fix(arborist): block symlink escape in patch applier

workspaces/arborist/lib/patch.js

@@ -29,6 +29,22 @@ const containedTarget = (cwd, file) => {
   if (!rel || rel.startsWith('..') || isAbsolute(rel)) {
     throw patchError(`patch path escapes the package directory: ${file}`, 'EPATCHUNSAFE', file)
   }
+  // Lexical containment is not enough: cwd holds a package extracted from a
+  // registry tarball, so a symlink the publisher planted there can redirect
+  // the write or delete outside the package even though rel looks contained.
+  // Reject a symlink at the target itself, and require the deepest existing
+  // ancestor's real path to stay inside the real package directory.
+  if (fs.lstatSync(target, { throwIfNoEntry: false })?.isSymbolicLink()) {
+    throw patchError(`patch path escapes the package directory: ${file}`, 'EPATCHUNSAFE', file)
+  }
+  let ancestor = dirname(target)
+  while (!fs.existsSync(ancestor)) {
+    ancestor = dirname(ancestor)
+  }
+  const realRel = relative(fs.realpathSync(cwd), fs.realpathSync(ancestor))
+  if (realRel.startsWith('..') || isAbsolute(realRel)) {
+    throw patchError(`patch path escapes the package directory: ${file}`, 'EPATCHUNSAFE', file)
+  }
   return target
 }
 

workspaces/arborist/test/patch.js

@@ -1,5 +1,5 @@
 const t = require('tap')
-const { readFileSync, existsSync } = require('node:fs')
+const { readFileSync, existsSync, symlinkSync } = require('node:fs')
 const { resolve } = require('node:path')
 const { createTwoFilesPatch } = require('diff')
 const { applyPatchToDir, patchIntegrity } = require('../lib/patch.js')
@@ -79,6 +79,45 @@ t.test('refuses to delete outside the package directory', async t => {
   )
 })
 
+t.test('refuses to write through a symlink that escapes the package', async t => {
+  const dir = t.testdir({
+    pkg: { 'index.js': 'x\n' },
+    outside: {},
+  })
+  symlinkSync(resolve(dir, 'outside'), resolve(dir, 'pkg', 'evil'))
+  await t.rejects(
+    applyPatchToDir({ patch: filePatch('evil/pwned.js', '', 'pwned\n'), cwd: resolve(dir, 'pkg') }),
+    { code: 'EPATCHUNSAFE' }
+  )
+  t.notOk(existsSync(resolve(dir, 'outside', 'pwned.js')), 'nothing written outside')
+})
+
+t.test('refuses to delete through a symlink that escapes the package', async t => {
+  const dir = t.testdir({
+    pkg: { 'index.js': 'x\n' },
+    outside: { 'secret.js': 'secret\n' },
+  })
+  symlinkSync(resolve(dir, 'outside'), resolve(dir, 'pkg', 'evil'))
+  await t.rejects(
+    applyPatchToDir({ patch: filePatch('evil/secret.js', 'secret\n', ''), cwd: resolve(dir, 'pkg') }),
+    { code: 'EPATCHUNSAFE' }
+  )
+  t.ok(existsSync(resolve(dir, 'outside', 'secret.js')), 'outside file untouched')
+})
+
+t.test('refuses a target that is itself a symlink out of the package', async t => {
+  const dir = t.testdir({
+    pkg: { 'index.js': 'x\n' },
+    outside: { 'secret.js': 'const v = 1\n' },
+  })
+  symlinkSync(resolve(dir, 'outside', 'secret.js'), resolve(dir, 'pkg', 'link.js'))
+  await t.rejects(
+    applyPatchToDir({ patch: filePatch('link.js', 'const v = 1\n', 'const v = 2\n'), cwd: resolve(dir, 'pkg') }),
+    { code: 'EPATCHUNSAFE' }
+  )
+  t.equal(readFileSync(resolve(dir, 'outside', 'secret.js'), 'utf8'), 'const v = 1\n', 'outside file untouched')
+})
+
 t.test('delete fails when the file drifted from the diff', async t => {
   const dir = t.testdir({ 'gone.js': 'different content\n' })
   await t.rejects(