fix: bound sequence output within the block buffer

src/index.ts

@@ -569,6 +569,8 @@ const rzb = (dat: Uint8Array, st: DZstdState, out?: Uint8Array) => {
             st.o[0] = off;
           } else off = st.o[0];
         }
+        // decoded output must stay within the block buffer
+        if (oubt + ll + ml > buf.length) err(0);
         for (let i = 0; i < ll; ++i) {
           buf[oubt + i] = buf[spl + i];
         }

tests/dos_malformed_sequences_test.ts

@@ -0,0 +1,16 @@
+import { assertThrows } from "https://deno.land/std@0.103.0/testing/asserts.ts";
+import * as fzstd from "../src/index.ts";
+
+// A 17-byte frame with a compressed block whose sequence section uses RLE-mode
+// FSE tables to force every sequence to emit match-length code 52 (baseline
+// 65539). The declared regenerated size (window) is 240 bytes, so the sequences
+// claim far more output than the block can hold. A conformant decoder rejects
+// this; without a bound the sequence loop performs unbounded out-of-bounds
+// copies (CPU denial-of-service).
+Deno.test("Malformed sequence overflow is rejected", () => {
+  const data = [
+    0x28, 0xB5, 0x2F, 0xFD, 0x20, 0xF0, 0x45, 0x00, 0x00,
+    0x01, 0x00, 0x7F, 0x54, 0x00, 0x00, 0x34, 0x80
+  ];
+  assertThrows(() => fzstd.decompress(new Uint8Array(data)));
+});