From ff75f3c1d46d9d76a91ce066af4c77555f62616e Mon Sep 17 00:00:00 2001
From: JinWang An <jinwang.an@samsung.com>
Date: Tue, 6 Apr 2021 15:38:05 +0900
Subject: [PATCH] [CVE-2020-27783] Prevent combinations of <math/svg> and
 <style> to sneak JavaScript through the HTML cleaner.

 A vulnerability (CVE-2020-27783) was discovered in the HTML Cleaner by Yaniv Nizry,
which allowed JavaScript to pass through.  The cleaner now removes more sneaky
"style" content.

Change-Id: Ic0411a54417df8d75cd2e86cf8ca36b7e2d80ab6
Signed-off-by: JinWang An <jinwang.an@samsung.com>
---
 src/lxml/html/clean.py             | 21 +++++++++++++--------
 src/lxml/html/tests/test_clean.py  | 10 ++++++++++
 src/lxml/html/tests/test_clean.txt | 12 ++++++++++--
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/src/lxml/html/clean.py b/src/lxml/html/clean.py
index aa9fc57..a15997a 100644
--- a/src/lxml/html/clean.py
+++ b/src/lxml/html/clean.py
@@ -61,13 +61,15 @@ __all__ = ['clean_html', 'clean', 'Cleaner', 'autolink', 'autolink_html',
 
 # This is an IE-specific construct you can have in a stylesheet to
 # run some Javascript:
-_css_javascript_re = re.compile(
-    r'expression\s*\(.*?\)', re.S|re.I)
+_css_javascript_javascript = re.compile(
+    r'expression\s*\(.*?\)', re.S|re.I).sub
 
 # Do I have to worry about @\nimport?
-_css_import_re = re.compile(
-    r'@\s*import', re.I)
+_replace_css_import = re.compile(
+    r'@\s*import', re.I).sub
 
+_looks_like_tag_content = re.compile(
+    r'</?[a-zA-Z]+|\son[a-zA-Z]+\s*=', re.ASCII).search
 # All kinds of schemes besides just javascript: that can cause
 # execution:
 _is_image_dataurl = re.compile(
@@ -292,8 +294,8 @@ class Cleaner(object):
             if not self.inline_style:
                 for el in _find_styled_elements(doc):
                     old = el.get('style')
-                    new = _css_javascript_re.sub('', old)
-                    new = _css_import_re.sub('', new)
+                    new = _replace_css_javascript('', old)
+                    new = _replace_css_import('', new)
                     if self._has_sneaky_javascript(new):
                         # Something tricky is going on...
                         del el.attrib['style']
@@ -305,9 +307,9 @@ class Cleaner(object):
                         el.drop_tree()
                         continue
                     old = el.text or ''
-                    new = _css_javascript_re.sub('', old)
+                    new = _replace_css_javascript('', old)
                     # The imported CSS can do anything; we just can't allow:
-                    new = _css_import_re.sub('', old)
+                    new = _replace_css_import('', new)
                     if self._has_sneaky_javascript(new):
                         # Something tricky is going on...
                         el.text = '/* deleted */'
@@ -509,6 +511,9 @@ class Cleaner(object):
             return True
         if 'expression(' in style:
             return True
+        if _looks_like_tag_content(style):
+            # e.g. '<math><style><img src=x onerror=alert(1)></style></math>'
+            return True
         return False
 
     def clean_html(self, html):
diff --git a/src/lxml/html/tests/test_clean.py b/src/lxml/html/tests/test_clean.py
index a193d99..e1a7a26 100644
--- a/src/lxml/html/tests/test_clean.py
+++ b/src/lxml/html/tests/test_clean.py
@@ -68,6 +68,16 @@ class CleanerTest(unittest.TestCase):
         s = lxml.html.fromstring('<invalid tag>child</another>')
         self.assertEqual('child', clean_html(s).text_content())
 
+    def test_sneaky_js_in_math_style(self):
+        # This gets parsed as <math> -> <style>"..."</style>
+        # thus passing any tag/script/whatever content through into the output.
+        html = '<math><style><img src=x onerror=alert(1)></style></math>'
+        s = lxml.html.fragment_fromstring(html)
+
+        self.assertEqual(
+            b'<math><style>/* deleted */</style></math>',
+            lxml.html.tostring(clean_html(s)))
+
 
 def test_suite():
     suite = unittest.TestSuite()
diff --git a/src/lxml/html/tests/test_clean.txt b/src/lxml/html/tests/test_clean.txt
index 2824f64..7df1f1d 100644
--- a/src/lxml/html/tests/test_clean.txt
+++ b/src/lxml/html/tests/test_clean.txt
@@ -104,7 +104,11 @@
 >>> print(Cleaner(page_structure=False, safe_attrs_only=False).clean_html(doc))
 <html>
   <head>
-    <style>/* deleted */</style>
+    <style>
+      body {background-image: url()};
+      div {background-image: url()};
+      div {color: };
+    </style>
   </head>
   <body>
     <a href="">a link</a>
@@ -168,7 +172,11 @@
     <link rel="alternate" type="text/rss" src="evil-rss">
     <link rel="alternate" type="text/rss" href="http://example.com">
     <link rel="stylesheet" type="text/rss" href="http://example.com">
-    <style>/* deleted */</style>
+    <style>
+      body {background-image: url()};
+      div {background-image: url()};
+      div {color: };
+    </style>
   </head>
   <body>
     <a href="">a link</a>
-- 
2.7.4