fix: stop exporting DecryptActivity and validate decrypt paths

DecryptActivity was exported without an intent filter, so any installed
app could start it with arbitrary FILE_PATH/REPO_PATH extras (and crash
it by omitting them). Nothing outside the app needs to launch it:
shortcuts and the decrypt action go through LaunchActivity.

LaunchActivity stays exported, so it now treats FILE_PATH as untrusted:
the path is forwarded only when it canonically resolves to a regular
file inside the password store, and REPO_PATH is always the real store
directory instead of the caller-supplied value. Anything else falls
back to the main screen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 62 insertions, 10 deletions

diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index d575c7f8..407697cf 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -51,7 +51,7 @@
 
     <activity
       android:name=".ui.crypto.DecryptActivity"
-      android:exported="true" />
+      android:exported="false" />
 
     <activity
       android:name=".ui.main.LaunchActivity"
diff --git a/app/src/main/java/app/passwordstore/ui/main/LaunchActivity.kt b/app/src/main/java/app/passwordstore/ui/main/LaunchActivity.kt
index 90434bb7..1a5e8a89 100644
--- a/app/src/main/java/app/passwordstore/ui/main/LaunchActivity.kt
+++ b/app/src/main/java/app/passwordstore/ui/main/LaunchActivity.kt
@@ -11,6 +11,7 @@ import android.os.Handler
 import android.os.Looper
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.content.edit
+import app.passwordstore.data.repo.PasswordRepository
 import app.passwordstore.ui.crypto.BasePGPActivity
 import app.passwordstore.ui.crypto.DecryptActivity
 import app.passwordstore.ui.passwords.PasswordStore
@@ -18,6 +19,7 @@ import app.passwordstore.util.auth.BiometricAuthenticator
 import app.passwordstore.util.auth.BiometricAuthenticator.Result
 import app.passwordstore.util.extensions.sharedPrefs
 import app.passwordstore.util.settings.PreferenceKeys
+import java.io.File
 
 @SuppressLint("CustomSplashScreen")
 class LaunchActivity : AppCompatActivity() {
@@ -53,17 +55,15 @@ class LaunchActivity : AppCompatActivity() {
   }
 
   private fun startTargetActivity(noAuth: Boolean) {
+    // This activity is exported, so the file path is untrusted input: only forward it to
+    // DecryptActivity when it points at a file inside the password store.
+    val repoDir = PasswordRepository.getRepositoryDirectory()
+    val filePath = intent.getStringExtra(BasePGPActivity.EXTRA_FILE_PATH)
     val intentToStart =
-      if (intent.action == ACTION_DECRYPT_PASS)
+      if (intent.action == ACTION_DECRYPT_PASS && isFileInRepo(filePath, repoDir))
         getDecryptIntent().apply {
-          putExtra(
-            BasePGPActivity.EXTRA_FILE_PATH,
-            intent.getStringExtra(BasePGPActivity.EXTRA_FILE_PATH),
-          )
-          putExtra(
-            BasePGPActivity.EXTRA_REPO_PATH,
-            intent.getStringExtra(BasePGPActivity.EXTRA_REPO_PATH),
-          )
+          putExtra(BasePGPActivity.EXTRA_FILE_PATH, filePath)
+          putExtra(BasePGPActivity.EXTRA_REPO_PATH, repoDir.absolutePath)
         }
       else Intent(this, PasswordStore::class.java).setAction(Intent.ACTION_VIEW)
     startActivity(intentToStart)
@@ -74,5 +74,12 @@ class LaunchActivity : AppCompatActivity() {
   companion object {
 
     const val ACTION_DECRYPT_PASS = "DECRYPT_PASS"
+
+    /** True when [path] resolves, after following `..` and symlinks, to a file under [repoDir]. */
+    fun isFileInRepo(path: String?, repoDir: File): Boolean {
+      if (path.isNullOrEmpty()) return false
+      val file = File(path).canonicalFile
+      return file.isFile && file.startsWith(repoDir.canonicalFile)
+    }
   }
 }
diff --git a/app/src/test/java/app/passwordstore/ui/main/LaunchActivityTest.kt b/app/src/test/java/app/passwordstore/ui/main/LaunchActivityTest.kt
new file mode 100644
index 00000000..3c30071a
--- /dev/null
+++ b/app/src/test/java/app/passwordstore/ui/main/LaunchActivityTest.kt
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+package app.passwordstore.ui.main
+
+import app.passwordstore.ui.main.LaunchActivity.Companion.isFileInRepo
+import java.io.File
+import java.nio.file.Files
+import kotlin.test.Test
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class LaunchActivityTest {
+
+  @Test
+  fun onlyAcceptsFilesInsideTheRepository() {
+    val root = Files.createTempDirectory("aps").toFile()
+    val repo = File(root, "store").apply { mkdirs() }
+    val inside =
+      File(repo, "site/login.gpg").apply {
+        parentFile!!.mkdirs()
+        writeText("x")
+      }
+    val outside = File(root, "secret.gpg").apply { writeText("x") }
+    val sibling =
+      File(root, "store-evil/login.gpg").apply {
+        parentFile!!.mkdirs()
+        writeText("x")
+      }
+    val link = File(repo, "link.gpg").toPath()
+    Files.createSymbolicLink(link, outside.toPath())
+
+    assertTrue(isFileInRepo(inside.absolutePath, repo))
+    assertFalse(isFileInRepo(outside.absolutePath, repo))
+    assertFalse(isFileInRepo("${repo.absolutePath}/../secret.gpg", repo))
+    assertFalse(isFileInRepo(sibling.absolutePath, repo))
+    assertFalse(isFileInRepo(link.toString(), repo))
+    assertFalse(isFileInRepo(repo.absolutePath, repo))
+    assertFalse(isFileInRepo(File(repo, "missing.gpg").absolutePath, repo))
+    assertFalse(isFileInRepo(null, repo))
+    assertFalse(isFileInRepo("", repo))
+    root.deleteRecursively()
+  }
+}