urllib3 allows an unbounded number of links in the decompression chain

@@ -0,0 +1,4 @@
+Fixed a security issue where an attacker could compose an HTTP response with
+virtually unlimited links in the ``Content-Encoding`` header, potentially
+leading to a denial of service (DoS) attack by exhausting system resources
+during decoding. The number of allowed chained encodings is now limited to 5.

@@ -306,8 +306,18 @@ class MultiDecoder(ContentDecoder):
         they were applied.
     """
 
+    # Maximum allowed number of chained HTTP encodings in the
+    # Content-Encoding header.
+    max_decode_links = 5
+
     def __init__(self, modes: str) -> None:
-        self._decoders = [_get_decoder(m.strip()) for m in modes.split(",")]
+        encodings = [m.strip() for m in modes.split(",")]
+        if len(encodings) > self.max_decode_links:
+            raise DecodeError(
+                "Too many content encodings in the chain: "
+                f"{len(encodings)} > {self.max_decode_links}"
+            )
+        self._decoders = [_get_decoder(e) for e in encodings]
 
     def flush(self) -> bytes:
         return self._decoders[0].flush()

@@ -847,6 +847,16 @@ def test_read_multi_decoding_deflate_deflate(self) -> None:
         assert r.read(9 * 37) == b"foobarbaz" * 37
         assert r.read() == b""
 
+    def test_read_multi_decoding_too_many_links(self) -> None:
+        fp = BytesIO(b"foo")
+        with pytest.raises(
+            DecodeError, match="Too many content encodings in the chain: 6 > 5"
+        ):
+            HTTPResponse(
+                fp,
+                headers={"content-encoding": "gzip, deflate, br, zstd, gzip, deflate"},
+            )
+
     def test_body_blob(self) -> None:
         resp = HTTPResponse(b"foo")
         assert resp.data == b"foo"