diff --git a/app/src/main/java/org/thoughtcrime/securesms/sharing/ShareRepository.java b/app/src/main/java/org/thoughtcrime/securesms/sharing/ShareRepository.java
index 821e5ff856..ae3e2c7a33 100644
--- a/app/src/main/java/org/thoughtcrime/securesms/sharing/ShareRepository.java
+++ b/app/src/main/java/org/thoughtcrime/securesms/sharing/ShareRepository.java
@@ -28,6 +28,7 @@ import org.thoughtcrime.securesms.mms.PartAuthority;
 import org.thoughtcrime.securesms.providers.BlobProvider;
 import org.thoughtcrime.securesms.util.MediaUtil;
 import org.thoughtcrime.securesms.util.TextSecurePreferences;
+import org.thoughtcrime.securesms.util.UriUtil;
 import org.thoughtcrime.securesms.util.Util;
 import org.whispersystems.libsignal.util.guava.Optional;
 
@@ -77,6 +78,10 @@ class ShareRepository {
       return ShareData.forPrimitiveTypes();
     }
 
+    if (!UriUtil.isValidExternalUri(context, uri)) {
+      throw new IOException("Invalid external URI!");
+    }
+
     mimeType = getMimeType(context, uri, mimeType);
 
     if (PartAuthority.isLocalUri(uri)) {
diff --git a/app/src/main/java/org/thoughtcrime/securesms/util/UriUtil.java b/app/src/main/java/org/thoughtcrime/securesms/util/UriUtil.java
new file mode 100644
index 0000000000..051b3e4287
--- /dev/null
+++ b/app/src/main/java/org/thoughtcrime/securesms/util/UriUtil.java
@@ -0,0 +1,33 @@
+package org.thoughtcrime.securesms.util;
+
+import android.content.ContentResolver;
+import android.content.Context;
+import android.net.Uri;
+
+import androidx.annotation.NonNull;
+
+import java.io.File;
+import java.io.IOException;
+
+public final class UriUtil {
+
+  /**
+   * Ensures that an external URI is valid and doesn't contain any references to internal files or
+   * any other trickiness.
+   */
+  public static boolean isValidExternalUri(@NonNull Context context, @NonNull Uri uri) {
+    if (ContentResolver.SCHEME_FILE.equals(uri.getScheme())) {
+      try {
+        File file = new File(uri.getPath());
+
+        return file.getCanonicalPath().equals(file.getPath()) &&
+               !file.getCanonicalPath().startsWith("/data")   &&
+               !file.getCanonicalPath().contains(context.getPackageName());
+      } catch (IOException e) {
+        return false;
+      }
+    } else {
+      return true;
+    }
+  }
+}
diff --git a/app/src/test/java/org/thoughtcrime/securesms/util/UriUtilTest_isValidExternalUri.java b/app/src/test/java/org/thoughtcrime/securesms/util/UriUtilTest_isValidExternalUri.java
new file mode 100644
index 0000000000..4f000eb21a
--- /dev/null
+++ b/app/src/test/java/org/thoughtcrime/securesms/util/UriUtilTest_isValidExternalUri.java
@@ -0,0 +1,55 @@
+package org.thoughtcrime.securesms.util;
+
+import android.app.Application;
+import android.content.Context;
+import android.net.Uri;
+
+import androidx.test.core.app.ApplicationProvider;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.ParameterizedRobolectricTestRunner;
+import org.robolectric.annotation.Config;
+
+import java.util.Arrays;
+import java.util.Collection;
+
+import static org.junit.Assert.assertEquals;
+
+@RunWith(ParameterizedRobolectricTestRunner.class)
+@Config(manifest = Config.NONE, application = Application.class)
+public class UriUtilTest_isValidExternalUri {
+
+  private final String  input;
+  private final boolean output;
+
+  @ParameterizedRobolectricTestRunner.Parameters
+  public static Collection<Object[]> data() {
+    return Arrays.asList(new Object[][]{
+        { "content://other.app.package.name.org/path/public.txt",             true  },
+        { "file:///sdcard/public.txt",                                        true  },
+        { "file:///data/data/org.thoughtcrime.securesms/private.txt",         false },
+        { "file:///any/path/with/package/name/org.thoughtcrime.securesms",    false },
+        { "file:///org.thoughtcrime.securesms/any/path/with/package/name",    false },
+        { "file:///any/path/../with/back/references/private.txt",             false },
+        { "file:///any/path/with/back/references/../private.txt",             false },
+        { "file:///../any/path/with/back/references/private.txt",             false },
+        { "file:///encoded/back/reference/%2F..%2F..path%2Fto%2Fprivate.txt", false },
+        { "file:///public/%2E%2E%2Fprivate%2Fprivate.txt",                    false },
+        { "file:///data/no/paths/in/data",                                    false },
+    });
+  }
+
+  public UriUtilTest_isValidExternalUri(String input, boolean output) {
+    this.input  = input;
+    this.output = output;
+  }
+
+  @Test
+  public void parse() {
+    Context context = ApplicationProvider.getApplicationContext();
+    Uri     uri     = Uri.parse(input);
+
+    assertEquals(output, UriUtil.isValidExternalUri(context, uri));
+  }
+}