Fixed #34170 -- Implemented Heal The Breach (HTB) in GzipMiddleware.

5 files changed, 106 insertions, 17 deletions

diff --git a/django/middleware/gzip.py b/django/middleware/gzip.py
index db8847a2f3..d91246c007 100644
--- a/django/middleware/gzip.py
+++ b/django/middleware/gzip.py
@@ -13,6 +13,8 @@ class GZipMiddleware(MiddlewareMixin):
     on the Accept-Encoding header.
     """
 
+    max_random_bytes = 100
+
     def process_response(self, request, response):
         # It's not worth attempting to compress really short responses.
         if not response.streaming and len(response.content) < 200:
@@ -31,11 +33,17 @@ class GZipMiddleware(MiddlewareMixin):
         if response.streaming:
             # Delete the `Content-Length` header for streaming content, because
             # we won't know the compressed size until we stream it.
-            response.streaming_content = compress_sequence(response.streaming_content)
+            response.streaming_content = compress_sequence(
+                response.streaming_content,
+                max_random_bytes=self.max_random_bytes,
+            )
             del response.headers["Content-Length"]
         else:
             # Return the compressed content only if it's actually shorter.
-            compressed_content = compress_string(response.content)
+            compressed_content = compress_string(
+                response.content,
+                max_random_bytes=self.max_random_bytes,
+            )
             if len(compressed_content) >= len(response.content):
                 return response
             response.content = compressed_content
diff --git a/django/utils/text.py b/django/utils/text.py
index dcfe3fba0e..86d3b52741 100644
--- a/django/utils/text.py
+++ b/django/utils/text.py
@@ -1,4 +1,6 @@
+import gzip
 import re
+import secrets
 import unicodedata
 from gzip import GzipFile
 from gzip import compress as gzip_compress
@@ -314,8 +316,23 @@ def phone2numeric(phone):
     return "".join(char2number.get(c, c) for c in phone.lower())
 
 
-def compress_string(s):
-    return gzip_compress(s, compresslevel=6, mtime=0)
+def _get_random_filename(max_random_bytes):
+    return b"a" * secrets.randbelow(max_random_bytes)
+
+
+def compress_string(s, *, max_random_bytes=None):
+    compressed_data = gzip_compress(s, compresslevel=6, mtime=0)
+
+    if not max_random_bytes:
+        return compressed_data
+
+    compressed_view = memoryview(compressed_data)
+    header = bytearray(compressed_view[:10])
+    header[3] = gzip.FNAME
+
+    filename = _get_random_filename(max_random_bytes) + b"\x00"
+
+    return bytes(header) + filename + compressed_view[10:]
 
 
 class StreamingBuffer(BytesIO):
@@ -327,9 +344,12 @@ class StreamingBuffer(BytesIO):
 
 
 # Like compress_string, but for iterators of strings.
-def compress_sequence(sequence):
+def compress_sequence(sequence, *, max_random_bytes=None):
     buf = StreamingBuffer()
-    with GzipFile(mode="wb", compresslevel=6, fileobj=buf, mtime=0) as zfile:
+    filename = _get_random_filename(max_random_bytes) if max_random_bytes else None
+    with GzipFile(
+        filename=filename, mode="wb", compresslevel=6, fileobj=buf, mtime=0
+    ) as zfile:
         # Output headers...
         yield buf.read()
         for item in sequence:
diff --git a/docs/ref/middleware.txt b/docs/ref/middleware.txt
index 3f55e36d36..a270b8b774 100644
--- a/docs/ref/middleware.txt
+++ b/docs/ref/middleware.txt
@@ -93,18 +93,33 @@ GZip middleware
 
 .. class:: GZipMiddleware
 
-.. warning::
+    .. attribute:: max_random_bytes
+
+        Defaults to 100. Subclass ``GZipMiddleware`` and override the attribute
+        to change the maximum number of random bytes that is included with
+        compressed responses.
+
+.. note::
 
-    Security researchers recently revealed that when compression techniques
-    (including ``GZipMiddleware``) are used on a website, the site may become
-    exposed to a number of possible attacks. Before using ``GZipMiddleware`` on
-    your site, you should consider very carefully whether you are subject to
-    these attacks. If you're in *any* doubt about whether you're affected, you
-    should avoid using ``GZipMiddleware``. For more details, see the `the BREACH
-    paper (PDF)`_ and `breachattack.com`_.
+    Security researchers revealed that when compression techniques (including
+    ``GZipMiddleware``) are used on a website, the site may become exposed to a
+    number of possible attacks.
 
