true);
+    expect(validateFetchUrl("https://random.example/x", { allowedHosts: [] }).ok).toBe(true);
+  });
+});
+
+describe("validateRedirectUrl", () => {
+  it("accepts an absolute https URL", () => {
+    expect(validateRedirectUrl("https://trails.cool/r/123").ok).toBe(true);
+  });
+
+  it("accepts a same-origin relative path", () => {
+    expect(validateRedirectUrl("/routes/abc").ok).toBe(true);
+  });
+
+  it("rejects javascript: scheme", () => {
+    expect(validateRedirectUrl("javascript:alert(1)").ok).toBe(false);
+  });
+
+  it("rejects protocol-relative //host URLs", () => {
+    // `<a href="//evil.example">` would resolve to https://evil.example
+    // when the page is on HTTPS. Explicitly reject to keep the
+    // "same-origin path" branch tight.
+    expect(validateRedirectUrl("//evil.example/x").ok).toBe(false);
+  });
+});
+
+describe("getCallbackAllowedHosts", () => {
+  beforeEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it("returns undefined when the env is unset", () => {
+    delete process.env.PLANNER_CALLBACK_ALLOWED_HOSTS;
+    expect(getCallbackAllowedHosts()).toBeUndefined();
+  });
+
+  it("splits, trims, and filters empty entries", () => {
+    vi.stubEnv("PLANNER_CALLBACK_ALLOWED_HOSTS", "a.test ,  b.test,, c.test ");
+    expect(getCallbackAllowedHosts()).toEqual(["a.test", "b.test", "c.test"]);
+  });
+});
diff --git a/apps/planner/app/lib/url-validation.server.ts b/apps/planner/app/lib/url-validation.server.ts
new file mode 100644
index 0000000..84e2964
--- /dev/null
+++ b/apps/planner/app/lib/url-validation.server.ts
@@ -0,0 +1,91 @@
+// Shared validation for query-param URLs that flow into either an
+// outbound fetch (callbackUrl) or a rendered link (returnUrl).
+// Keeps the new-session and session-detail loaders honest without
+// scattering ad-hoc string checks.
+
+const SAFE_SCHEMES = new Set(["http:", "https:"]);
+
+/**
+ * Result type so callers can decide whether to reject the request or
+ * just drop the value (e.g. returnUrl is optional UX; callbackUrl is
+ * load-bearing).
+ */
+export interface UrlValidationResult {
+  ok: boolean;
+  reason?: string;
+  url?: URL;
+}
+
+/**
+ * Validate a URL string for use as a fetch target. Requires absolute
+ * http(s) URL. If `allowedHosts` is provided, the host must be on the
+ * list — matches behavior of standard "open redirect" allowlists.
+ */
+export function validateFetchUrl(
+  raw: string,
+  opts: { allowedHosts?: string[]; maxLength?: number } = {},
+): UrlValidationResult {
+  const maxLength = opts.maxLength ?? 2048;
+  if (raw.length > maxLength) {
+    return { ok: false, reason: `url exceeds ${maxLength} chars` };
+  }
+  let parsed: URL;
+  try {
+    parsed = new URL(raw);
+  } catch {
+    return { ok: false, reason: "not a valid absolute URL" };
+  }
+  if (!SAFE_SCHEMES.has(parsed.protocol)) {
+    return { ok: false, reason: `disallowed scheme ${parsed.protocol}` };
+  }
+  if (opts.allowedHosts && opts.allowedHosts.length > 0) {
+    if (!opts.allowedHosts.includes(parsed.host)) {
+      return { ok: false, reason: `host ${parsed.host} not on allowlist` };
+    }
+  }
+  return { ok: true, url: parsed };
+}
+
+/**
+ * Validate a URL string for use as a rendered `<a href>`. Accepts
+ * either a same-origin relative path (starts with `/` and doesn't
+ * start with `//`) or an absolute http(s) URL. The browser still
+ * follows the link, so the goal is to refuse `javascript:` and
+ * `data:` schemes which would execute on click.
