diff --git a/CHANGELOG.md b/CHANGELOG.md
index a9e1e15394b3..bee7ca749dad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure adjacent rules are merged together after handling nesting when generating optimized CSS ([#14873](https://github.com/tailwindlabs/tailwindcss/pull/14873))
 - _Upgrade (experimental)_: Install `@tailwindcss/postcss` next to `tailwindcss` ([#14830](https://github.com/tailwindlabs/tailwindcss/pull/14830))
 - _Upgrade (experimental)_: Remove whitespace around `,` separator when print arbitrary values ([#14838](https://github.com/tailwindlabs/tailwindcss/pull/14838))
+- _Upgrade (experimental)_: Fix crash during upgrade when content globs escape root of project ([#14896](https://github.com/tailwindlabs/tailwindcss/pull/14896))
 
 ### Changed
 
diff --git a/integrations/upgrade/js-config.test.ts b/integrations/upgrade/js-config.test.ts
index bee8044cbad7..398402139853 100644
--- a/integrations/upgrade/js-config.test.ts
+++ b/integrations/upgrade/js-config.test.ts
@@ -1,3 +1,4 @@
+import path from 'node:path'
 import { describe, expect } from 'vitest'
 import { css, html, json, test, ts } from '../utils'
 
@@ -938,6 +939,98 @@ test(
   },
 )
 
+test(
+  'migrate sources when pointing to folders outside the project root',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "@tailwindcss/upgrade": "workspace:^"
+          }
+        }
+      `,
+
+      'frontend/tailwind.config.ts': ts`
+        export default {
+          content: {
+            relative: true,
+            files: ['./src/**/*.html', '../backend/mails/**/*.blade.php'],
+          },
+          theme: {
+            extend: {
+              colors: {
+                primary: 'red',
+              },
+            },
+          },
+        }
+      `,
+      'frontend/src/input.css': css`
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+        @config "../tailwind.config.ts";
+      `,
+      'frontend/src/index.html': html`<div class="!text-primary"></div>`,
+
+      'backend/mails/welcome.blade.php': html`<div class="!text-primary"></div>`,
+    },
+  },
+  async ({ root, exec, fs }) => {
+    await exec('npx @tailwindcss/upgrade', {
+      cwd: path.join(root, 'frontend'),
+    })
+
+    expect(await fs.dumpFiles('frontend/**/*.css')).toMatchInlineSnapshot(`
+      "
+      --- frontend/src/input.css ---
+      @import 'tailwindcss';
+
+      @source '../../backend/mails/**/*.blade.php';
+
+      @theme {
+        --color-primary: red;
+      }
+
+      /*
+        The default border color has changed to \`currentColor\` in Tailwind CSS v4,
+        so we've added these compatibility styles to make sure everything still
+        looks the same as it did with Tailwind CSS v3.
+
+        If we ever want to remove these styles, we need to add an explicit border
+        color utility to any element that depends on these defaults.
+      */
+      @layer base {
+        *,
+        ::after,
+        ::before,
+        ::backdrop,
+        ::file-selector-button {
+          border-color: var(--color-gray-200, currentColor);
+        }
+      }
+
+      /*
+        Form elements have a 1px border by default in Tailwind CSS v4, so we've
+        added these compatibility styles to make sure everything still looks the
+        same as it did with Tailwind CSS v3.
+
+        If we ever want to remove these styles, we need to add \`border-0\` to
+        any form elements that shouldn't have a border.
+      */
+      @layer base {
+        input:where(:not([type='button'], [type='reset'], [type='submit'])),
+        select,
+        textarea {
+          border-width: 0;
+        }
+      }
+      "
+    `)
+  },
+)
+
 describe('border compatibility', () => {
   test(
     'migrate border compatibility',
diff --git a/packages/@tailwindcss-upgrade/package.json b/packages/@tailwindcss-upgrade/package.json
index b57b7b0c6307..217824d8e546 100644
--- a/packages/@tailwindcss-upgrade/package.json
+++ b/packages/@tailwindcss-upgrade/package.json
@@ -29,6 +29,7 @@
   "dependencies": {
     "@tailwindcss/node": "workspace:^",
     "@tailwindcss/oxide": "workspace:^",
+    "braces": "^3.0.3",
     "dedent": "1.5.3",
     "enhanced-resolve": "^5.17.1",
     "globby": "^14.0.2",
@@ -44,6 +45,7 @@
     "tree-sitter-typescript": "^0.23.0"
   },
   "devDependencies": {
+    "@types/braces": "^3.0.4",
     "@types/node": "catalog:",
     "@types/postcss-import": "^14.0.3"
   }
diff --git a/packages/@tailwindcss-upgrade/src/index.ts b/packages/@tailwindcss-upgrade/src/index.ts
index b2c9df5cc966..616ce65bb1a1 100644
--- a/packages/@tailwindcss-upgrade/src/index.ts
+++ b/packages/@tailwindcss-upgrade/src/index.ts
@@ -21,6 +21,7 @@ import { migrate as migrateTemplate } from './template/migrate'
 import { prepareConfig } from './template/prepare-config'
 import { args, type Arg } from './utils/args'
 import { isRepoDirty } from './utils/git'
+import { hoistStaticGlobParts } from './utils/hoist-static-glob-parts'
 import { pkg } from './utils/packages'
 import { eprintln, error, header, highlight, info, success } from './utils/renderer'
 
