urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)

@@ -1,3 +1,16 @@
+2.6.3 (TBD)
+==================
+
+Bugfixes
+--------
+
+- Fixed a high-severity security issue where decompression-bomb safeguards of
+  the streaming API were bypassed when HTTP redirects were followed.
+  (`GHSA-38jv-5279-wg99 <https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99>`__)
+
+TODO: add other entries.
+
+
 2.6.2 (2025-12-11)
 ==================
 

@@ -233,10 +233,16 @@ async def redirect() -> ResponseReturnValue:
     values = await request.values
     target = values.get("target", "/")
     status = values.get("status", "303 See Other")
+    compressed = values.get("compressed") == "true"
     status_code = status.split(" ")[0]
 
     headers = [("Location", target)]
-    return await make_response("", status_code, headers)
+    if compressed:
+        headers.append(("Content-Encoding", "gzip"))
+        data = gzip.compress(b"foo")
+    else:
+        data = b""
+    return await make_response(data, status_code, headers)
 
 
 @hypercorn_app.route("/redirect_after")

@@ -797,7 +797,11 @@ def drain_conn(self) -> None:
         Unread data in the HTTPResponse connection blocks the connection from being released back to the pool.
         """
         try:
-            self.read()
+            self.read(
+                # Do not spend resources decoding the content unless
+                # decoding has already been initiated.
+                decode_content=self._has_decoded_content,
+            )
         except (HTTPError, OSError, BaseSSLError, HTTPException):
             pass
 

@@ -508,6 +508,25 @@ def test_redirect(self) -> None:
             assert r.status == 200
             assert r.data == b"Dummy server!"
 
+    @mock.patch("urllib3.response.GzipDecoder.decompress")
+    def test_no_decoding_with_redirect_when_preload_disabled(
+        self, gzip_decompress: mock.MagicMock
+    ) -> None:
+        """
+        Test that urllib3 does not attempt to decode a gzipped redirect
+        response when `preload_content` is set to `False`.
+        """
+        with HTTPConnectionPool(self.host, self.port) as pool:
+            # Three requests are expected: two redirects and one final / 200 OK.
+            response = pool.request(
+                "GET",
+                "/redirect",
+                fields={"target": "/redirect?compressed=true", "compressed": "true"},
+                preload_content=False,
+            )
+        assert response.status == 200
+        gzip_decompress.assert_not_called()
+
     def test_303_redirect_makes_request_lose_body(self) -> None:
         with HTTPConnectionPool(self.host, self.port) as pool:
             response = pool.request(