Strip the full set of Unicode bidi controls from attachment filenames.

core/ui/src/main/java/org/signal/core/ui/util/StorageUtil.java

@@ -15,6 +15,7 @@ import androidx.core.content.ContextCompat;
 
 import org.signal.core.ui.CoreUiDependencies;
 import org.signal.core.ui.R;
+import org.signal.core.util.BidiUtil;
 import org.signal.core.util.NoExternalStorageException;
 import org.signal.core.util.permissions.PermissionCompat;
 import org.signal.core.ui.permissions.Permissions;
@@ -179,11 +180,6 @@ public class StorageUtil {
   }
 
   public static @Nullable String getCleanFileName(@Nullable String fileName) {
-    if (fileName == null) return null;
-
-    fileName = fileName.replace('\u202D', '\uFFFD');
-    fileName = fileName.replace('\u202E', '\uFFFD');
-
-    return fileName;
+    return BidiUtil.replaceBidiCharacters(fileName);
   }
 }

core/util-jvm/src/main/java/org/signal/core/util/BidiUtil.kt

@@ -10,6 +10,9 @@ import java.util.regex.Pattern
 object BidiUtil {
   private val ALL_ASCII_PATTERN: Pattern = Pattern.compile("^[\\x00-\\x7F]*$")
 
+  /** The full set of Unicode bidirectional control characters (overrides, isolates, and indicators). */
+  private const val DIRECTIONAL_CHARACTERS = "[\\u200f\\u2066\\u2067\\u2068\\u2069\\u202a\\u202b\\u202c\\u202d\\u202e]"
+
   object BidiCodepoint {
     const val LRI = "\u2066"
 
@@ -160,6 +163,22 @@ object BidiUtil {
   }
 
   fun stripAllDirectionalCharacters(text: String): String {
-    return text.replace("[\\u200f\\u2066\\u2067\\u2068\\u2069\\u202a\\u202b\\u202c\\u202d\\u202e]".toRegex(), "")
+    return text.replace(DIRECTIONAL_CHARACTERS.toRegex(), "")
+  }
+
+  /**
+   * Neutralizes the full set of Unicode bidirectional control characters by replacing each one with
+   * the Unicode replacement character (U+FFFD).
+   *
+   * Unlike [stripAllDirectionalCharacters], this preserves a visible indication that a character was
+   * removed. This is useful when sanitizing strings for display where a directional override or
+   * isolate could otherwise be used to spoof content, such as faking an attachment's file extension
+   * (e.g. "document<RLO>txt.exe" rendering as "documentexe.txt").
+   */
+  @JvmStatic
+  fun replaceBidiCharacters(text: String?): String? {
+    if (text == null) return null
+
+    return text.replace(DIRECTIONAL_CHARACTERS.toRegex(), "<EFBFBD>")
   }
 }

core/util-jvm/src/test/java/org/signal/core/util/BidiUtilTest.kt

@@ -0,0 +1,58 @@
+package org.signal.core.util
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class BidiUtilTest {
+
+  private val replacement = "<EFBFBD>"
+
+  @Test
+  fun replaceBidiCharacters_nullInput_returnsNull() {
+    assertNull(BidiUtil.replaceBidiCharacters(null))
+  }
+
+  @Test
+  fun replaceBidiCharacters_plainText_isUnchanged() {
+    assertEquals("document.txt", BidiUtil.replaceBidiCharacters("document.txt"))
+  }
+
+  @Test
+  fun replaceBidiCharacters_overrides_areReplaced() {
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‪")) // LRE
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‫")) // RLE
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‬")) // PDF
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‭")) // LRO
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‮")) // RLO
+  }
+
+  @Test
+  fun replaceBidiCharacters_isolates_areReplaced() {
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("⁦")) // LRI
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("⁧")) // RLI
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("⁨")) // FSI
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("⁩")) // PDI
+  }
+
+  @Test
+  fun replaceBidiCharacters_directionalIndicator_isReplaced() {
+    assertEquals("$replacement", BidiUtil.replaceBidiCharacters("‏")) // RLM
+  }
+
+  @Test
+  fun replaceBidiCharacters_spoofedExtension_isNeutralized() {
+    // "document<RLO>txt.exe<PDI>" would render as "documentexe.txt" without sanitization.
+    val malicious = "document‮txt.exe⁩"
+    val cleaned = BidiUtil.replaceBidiCharacters(malicious)
+
+    assertEquals("document${replacement}txt.exe$replacement", cleaned)
+    assertEquals(0, cleaned!!.count { it == '‮' || it == '⁩' })
+  }
+
+  @Test
+  fun replaceBidiCharacters_multipleControls_allReplaced() {
+    val input = "a‫b⁦c‮d"
+    assertEquals("a${replacement}b${replacement}c${replacement}d", BidiUtil.replaceBidiCharacters(input))
+  }
+}