diff --git a/server/publish/runtime/packageServer.ts b/server/publish/runtime/packageServer.ts
index 15ad2854c..9f61410f4 100644
--- a/server/publish/runtime/packageServer.ts
+++ b/server/publish/runtime/packageServer.ts
@@ -32,6 +32,7 @@
  */
 import { existsSync } from 'node:fs'
 import { resolve as resolvePath } from 'node:path'
+import { isPathWithin } from '../../util/pathWithin'
 import { nodeModulesDirForHash, sentinelPathForHash } from './dependencyCache'
 
 const RUNTIME_PACKAGE_PREFIX = '/_instatic/runtime/cache/'
@@ -76,8 +77,10 @@ function resolveCacheFilePath(pathname: string): { hash: string; absPath: string
   const absPath = resolvePath(nodeModulesDir, subPath)
 
   // Final containment check — the resolved path must live inside
-  // node_modules/. Any escape attempt returns null.
-  if (!absPath.startsWith(`${nodeModulesDir}/`)) return null
+  // node_modules/. Any escape returns null. Decided by `relative()`, not a
+  // string prefix: a hard-coded `/` rejected every legitimate path on Windows
+  // (GHSA-hwp9), where `resolvePath` yields `\` separators.
+  if (!isPathWithin(nodeModulesDir, absPath)) return null
 
   return { hash, absPath }
 }
diff --git a/server/publish/runtime/virtualSiteWorkspace.ts b/server/publish/runtime/virtualSiteWorkspace.ts
index 053d802a9..93e60115d 100644
--- a/server/publish/runtime/virtualSiteWorkspace.ts
+++ b/server/publish/runtime/virtualSiteWorkspace.ts
@@ -3,6 +3,7 @@ import { tmpdir } from 'node:os'
 import { dirname, join, resolve } from 'node:path'
 import type { SiteDocument } from '@core/page-tree'
 import { isSafePath, normalizePath } from '@core/files/pathValidation'
+import { isPathWithin } from '../../util/pathWithin'
 
 interface SiteScriptWorkspace {
   rootDir: string
@@ -23,7 +24,9 @@ export async function materializeSiteScriptWorkspace(site: SiteDocument): Promis
       if (!isSafePath(normalized)) continue
 
       const absolutePath = resolve(rootDir, normalized)
-      if (!absolutePath.startsWith(rootDir)) continue
+      // Bare `startsWith(rootDir)` would also accept a sibling directory whose
+      // name merely begins with the root. Use the shared containment rule.
+      if (!isPathWithin(rootDir, absolutePath)) continue
 
       await mkdir(dirname(absolutePath), { recursive: true })
       await writeFile(absolutePath, file.content, 'utf8')
diff --git a/server/util/pathWithin.ts b/server/util/pathWithin.ts
index 812ec2a1d..6d4eab14c 100644
--- a/server/util/pathWithin.ts
+++ b/server/util/pathWithin.ts
@@ -1,19 +1,38 @@
-import { isAbsolute, relative } from 'node:path'
+import { isAbsolute, relative, sep } from 'node:path'
 
