diff --git a/packages/parsers/src/assetPaths.test.ts b/packages/parsers/src/assetPaths.test.ts
new file mode 100644
index 0000000000..9e2a530e73
--- /dev/null
+++ b/packages/parsers/src/assetPaths.test.ts
@@ -0,0 +1,23 @@
+import { resolve } from "node:path";
+import { describe, expect, it } from "vitest";
+
+import { isPathInside } from "./assetPaths.js";
+
+describe("isPathInside", () => {
+  it("accepts the parent itself and nested children", () => {
+    expect(isPathInside(resolve("/project"), resolve("/project"))).toBe(true);
+    expect(isPathInside(resolve("/project/assets/logo.png"), resolve("/project"))).toBe(true);
+  });
+
+  it("rejects paths above the parent and siblings sharing its prefix", () => {
+    expect(isPathInside(resolve("/project/../secret.txt"), resolve("/project"))).toBe(false);
+    expect(isPathInside(resolve("/"), resolve("/project"))).toBe(false);
+    expect(isPathInside(resolve("/project-other/logo.png"), resolve("/project"))).toBe(false);
+  });
+
+  it("accepts a child whose name starts with two dots", () => {
+    // `..assets` is an ordinary directory name, not a parent-dir segment.
+    expect(isPathInside(resolve("/project/..assets/logo.png"), resolve("/project"))).toBe(true);
+    expect(isPathInside(resolve("/project/..intro.mp4"), resolve("/project"))).toBe(true);
+  });
+});
diff --git a/packages/parsers/src/assetPaths.ts b/packages/parsers/src/assetPaths.ts
index 390bc66492..a4821134a2 100644
--- a/packages/parsers/src/assetPaths.ts
+++ b/packages/parsers/src/assetPaths.ts
@@ -36,12 +36,14 @@ export function isNonRelativeUrl(val: string): boolean {
  * Cross-platform containment check: is `childPath` inside `parentPath`?
  * Equality counts as "inside".
  */
+const LEADING_PARENT_SEGMENT_RE = /^\.\.(?:[\\/]|$)/;
+
 export function isPathInside(childPath: string, parentPath: string): boolean {
   const absChild = resolve(childPath);
   const absParent = resolve(parentPath);
   if (absChild === absParent) return true;
   const rel = relative(absParent, absChild);
-  return rel !== "" && !rel.startsWith("..") && !isAbsolute(rel);
+  return rel !== "" && !LEADING_PARENT_SEGMENT_RE.test(rel) && !isAbsolute(rel);
 }
 
 export {
diff --git a/packages/producer/src/utils/paths.test.ts b/packages/producer/src/utils/paths.test.ts
index abbffb0001..fee8a379f4 100644
--- a/packages/producer/src/utils/paths.test.ts
+++ b/packages/producer/src/utils/paths.test.ts
@@ -49,6 +49,29 @@ describe("isPathInside", () => {
     expect(isPathInside(resolve("/foo/bar/../../etc/passwd"), resolve("/foo/bar"))).toBe(false);
   });
 
+  it("returns true for a child whose name starts with two dots", () => {
+    // `..assets` is an ordinary directory name, not a parent-dir segment.
+    expect(isPathInside(resolve("/foo/bar/..assets/logo.png"), resolve("/foo/bar"))).toBe(true);
+    expect(isPathInside(resolve("/foo/bar/..intro.mp4"), resolve("/foo/bar"))).toBe(true);
+    expect(
+      isPathInside(
+        win32.resolve("C:\\project\\..fonts\\Inter.woff2"),
+        win32.resolve("C:\\project"),
+        {
+          pathModule: win32,
+        },
+      ),
+    ).toBe(true);
+  });
+
+  it("still rejects a Windows child above the parent", () => {
+    expect(
+      isPathInside(win32.resolve("C:\\project\\..\\secret.txt"), win32.resolve("C:\\project"), {
+        pathModule: win32,
+      }),
+    ).toBe(false);
+  });
+
   it("normalises trailing slashes on parent", () => {
     expect(isPathInside(resolve("/foo/bar/baz"), resolve("/foo/bar/"))).toBe(true);
   });
diff --git a/packages/producer/src/utils/paths.ts b/packages/producer/src/utils/paths.ts
index 7ff314517f..14327a081a 100644
--- a/packages/producer/src/utils/paths.ts
+++ b/packages/producer/src/utils/paths.ts
@@ -28,6 +28,8 @@ type PathModuleLike = {
   isAbsolute: (path: string) => boolean;
 };
 
+const LEADING_PARENT_SEGMENT_RE = /^\.\.(?:[\\/]|$)/;
+
 type IsPathInsideOptions = {
   pathModule?: PathModuleLike;
 };
@@ -59,7 +61,7 @@ export function isPathInside(
   // `relative()` returns "" when paths are equal, ".." or "..\\foo" when child
   // is above the parent, and an absolute path when they live on different
   // drives/volumes (Windows) — none of which count as "inside".
-  return rel !== "" && !rel.startsWith("..") && !isPathAbsolute(rel);
+  return rel !== "" && !LEADING_PARENT_SEGMENT_RE.test(rel) && !isPathAbsolute(rel);
 }
 
 /**