+ */
+export function validateRedirectUrl(
+  raw: string,
+  opts: { maxLength?: number } = {},
+): UrlValidationResult {
+  const maxLength = opts.maxLength ?? 2048;
+  if (raw.length > maxLength) {
+    return { ok: false, reason: `url exceeds ${maxLength} chars` };
+  }
+  // Same-origin relative path. `//host` would be protocol-relative and
+  // bypass the scheme check, so reject those explicitly.
+  if (raw.startsWith("/") && !raw.startsWith("//")) {
+    return { ok: true };
+  }
+  let parsed: URL;
+  try {
+    parsed = new URL(raw);
+  } catch {
+    return { ok: false, reason: "not a valid absolute or relative URL" };
+  }
+  if (!SAFE_SCHEMES.has(parsed.protocol)) {
+    return { ok: false, reason: `disallowed scheme ${parsed.protocol}` };
+  }
+  return { ok: true, url: parsed };
+}
+
+/**
+ * Parse the `PLANNER_CALLBACK_ALLOWED_HOSTS` env (comma-separated
+ * hostnames). When set, callbackUrl hosts must match. When unset,
+ * no allowlist is applied — useful in dev/self-hosted where the
+ * journal lives somewhere unpredictable.
+ */
+export function getCallbackAllowedHosts(): string[] | undefined {
+  const raw = process.env.PLANNER_CALLBACK_ALLOWED_HOSTS;
+  if (!raw) return undefined;
+  return raw.split(",").map((s) => s.trim()).filter(Boolean);
+}
diff --git a/apps/planner/app/routes/new.tsx b/apps/planner/app/routes/new.tsx
index 121e0f6..739b6d9 100644
--- a/apps/planner/app/routes/new.tsx
+++ b/apps/planner/app/routes/new.tsx
@@ -3,6 +3,16 @@ import type { Route } from "./+types/new";
 import { createSession, initializeSessionWithWaypoints, initializeSessionWithNoGoAreas } from "~/lib/sessions";
 import { parseGpxAsync, extractWaypoints } from "@trails-cool/gpx";
 import { checkRateLimit } from "~/lib/rate-limit";
+import {
+  validateFetchUrl,
+  validateRedirectUrl,
+  getCallbackAllowedHosts,
+} from "~/lib/url-validation.server";
+
+// Query-param hard caps so an attacker can't shovel multi-MB strings
+// into our URL parser / session-storage path.
+const MAX_GPX_QUERY_LEN = 2_000_000; // GPX intentionally allowed to be large (full route)
+const MAX_URL_PARAM_LEN = 2048;
 
 export async function loader({ request }: Route.LoaderArgs) {
   const url = new URL(request.url);
@@ -25,6 +35,31 @@ export async function loader({ request }: Route.LoaderArgs) {
   const returnUrl = url.searchParams.get("returnUrl");
   const gpxEncoded = url.searchParams.get("gpx");
 
+  // Validate the callback URL — it will be used as a fetch target on
+  // save-to-journal. Reject non-http(s) schemes (no `file:` / `javascript:`),
+  // and if PLANNER_CALLBACK_ALLOWED_HOSTS is set, restrict to those hosts.
