diff --git a/packages/theme/src/cli/utilities/theme-fs.test.ts b/packages/theme/src/cli/utilities/theme-fs.test.ts
index f2ded99a372..1c730203022 100644
--- a/packages/theme/src/cli/utilities/theme-fs.test.ts
+++ b/packages/theme/src/cli/utilities/theme-fs.test.ts
@@ -27,6 +27,7 @@ import {Operation, type Checksum, type ThemeAsset} from '@shopify/cli-kit/node/t
 import {dirname, joinPath} from '@shopify/cli-kit/node/path'
 import {recordError} from '@shopify/cli-kit/node/analytics'
 import {AdminSession} from '@shopify/cli-kit/node/session'
+import {AbortError} from '@shopify/cli-kit/node/error'
 
 import EventEmitter from 'events'
 import {fileURLToPath} from 'node:url'
@@ -337,6 +338,24 @@ describe('theme-fs', () => {
         writeFileSpy.mockRestore()
       })
     })
+
+    test('"write" rejects keys that resolve outside of the theme directory', async () => {
+      await inTemporaryDirectory(async (tmpDir) => {
+        // Given
+        const root = joinPath(tmpDir, 'theme')
+        await mkdir(root)
+        const themeFileSystem = mountThemeFileSystem(root)
+        await themeFileSystem.ready()
+        const escapingKey = '../escaped.liquid'
+
+        // When
+        const writePromise = themeFileSystem.write({key: escapingKey, checksum: '1010', value: 'content'})
+
+        // Then
+        await expect(writePromise).rejects.toThrow(AbortError)
+        await expect(fileExists(joinPath(tmpDir, 'escaped.liquid'))).resolves.toBe(false)
+      })
+    })
   })
 
   describe('themeFileSystem.read', async () => {
diff --git a/packages/theme/src/cli/utilities/theme-fs.ts b/packages/theme/src/cli/utilities/theme-fs.ts
index 4cc4e3c3f7e..94a5307dbc8 100644
--- a/packages/theme/src/cli/utilities/theme-fs.ts
+++ b/packages/theme/src/cli/utilities/theme-fs.ts
@@ -6,11 +6,12 @@ import {triggerBrowserFullReload} from './theme-environment/hot-reload/server.js
 import {getListingFilePath, updateSettingsDataForListing} from './theme-listing.js'
 import {DEFAULT_IGNORE_PATTERNS, timestampDateFormat} from '../constants.js'
 import {glob, readFile, ReadOptions, fileExists, mkdir, writeFile, removeFile} from '@shopify/cli-kit/node/fs'
-import {joinPath, basename, relativePath} from '@shopify/cli-kit/node/path'
+import {joinPath, basename, relativePath, resolvePath, isSubpath} from '@shopify/cli-kit/node/path'
 import {lookupMimeType, setMimeTypes} from '@shopify/cli-kit/node/mimes'
 import {outputContent, outputDebug, outputInfo, outputToken, outputWarn} from '@shopify/cli-kit/node/output'
 import {buildThemeAsset} from '@shopify/cli-kit/node/themes/factories'
 import {recordError} from '@shopify/cli-kit/node/analytics'
+import {AbortError} from '@shopify/cli-kit/node/error'
 import {AdminSession} from '@shopify/cli-kit/node/session'
 import {bulkUploadThemeAssets, deleteThemeAssets} from '@shopify/cli-kit/node/themes/api'
 
@@ -395,8 +396,26 @@ export function handleSyncUpdate(
   }
 }
 
+/**
+ * Resolves a theme file key against the theme root, ensuring the result stays inside it.
+ *
+ * File keys come from the remote theme, so a key containing `..` segments would otherwise
+ * make `joinPath` resolve to a path outside the theme directory and let a remote response
+ * read or overwrite arbitrary files on the developer's machine.
+ */
+function resolveThemeFilePath(root: string, key: Key): string {
+  const absoluteRoot = resolvePath(root)
+  const absolutePath = resolvePath(absoluteRoot, key)
+
+  if (!isSubpath(absoluteRoot, absolutePath)) {
+    throw new AbortError(`Theme file "${key}" is outside of the theme directory.`)
+  }
+
+  return absolutePath
+}
+
 async function writeThemeFile(root: string, {key, attachment, value}: ThemeAsset) {
-  const absolutePath = joinPath(root, key)
+  const absolutePath = resolveThemeFilePath(root, key)
 
   await ensureDirExists(absolutePath)
 
@@ -411,7 +430,7 @@ async function writeThemeFile(root: string, {key, attachment, value}: ThemeAsset
 
 export async function readThemeFile(root: string, path: Key): Promise<string | Buffer | undefined> {
   const options: ReadOptions = isTextFile(path) ? {encoding: 'utf8'} : {}
-  const absolutePath = joinPath(root, path)
+  const absolutePath = resolveThemeFilePath(root, path)
 
   const themeFileExists = await fileExists(absolutePath)
   if (!themeFileExists) {
@@ -444,7 +463,7 @@ async function readThemeFileWithListing(
 }
 
 async function removeThemeFile(root: string, path: Key) {
-  const absolutePath = joinPath(root, path)
+  const absolutePath = resolveThemeFilePath(root, path)
 
   const themeFileExists = await fileExists(absolutePath)
   if (!themeFileExists) {