-    .. _the BREACH paper (PDF): https://www.breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf
+    To mitigate attacks, Django implements a technique called *Heal The Breach
+    (HTB)*. It adds up to 100 bytes (see
+    :attr:`.max_random_bytes`) of random bytes to each response
+    to make the attacks less effective.
+
+    For more details, see the `BREACH paper (PDF)`_, `breachattack.com`_, and
+    the `Heal The Breach (HTB) paper`_.
+
+    .. _BREACH paper (PDF): https://www.breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf
     .. _breachattack.com: https://www.breachattack.com/
+    .. _Heal The Breach (HTB) paper: https://ieeexplore.ieee.org/document/9754554
+
+.. versionchanged:: 4.2
+
+    Mitigation for the BREACH attack was added.
 
 The ``django.middleware.gzip.GZipMiddleware`` compresses content for browsers
 that understand GZip compression (all modern browsers).
diff --git a/docs/releases/4.2.txt b/docs/releases/4.2.txt
index 077762275a..682fce2a53 100644
--- a/docs/releases/4.2.txt
+++ b/docs/releases/4.2.txt
@@ -40,6 +40,16 @@ in the future.
 .. _psycopg: https://www.psycopg.org/psycopg3/
 .. _psycopg library: https://pypi.org/project/psycopg/
 
+Mitigation for the BREACH attack
+--------------------------------
+
+:class:`~django.middleware.gzip.GZipMiddleware` now includes a mitigation for
+the BREACH attack. It will add up to 100 random bytes to gzip responses to make
+BREACH attacks harder. Read more about the mitigation technique in the `Heal
+The Breach (HTB) paper`_.
+
+.. _Heal The Breach (HTB) paper: https://ieeexplore.ieee.org/document/9754554
+
 Minor features
 --------------
 
diff --git a/tests/middleware/tests.py b/tests/middleware/tests.py
index 50ba268832..1b8efe1a3e 100644
--- a/tests/middleware/tests.py
+++ b/tests/middleware/tests.py
@@ -3,6 +3,7 @@ import random
 import re
 import struct
 from io import BytesIO
+from unittest import mock
 from urllib.parse import quote
 
 from django.conf import settings
@@ -978,12 +979,47 @@ class GZipMiddlewareTest(SimpleTestCase):
         ConditionalGetMiddleware from recognizing conditional matches
         on gzipped content).
         """
-        r1 = GZipMiddleware(self.get_response)(self.req)
-        r2 = GZipMiddleware(self.get_response)(self.req)
+
+        class DeterministicGZipMiddleware(GZipMiddleware):
+            max_random_bytes = 0
+
+        r1 = DeterministicGZipMiddleware(self.get_response)(self.req)
+        r2 = DeterministicGZipMiddleware(self.get_response)(self.req)
         self.assertEqual(r1.content, r2.content)
         self.assertEqual(self.get_mtime(r1.content), 0)
         self.assertEqual(self.get_mtime(r2.content), 0)
 
+    def test_random_bytes(self):
+        """A random number of bytes is added to mitigate the BREACH attack."""
+        with mock.patch(
+            "django.utils.text.secrets.randbelow", autospec=True, return_value=3
+        ):
+            r = GZipMiddleware(self.get_response)(self.req)
+        # The fourth byte of a gzip stream contains flags.
+        self.assertEqual(r.content[3], gzip.FNAME)
+        # A 3 byte filename "aaa" and a null byte are added.
+        self.assertEqual(r.content[10:14], b"aaa\x00")
+        self.assertEqual(self.decompress(r.content), self.compressible_string)
+
+    def test_random_bytes_streaming_response(self):
+        """A random number of bytes is added to mitigate the BREACH attack."""
+
+        def get_stream_response(request):
+            resp = StreamingHttpResponse(self.sequence)
+            resp["Content-Type"] = "text/html; charset=UTF-8"
+            return resp
+
+        with mock.patch(
+            "django.utils.text.secrets.randbelow", autospec=True, return_value=3
+        ):
+            r = GZipMiddleware(get_stream_response)(self.req)
+            content = b"".join(r)
+        # The fourth byte of a gzip stream contains flags.
+        self.assertEqual(content[3], gzip.FNAME)
+        # A 3 byte filename "aaa" and a null byte are added.
+        self.assertEqual(content[10:14], b"aaa\x00")
+        self.assertEqual(self.decompress(content), b"".join(self.sequence))
+
 
 class ETagGZipMiddlewareTest(SimpleTestCase):
     """