+  if (callbackUrl) {
+    const v = validateFetchUrl(callbackUrl, {
+      allowedHosts: getCallbackAllowedHosts(),
+      maxLength: MAX_URL_PARAM_LEN,
+    });
+    if (!v.ok) {
+      throw data({ error: `Invalid callback URL: ${v.reason}` }, { status: 400 });
+    }
+  }
+  if (token && token.length > MAX_URL_PARAM_LEN) {
+    throw data({ error: "callback token too long" }, { status: 400 });
+  }
+  if (returnUrl) {
+    const v = validateRedirectUrl(returnUrl, { maxLength: MAX_URL_PARAM_LEN });
+    if (!v.ok) {
+      throw data({ error: `Invalid returnUrl: ${v.reason}` }, { status: 400 });
+    }
+  }
+  if (gpxEncoded && gpxEncoded.length > MAX_GPX_QUERY_LEN) {
+    throw data({ error: "GPX payload too large" }, { status: 413 });
+  }
+
   // Create a session with callback info
   const session = await createSession({
     callbackUrl: callbackUrl ?? undefined,
diff --git a/apps/planner/app/routes/session.$id.tsx b/apps/planner/app/routes/session.$id.tsx
index fdcf518..99979e7 100644
--- a/apps/planner/app/routes/session.$id.tsx
+++ b/apps/planner/app/routes/session.$id.tsx
@@ -28,21 +28,47 @@ export async function loader({ params }: Route.LoaderArgs) {
   });
 }
 
+// URL-param caps prevent a hostile link from feeding multi-MB JSON
+// into JSON.parse on the client (and from there into the Yjs doc).
+// Picked generously enough to accept realistic multi-day routes
+// (~hundreds of waypoints) but small enough to refuse abuse.
+const MAX_WAYPOINTS_LEN = 50_000;
+const MAX_NOGO_LEN = 50_000;
+const MAX_NOTES_LEN = 10_000;
+const MAX_RETURN_URL_LEN = 2048;
+
+function safeParseJson<T>(raw: string, maxLen: number): T | undefined {
+  if (raw.length > maxLen) return undefined;
+  try { return JSON.parse(raw) as T; } catch { return undefined; }
+}
+
+function safeReturnUrl(raw: string | null): string | undefined {
+  if (!raw || raw.length > MAX_RETURN_URL_LEN) return undefined;
+  // Same-origin relative path or http(s) absolute URL only. Rejects
+  // `javascript:` and protocol-relative `//host` so the value is safe
+  // to drop into an `<a href>`.
+  if (raw.startsWith("/") && !raw.startsWith("//")) return raw;
+  try {
+    const parsed = new URL(raw);
+    if (parsed.protocol === "http:" || parsed.protocol === "https:") return raw;
+  } catch { /* fall through */ }
+  return undefined;
+}
+
 export default function SessionPage({ loaderData }: Route.ComponentProps) {
   const { id } = useParams();
   const [searchParams] = useSearchParams();
-  const returnUrl = searchParams.get("returnUrl") ?? undefined;
+  const returnUrl = safeReturnUrl(searchParams.get("returnUrl"));
   const waypointsParam = searchParams.get("waypoints");
-  let initialWaypoints: Array<{ lat: number; lon: number; name?: string; isDayBreak?: boolean }> | undefined;
-  if (waypointsParam) {
-    try { initialWaypoints = JSON.parse(waypointsParam); } catch { /* ignore */ }
-  }
+  const initialWaypoints = waypointsParam
+    ? safeParseJson<Array<{ lat: number; lon: number; name?: string; isDayBreak?: boolean }>>(waypointsParam, MAX_WAYPOINTS_LEN)
+    : undefined;
   const noGoParam = searchParams.get("noGoAreas");
-  let initialNoGoAreas: Array<{ points: Array<{ lat: number; lon: number }> }> | undefined;
-  if (noGoParam) {
-    try { initialNoGoAreas = JSON.parse(noGoParam); } catch { /* ignore */ }
-  }
-  const initialNotes = searchParams.get("notes") ?? undefined;
+  const initialNoGoAreas = noGoParam
+    ? safeParseJson<Array<{ points: Array<{ lat: number; lon: number }> }>>(noGoParam, MAX_NOGO_LEN)
+    : undefined;
+  const notesRaw = searchParams.get("notes");
+  const initialNotes = notesRaw && notesRaw.length <= MAX_NOTES_LEN ? notesRaw : undefined;
 
   return (
     <div className="flex h-full flex-col">