@@ -143,11 +144,11 @@ async function run() {
       info('Migrating templates using the provided configuration file.')
       for (let config of configBySheet.values()) {
         let set = new Set<string>()
-        for (let { pattern, base } of config.globs) {
-          let files = await globby([pattern], {
+        for (let globEntry of config.globs.flatMap((entry) => hoistStaticGlobParts(entry))) {
+          let files = await globby([globEntry.pattern], {
             absolute: true,
             gitignore: true,
-            cwd: base,
+            cwd: globEntry.base,
           })
 
           for (let file of files) {
diff --git a/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.test.ts b/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.test.ts
new file mode 100644
index 000000000000..1e6d8b1e861e
--- /dev/null
+++ b/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.test.ts
@@ -0,0 +1,48 @@
+import { expect, it } from 'vitest'
+import { hoistStaticGlobParts } from './hoist-static-glob-parts'
+
+it.each([
+  // A basic glob
+  [
+    { base: '/projects/project-a', pattern: './src/**/*.html' },
+    [{ base: '/projects/project-a/src', pattern: '**/*.html' }],
+  ],
+
+  // A glob pointing to a folder should result in `**/*`
+  [
+    { base: '/projects/project-a', pattern: './src' },
+    [{ base: '/projects/project-a/src', pattern: '**/*' }],
+  ],
+
+  // A glob pointing to a file, should result in the file as the pattern
+  [
+    { base: '/projects/project-a', pattern: './src/index.html' },
+    [{ base: '/projects/project-a/src', pattern: 'index.html' }],
+  ],
+
+  // A glob going up a directory, should result in the new directory as the base
+  [
+    { base: '/projects/project-a', pattern: '../project-b/src/**/*.html' },
+    [{ base: '/projects/project-b/src', pattern: '**/*.html' }],
+  ],
+
+  // A glob with curlies, should be expanded to multiple globs
+  [
+    { base: '/projects/project-a', pattern: '../project-{b,c}/src/**/*.html' },
+    [
+      { base: '/projects/project-b/src', pattern: '**/*.html' },
+      { base: '/projects/project-c/src', pattern: '**/*.html' },
+    ],
+  ],
+  [
+    { base: '/projects/project-a', pattern: '../project-{b,c}/src/**/*.{js,html}' },
+    [
+      { base: '/projects/project-b/src', pattern: '**/*.js' },
+      { base: '/projects/project-b/src', pattern: '**/*.html' },
+      { base: '/projects/project-c/src', pattern: '**/*.js' },
+      { base: '/projects/project-c/src', pattern: '**/*.html' },
+    ],
+  ],
+])('should hoist the static parts of the glob: %s', (input, output) => {
+  expect(hoistStaticGlobParts(input)).toEqual(output)
+})
diff --git a/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.ts b/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.ts
new file mode 100644
index 000000000000..0c8ab4629fd6
--- /dev/null
+++ b/packages/@tailwindcss-upgrade/src/utils/hoist-static-glob-parts.ts
@@ -0,0 +1,79 @@
+import braces from 'braces'
+import path from 'node:path'
+
+interface GlobEntry {
+  base: string
+  pattern: string
+}
+
+export function hoistStaticGlobParts(entry: GlobEntry): GlobEntry[] {
+  return braces(entry.pattern, { expand: true }).map((pattern) => {
+    let clone = { ...entry }
+    let [staticPart, dynamicPart] = splitPattern(pattern)
+
+    // Move static part into the `base`.
+    if (staticPart !== null) {
