diff --git a/dexlib2/src/main/java/com/android/tools/smali/dexlib2/dexbacked/DexBackedDexFile.java b/dexlib2/src/main/java/com/android/tools/smali/dexlib2/dexbacked/DexBackedDexFile.java
index 4772f306..9f57db40 100644
--- a/dexlib2/src/main/java/com/android/tools/smali/dexlib2/dexbacked/DexBackedDexFile.java
+++ b/dexlib2/src/main/java/com/android/tools/smali/dexlib2/dexbacked/DexBackedDexFile.java
@@ -174,6 +174,10 @@ protected DexBackedDexFile(@Nullable Opcodes opcodes,
                         "Invalid container_size %d (exceeds buffer length %d)",
                         containerSize, buf.length - offset));
             }
+        } else if (CDexBackedDexFile.isCdex(buf, offset + header_offset)) {
+            int dataStart = dexBuffer.readSmallUint(header_offset + HeaderItem.DATA_START_OFFSET);
+            int dataSize = dexBuffer.readSmallUint(header_offset + HeaderItem.DATA_SIZE_OFFSET);
+            this.containerSize = Math.min(buf.length - offset, Math.max(this.fileSize, dataStart + dataSize));
         } else {
             this.containerSize = this.fileSize;
         }
@@ -185,9 +189,15 @@ protected DexBackedDexFile(@Nullable Opcodes opcodes,
              throw new DexUtil.InvalidFile(String.format("Invalid mapOffset %d", mapOffset));
         }
 
+        int baseDataOffset = getBaseDataOffset();
+        if (baseDataOffset > containerSize - 4 - mapOffset) {
+             throw new DexUtil.InvalidFile(String.format("Invalid mapOffset %d", mapOffset));
+        }
+
         // Eagerly read mapSize to validate map bounds
-        int mapSize = dexBuffer.readSmallUint(mapOffset);
-        if (mapOffset + 4 + mapSize * MapItem.ITEM_SIZE > containerSize) {
+        int mapSize = dataBuffer.readSmallUint(mapOffset);
+        int remainingSize = containerSize - baseDataOffset - mapOffset - 4;
+        if (mapSize > remainingSize / MapItem.ITEM_SIZE) {
              throw new DexUtil.InvalidFile("Map extends beyond container bounds");
         }
     }
diff --git a/dexlib2/src/test/java/com/android/tools/smali/dexlib2/dexbacked/CDexTest.java b/dexlib2/src/test/java/com/android/tools/smali/dexlib2/dexbacked/CDexTest.java
new file mode 100644
index 00000000..4838f174
--- /dev/null
+++ b/dexlib2/src/test/java/com/android/tools/smali/dexlib2/dexbacked/CDexTest.java
@@ -0,0 +1,85 @@
+/*
+ * Copyright 2026, Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.android.tools.smali.dexlib2.dexbacked;
+
+import com.android.tools.smali.dexlib2.dexbacked.raw.HeaderItem;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class CDexTest {
+
+    @Test
+    public void testSyntheticCdexHeader() throws Exception {
+        // CDEX header layout (112 bytes min header):
+        // 0-7: magic "cdex001\0"
+        // 32-35: file_size = 112 (0x70)
+        // 36-39: header_size = 112 (0x70)
+        // 40-43: endian_tag = 0x12345678
+        // 52-55: map_off = 120 (0x78)  <- Notice map_off is relative to data_off (112)
+        // 104-107: data_size = 128
+        // 108-111: data_off = 112
+        //
+        // In CDEX, map item is at absolute offset data_off + map_off = 112 + 120 = 232.
+        // At absolute offset map_off (120), write invalid large uint (0xFFFFFFFF) to ensure
+        // mapSize is read from dataBuffer (offset 232) and NOT dexBuffer (offset 120).
+        byte[] buf = new byte[256];
+        byte[] magic = new byte[]{'c', 'd', 'e', 'x', '0', '0', '1', 0};
+        System.arraycopy(magic, 0, buf, 0, 8);
+        int fileSize = 112;
+        int dataStart = 112;
+        int dataSize = 128;
+        int mapOffset = 120;
+
+        writeInt(buf, HeaderItem.FILE_SIZE_OFFSET, fileSize);
+        writeInt(buf, HeaderItem.HEADER_SIZE_OFFSET, fileSize);
+        writeInt(buf, HeaderItem.ENDIAN_TAG_OFFSET, HeaderItem.LITTLE_ENDIAN_TAG);
+        writeInt(buf, HeaderItem.MAP_OFFSET, mapOffset);
+        writeInt(buf, HeaderItem.DATA_SIZE_OFFSET, dataSize);
+        writeInt(buf, HeaderItem.DATA_START_OFFSET, dataStart);
+
+        writeInt(buf, mapOffset, 0xFFFFFFFF); // Trap if reading from dexBuffer at mapOffset (120)
+        writeInt(buf, dataStart + mapOffset, 0); // Correct map_size = 0 at dataStart + mapOffset (232)
+
+        CDexBackedDexFile cdex = new CDexBackedDexFile(null, buf, 0, false);
+        Assert.assertNotNull(cdex);
+    }
+
+    private static void writeInt(byte[] buf, int offset, int value) {
+        buf[offset] = (byte) value;
+        buf[offset + 1] = (byte) (value >> 8);
+        buf[offset + 2] = (byte) (value >> 16);
+        buf[offset + 3] = (byte) (value >> 24);
+    }
+}