From 24d7b67eac89f94e11003424bcf0d8f7b72222a8 Mon Sep 17 00:00:00 2001
From: Illia Volochii <illia.volochii@gmail.com>
Date: Fri, 5 Dec 2025 16:41:33 +0200
Subject: [PATCH] Merge commit from fork

* Add a hard-coded limit for the decompression chain

* Reuse new list

Reference:github.com/urllib3/urllib3/commit/24d7b67e.patch
Conflict:no
---
 changelog/GHSA-gm62-xv2j-4w53.security.rst |  4 ++++
 src/urllib3/response.py                    | 12 +++++++++++-
 test/test_response.py                      | 10 ++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 changelog/GHSA-gm62-xv2j-4w53.security.rst

diff --git a/changelog/GHSA-gm62-xv2j-4w53.security.rst b/changelog/GHSA-gm62-xv2j-4w53.security.rst
new file mode 100644
index 0000000000..6646eaa3a6
--- /dev/null
+++ b/changelog/GHSA-gm62-xv2j-4w53.security.rst
@@ -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.
diff --git a/src/urllib3/response.py b/src/urllib3/response.py
index 4ba421369f..069f726cb8 100644
--- a/src/urllib3/response.py
+++ b/src/urllib3/response.py
@@ -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()
diff --git a/test/test_response.py b/test/test_response.py
index 9592fdd940..d824ae70b4 100644
--- a/test/test_response.py
+++ b/test/test_response.py
@@ -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"