+      clone.base = path.resolve(entry.base, staticPart)
+    } else {
+      clone.base = path.resolve(entry.base)
+    }
+
+    // Move dynamic part into the `pattern`.
+    if (dynamicPart === null) {
+      clone.pattern = '**/*'
+    } else {
+      clone.pattern = dynamicPart
+    }
+
+    // If the pattern looks like a file, move the file name from the `base` to
+    // the `pattern`.
+    let file = path.basename(clone.base)
+    if (file.includes('.')) {
+      clone.pattern = file
+      clone.base = path.dirname(clone.base)
+    }
+
+    return clone
+  })
+}
+
+// Split a glob pattern into a `static` and `dynamic` part.
+//
+// Assumption: we assume that all globs are expanded, which means that the only
+// dynamic parts are using `*`.
+//
+// E.g.:
+//  Original input: `../project-b/**/*.{html,js}`
+//  Expanded input: `../project-b/**/*.html` & `../project-b/**/*.js`
+//  Split on first input: ("../project-b", "**/*.html")
+//  Split on second input: ("../project-b", "**/*.js")
+function splitPattern(pattern: string): [staticPart: string | null, dynamicPart: string | null] {
+  // No dynamic parts, so we can just return the input as-is.
+  if (!pattern.includes('*')) {
+    return [pattern, null]
+  }
+
+  let lastSlashPosition: number | null = null
+
+  for (let i = 0; i < pattern.length; i++) {
+    let c = pattern[i];
+    if (c === '/') {
+      lastSlashPosition = i
+    }
+
+    if (c === '*' || c === '!') {
+      break
+    }
+  }
+
+  // Very first character is a `*`, therefore there is no static part, only a
+  // dynamic part.
+  if (lastSlashPosition === null) {
+    return [null, pattern]
+  }
+
+  let staticPart = pattern.slice(0, lastSlashPosition).trim()
+  let dynamicPart = pattern.slice(lastSlashPosition + 1).trim()
+
+  return [staticPart || null, dynamicPart || null]
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 8505e5b5c405..4545fe4cddc4 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -10,7 +10,7 @@ catalogs:
       specifier: ^20.14.8
       version: 20.14.13
     lightningcss:
-      specifier: ^1.26.0
+      specifier: ^1.28.1
       version: 1.26.0
     vite:
       specifier: ^5.4.0
@@ -284,6 +284,9 @@ importers:
       '@tailwindcss/oxide':
         specifier: workspace:^
         version: link:../../crates/node
+      braces:
+        specifier: ^3.0.3
+        version: 3.0.3
       dedent:
         specifier: 1.5.3
         version: 1.5.3
@@ -324,6 +327,9 @@ importers:
         specifier: ^0.23.0
         version: 0.23.0(tree-sitter@0.22.0)
     devDependencies:
+      '@types/braces':
+        specifier: ^3.0.4
+        version: 3.0.4
       '@types/node':
         specifier: 'catalog:'
         version: 20.14.13
@@ -1304,6 +1310,9 @@ packages:
   '@types/babel__traverse@7.20.6':
     resolution: {integrity: sha512-r1bzfrm0tomOI8g1SzvCaQHo6Lcv6zu0EA+W2kHrt8dyrHQxGzBBL4kdkzIS+jBMV+EYcMAEAqXqYaLJq5rOZg==}
 
+  '@types/braces@3.0.4':
+    resolution: {integrity: sha512-0WR3b8eaISjEW7RpZnclONaLFDf7buaowRHdqLp4vLj54AsSAYWfh3DRbfiYJY9XDxMgx1B4sE1Afw2PGpuHOA==}
+
   '@types/bun@1.1.11':
     resolution: {integrity: sha512-0N7D/H/8sbf9JMkaG5F3+I/cB4TlhKTkO9EskEWP8XDr8aVcDe4EywSnU4cnyZy6tar1dq70NeFNkqMEUigthw==}
 
@@ -4120,6 +4129,8 @@ snapshots:
     dependencies:
       '@babel/types': 7.25.2
 
+  '@types/braces@3.0.4': {}
+
   '@types/bun@1.1.11':
     dependencies:
       bun-types: 1.1.30