From: Stefan Behnel <stefan_ml@behnel.de>
Date: Thu, 11 Nov 2021 12:20:57 +0100
Subject: Cleaner: Prevent "@import" from re-occurring in the CSS after
 replacements, e.g. "@@importimport".
Origin: https://github.com/lxml/lxml/commit/12fa9669007180a7bb87d990c375cf91ca5b664a
Bug-Debian: https://bugs.debian.org/1001885
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2021-43818

Reported as GHSL-2021-1037
---
 src/lxml/html/clean.py            |  2 ++
 src/lxml/html/tests/test_clean.py | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/src/lxml/html/clean.py b/src/lxml/html/clean.py
index 0494357e5b01..25844e873309 100644
--- a/src/lxml/html/clean.py
+++ b/src/lxml/html/clean.py
@@ -541,6 +541,8 @@ class Cleaner(object):
             return True
         if 'expression(' in style:
             return True
+        if '@import' in style:
+            return True
         if '</noscript' in style:
             # e.g. '<noscript><style><a title="</noscript><img src=x onerror=alert(1)>">'
             return True
diff --git a/src/lxml/html/tests/test_clean.py b/src/lxml/html/tests/test_clean.py
index 45c2e83abb13..d395d5141eb8 100644
--- a/src/lxml/html/tests/test_clean.py
+++ b/src/lxml/html/tests/test_clean.py
@@ -123,6 +123,26 @@ class CleanerTest(unittest.TestCase):
             b'<math><style>/* deleted */</style></math>',
             lxml.html.tostring(clean_html(s)))
 
+    def test_sneaky_import_in_style(self):
+        # Prevent "@@importimport" -> "@import" replacement.
+        style_codes = [
+            "@@importimport(extstyle.css)",
+            "@ @  import import(extstyle.css)",
+            "@ @ importimport(extstyle.css)",
+            "@@  import import(extstyle.css)",
+            "@ @import import(extstyle.css)",
+            "@@importimport()",
+        ]
+        for style_code in style_codes:
+            html = '<style>%s</style>' % style_code
+            s = lxml.html.fragment_fromstring(html)
+
+            cleaned = lxml.html.tostring(clean_html(s))
+            self.assertEqual(
+                b'<style>/* deleted */</style>',
+                cleaned,
+                "%s  ->  %s" % (style_code, cleaned))
+
     def test_formaction_attribute_in_button_input(self):
         # The formaction attribute overrides the form's action and should be
         # treated as a malicious link attribute
-- 
2.34.1