Add regression test and fuzz coverage for zero first offset in lists

crates/ssz/tests/encode_decode.rs

@@ -150,6 +150,31 @@ fn vec_of_vecs_empty() {
     assert_eq!(Vec::<Vec<u8>>::from_ssz_bytes(&encoded).unwrap(), val);
 }
 
+// A zero first offset would imply 0 elements while the offset/scope boundary
+// describes 1 element spanning the whole input; it must be rejected up front.
+// See https://github.com/leanEthereum/leanSpec/issues/1175
+#[test]
+fn vec_of_vecs_zero_first_offset() {
+    let bytes = [0x00, 0x00, 0x00, 0x00, 0xAA, 0xBB, 0xCC, 0xDD];
+    assert_eq!(
+        Vec::<Vec<u8>>::from_ssz_bytes(&bytes),
+        Err(DecodeError::InvalidFirstOffset {
+            expected: 4,
+            got: 0
+        })
+    );
+
+    // Same with no trailing data: only an offset table pointing at itself.
+    let bytes = [0x00, 0x00, 0x00, 0x00];
+    assert_eq!(
+        Vec::<Vec<u8>>::from_ssz_bytes(&bytes),
+        Err(DecodeError::InvalidFirstOffset {
+            expected: 4,
+            got: 0
+        })
+    );
+}
+
 // ── ContainerEncoder / ContainerDecoder ──
 
 /// Simulates a container: { a: u32, b: Vec<u8>, c: u16 }

fuzz/fuzz_targets/offset_adversarial.rs

@@ -34,6 +34,11 @@ struct FixedThenVar {
     data: Vec<u8>,
 }
 
+#[derive(Debug, PartialEq, SszEncode, SszDecode)]
+struct VarItemsField {
+    items: Vec<Vec<u8>>,
+}
+
 fuzz_target!(|data: &[u8]| {
     // Attempt decode of each struct — must never panic regardless of input.
     // If decode succeeds, roundtrip must hold.
@@ -61,4 +66,18 @@ fuzz_target!(|data: &[u8]| {
         let decoded = FixedThenVar::from_ssz_bytes(&encoded).unwrap();
         assert_eq!(decoded, v, "FixedThenVar roundtrip");
     }
+
+    // Lists of variable-length items have their own offset table, exercising
+    // decode_variable_length_items_with_max rather than ContainerDecoder.
+    if let Ok(v) = Vec::<Vec<u8>>::from_ssz_bytes(data) {
+        let encoded = v.to_ssz();
+        let decoded = Vec::<Vec<u8>>::from_ssz_bytes(&encoded).unwrap();
+        assert_eq!(decoded, v, "Vec<Vec<u8>> roundtrip");
+    }
+
+    if let Ok(v) = VarItemsField::from_ssz_bytes(data) {
+        let encoded = v.to_ssz();
+        let decoded = VarItemsField::from_ssz_bytes(&encoded).unwrap();
+        assert_eq!(decoded, v, "VarItemsField roundtrip");
+    }
 });