 /**
- * Defense-in-depth filesystem path containment. Schema-level patterns may
- * exclude `..` segments and absolute paths, but filesystem sinks recompose
- * paths via `path.join` — so re-assert the resolved `child` stays strictly
- * under `rootDir` after composition. Throws on the root itself, any `..`
- * escape, or an absolute path that lands outside the root.
+ * Filesystem path containment — the one rule, in two ergonomics.
  *
- * Used by every untrusted-path write sink: plugin asset extraction
+ * Schema-level patterns may exclude `..` segments and absolute paths, but
+ * filesystem sinks recompose paths via `path.join` / `path.resolve` — so
+ * re-assert the resolved `child` stays strictly under `rootDir` after
+ * composition. The root itself does not count as "within".
+ *
+ * Containment is decided by `relative()`, never by a string prefix. A prefix
+ * test has to hard-code a separator, which is wrong twice over: it says `/` on
+ * Windows (where `relative` and `resolve` produce `\`, so every legitimate path
+ * is rejected — GHSA-hwp9), and without a separator it accepts a sibling whose
+ * name merely starts with the root (`/srv/site-evil` under `/srv/site`).
+ * `relative()` is platform-aware and handles both.
+ *
+ * Used by every untrusted-path sink: plugin asset extraction
  * (`server/plugins/runtime.ts`, `pack.ts`), the plugin admin upload route,
- * and site-bundle media import.
+ * site-bundle media import, the runtime package server, and the site-script
+ * workspace.
  */
-export function assertPathWithin(rootDir: string, child: string): void {
+export function isPathWithin(rootDir: string, child: string): boolean {
   const rel = relative(rootDir, child)
-  if (rel === '' || rel.startsWith('..') || isAbsolute(rel)) {
+  // '' means child IS the root; an absolute result means different roots
+  // entirely (separate drives on Windows).
+  if (rel === '' || isAbsolute(rel)) return false
+  // Compare against a full segment, so a child legitimately named `..foo`
+  // is not mistaken for an escape.
+  return rel !== '..' && !rel.startsWith(`..${sep}`)
+}
+
+/** Throwing form of {@link isPathWithin}, for sinks that treat an escape as fatal. */
+export function assertPathWithin(rootDir: string, child: string): void {
+  if (!isPathWithin(rootDir, child)) {
     throw new Error(`Path "${child}" escapes root "${rootDir}"`)
   }
 }
diff --git a/src/__tests__/server/pathWithin.test.ts b/src/__tests__/server/pathWithin.test.ts
new file mode 100644
index 000000000..5549c3e73
--- /dev/null
+++ b/src/__tests__/server/pathWithin.test.ts
@@ -0,0 +1,76 @@
+import { describe, expect, it } from 'bun:test'
+import { readFile } from 'node:fs/promises'
+import { fileURLToPath } from 'node:url'
+import { join, dirname } from 'node:path'
+import { isPathWithin, assertPathWithin } from '../../../server/util/pathWithin'
+
+const ROOT = join(dirname(fileURLToPath(import.meta.url)), '..', '..', '..')
+
+/**
+ * Path containment is decided by `relative()`, never by a string prefix.
+ * Both prefix spellings are wrong in a different direction:
+ *
+ *   `startsWith(`${root}/`)`  — hard-codes a POSIX separator, so on Windows
+ *                               (where `resolve` yields `\`) it rejects every
+ *                               legitimate path. That was GHSA-hwp9: the
+ *                               runtime package server 404'd all assets on
+ *                               Windows hosts.
+ *   `startsWith(root)`        — no separator at all, so a sibling directory
+ *                               whose name merely begins with the root slips
+ *                               through.
+ */
+describe('isPathWithin', () => {
+  it('accepts a real descendant', () => {
+    expect(isPathWithin('/srv/site', '/srv/site/build/app.js')).toBe(true)
+    expect(isPathWithin('/srv/site', '/srv/site/a')).toBe(true)
+  })
+
+  it('rejects the root itself', () => {
+    expect(isPathWithin('/srv/site', '/srv/site')).toBe(false)
+  })
+
+  it('rejects a traversal escape', () => {
+    expect(isPathWithin('/srv/site', '/srv/site/../secrets')).toBe(false)
+    expect(isPathWithin('/srv/site', '/srv')).toBe(false)
+    expect(isPathWithin('/srv/site', '/etc/passwd')).toBe(false)
+  })
+
+  it('rejects a sibling that merely shares the root as a name prefix', () => {
+    // The bare-`startsWith(root)` failure mode.
+    expect(isPathWithin('/srv/site', '/srv/site-evil/app.js')).toBe(false)
+    expect(isPathWithin('/srv/site', '/srv/sitedata')).toBe(false)
+  })
+
+  it('accepts a descendant whose name legitimately begins with dots', () => {
+    // A naive `rel.startsWith('..')` would call this an escape.
+    expect(isPathWithin('/srv/site', '/srv/site/..foo')).toBe(true)
+    expect(isPathWithin('/srv/site', '/srv/site/...bar/baz')).toBe(true)
+  })
+
+  it('assertPathWithin throws exactly when isPathWithin is false', () => {
+    expect(() => assertPathWithin('/srv/site', '/srv/site/ok')).not.toThrow()
+    expect(() => assertPathWithin('/srv/site', '/srv/site-evil')).toThrow(/escapes root/)
+    expect(() => assertPathWithin('/srv/site', '/srv/site')).toThrow(/escapes root/)
+  })
+})
+
+describe('no separator-naive containment checks remain (GHSA-hwp9)', () => {
+  const SINKS = [
+    'server/publish/runtime/packageServer.ts',
+    'server/publish/runtime/virtualSiteWorkspace.ts',
+    'server/util/pathWithin.ts',
+  ]
+
+  it('containment sinks use the shared helper, not a string prefix', async () => {
+    for (const relPath of SINKS) {
+      const source = await readFile(join(ROOT, relPath), 'utf-8')
+      const code = source
+        .replace(/\/\*[\s\S]*?\*\//g, '')
+        .replace(/\/\/[^\n]*/g, '')
+      // A containment decision written as `startsWith(`${someDir}/`)` or
+      // `startsWith(someDir)` is the bug this suite exists to prevent.
+      expect(code).not.toMatch(/startsWith\(\s*`\$\{\w*[Dd]ir\w*\}/)
+      expect(code).not.toMatch(/startsWith\(\s*\w*(?:Dir|Root|rootDir)\w*\s*\)/)
+    }
+  })
+})