diff --git a/src/runtime/internal/route-rules.ts b/src/runtime/internal/route-rules.ts
index d0162a18e2..a056395546 100644
--- a/src/runtime/internal/route-rules.ts
+++ b/src/runtime/internal/route-rules.ts
@@ -1,4 +1,4 @@
-import { proxyRequest, redirect as sendRedirect, requireBasicAuth } from "h3";
+import { HTTPError, proxyRequest, redirect as sendRedirect, requireBasicAuth } from "h3";
 import type { BasicAuthOptions, EventHandler, Middleware } from "h3";
 import type { MatchedRouteRule, NitroRouteRules } from "nitro/types";
 import { joinURL, withQuery, withoutBase } from "ufo";
@@ -29,6 +29,9 @@ export const redirect: RouteRuleCtor<"redirect"> = ((m) =>
       let targetPath = event.url.pathname + event.url.search;
       const strpBase = (m.options as any)._redirectStripBase;
       if (strpBase) {
+        if (!isPathInScope(event.url.pathname, strpBase)) {
+          throw new HTTPError({ status: 400 });
+        }
         targetPath = withoutBase(targetPath, strpBase);
       }
       target = joinURL(target.slice(0, -3), targetPath);
@@ -49,6 +52,9 @@ export const proxy: RouteRuleCtor<"proxy"> = ((m) =>
       let targetPath = event.url.pathname + event.url.search;
       const strpBase = (m.options as any)._proxyStripBase;
       if (strpBase) {
+        if (!isPathInScope(event.url.pathname, strpBase)) {
+          throw new HTTPError({ status: 400 });
+        }
         targetPath = withoutBase(targetPath, strpBase);
       }
       target = joinURL(target.slice(0, -3), targetPath);
@@ -96,3 +102,22 @@ export const basicAuth: RouteRuleCtor<"auth"> = /* @__PURE__ */ Object.assign(
     }) satisfies RouteRuleCtor<"auth">,
   { order: -1 }
 );
+
+// Check whether `pathname`, after canonicalization, stays within `base`.
+// Prevents match/forward differentials where an encoded traversal like `..%2f`
+// bypasses the `/**` scope at match time but escapes the base once the
+// downstream (proxy upstream or redirect target) decodes `%2f` → `/`
+// (GHSA-5w89-w975-hf9q).
+//
+// WHATWG URL keeps `%2F` and `%5C` opaque in paths, so we pre-decode those,
+// then let `new URL` resolve `.`/`..`/`%2E%2E` segments and normalize `\`.
+export function isPathInScope(pathname: string, base: string): boolean {
+  let canonical: string;
+  try {
+    const pre = pathname.replace(/%2f/gi, "/").replace(/%5c/gi, "\\");
+    canonical = new URL(pre, "http://_").pathname;
+  } catch {
+    return false;
+  }
+  return !base || canonical === base || canonical.startsWith(base + "/");
+}
diff --git a/test/unit/route-rules.test.ts b/test/unit/route-rules.test.ts
index 29210731ca..a749dd6168 100644
--- a/test/unit/route-rules.test.ts
+++ b/test/unit/route-rules.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import { normalizeRouteRules } from "../../src/config/resolvers/route-rules.ts";
+import { isPathInScope } from "../../src/runtime/internal/route-rules.ts";
 
 describe("normalizeRouteRules - swr", () => {
   it("swr: true enables SWR", () => {
@@ -29,3 +30,45 @@ describe("normalizeRouteRules - swr", () => {
     expect(withFalse["/api/**"].cache).toBeUndefined();
   });
 });
+
+// Regression for GHSA-5w89-w975-hf9q: an encoded traversal like `..%2f` must
+// not let a request escape a `/**` proxy/redirect scope once the downstream
+// decodes it.
+describe("isPathInScope", () => {
+  it("accepts in-scope paths", () => {
+    expect(isPathInScope("/api/orders/list.json", "/api/orders")).toBe(true);
+    expect(isPathInScope("/api/orders/", "/api/orders")).toBe(true);
+    expect(isPathInScope("/api/orders", "/api/orders")).toBe(true);
+  });
+
+  it("rejects encoded slash traversal (%2f)", () => {
+    expect(isPathInScope("/api/orders/..%2fadmin%2fconfig.json", "/api/orders")).toBe(false);
+    expect(isPathInScope("/api/orders/..%2Fadmin", "/api/orders")).toBe(false);
+  });
+
+  it("rejects encoded backslash traversal (%5c)", () => {
+    expect(isPathInScope("/api/orders/..%5cadmin", "/api/orders")).toBe(false);
+  });
+
+  it("rejects double-encoded dot-segments (%2E%2E)", () => {
+    expect(isPathInScope("/api/orders/%2E%2E%2Fadmin", "/api/orders")).toBe(false);
+  });
+
+  it("rejects literal traversal above scope", () => {
+    expect(isPathInScope("/api/orders/../admin", "/api/orders")).toBe(false);
+    expect(isPathInScope("/api/orders/../../etc/passwd", "/api/orders")).toBe(false);
+  });
+
+  it("keeps traversal confined within scope", () => {
+    expect(isPathInScope("/api/orders/foo/../bar", "/api/orders")).toBe(true);
+    expect(isPathInScope("/api/orders/foo%2f..%2fbar", "/api/orders")).toBe(true);
+  });
+
+  it("does not confuse sibling prefix with scope", () => {
+    expect(isPathInScope("/api/ordersX/list.json", "/api/orders")).toBe(false);
+  });
+
+  it("allows anything for an empty base (catch-all /**)", () => {
+    expect(isPathInScope("/anything/here", "")).toBe(true);
+  });
+});