From 69a747356655158fdf9abaecea5feafb3bd6b5f5 Mon Sep 17 00:00:00 2001
From: Stefan Behnel <stefan_ml@behnel.de>
Date: Sat, 11 Dec 2021 12:19:21 +0100
Subject: [PATCH] Cleaner: cover some more cases where scripts could sneak
 through in specially crafted style content.

---
 src/lxml/html/clean.py            | 20 ++++++------
 src/lxml/html/tests/test_clean.py | 65 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 73 insertions(+), 12 deletions(-)

diff --git a/src/lxml/html/clean.py b/src/lxml/html/clean.py
index 4df10c2..0e96627 100644
--- a/src/lxml/html/clean.py
+++ b/src/lxml/html/clean.py
@@ -74,22 +74,20 @@ _looks_like_tag_content = re.compile(
 # All kinds of schemes besides just javascript: that can cause
 # execution:
 _find_image_dataurls = re.compile(
-    r'^data:image/(.+);base64,', re.I).findall
-_is_possibly_malicious_scheme = re.compile(
+    r'data:image/(.+);base64,', re.I).findall
+_possibly_malicious_schemes = re.compile(
     r'(javascript|jscript|livescript|vbscript|data|about|mocha):',
     re.I).findall
 # SVG images can contain script content
-_is_unsafe_image_type = re.compile(r"(xml|svg)", re.I).findall
+_is_unsafe_image_type = re.compile(r"(xml|svg)", re.I).search
 
-def _is_javascript_scheme(s):
-    is_image_url = False
+def _has_javascript_scheme(s):
+    safe_image_urls = 0
     for image_type in _find_image_dataurls(s):
-        is_image_url = True
         if _is_unsafe_image_type(image_type):
             return True
-    if is_image_url:
-        return False
-    return bool(_is_possibly_malicious_scheme(s))
+        safe_image_urls += 1
+    return len(_possibly_malicious_schemes(s)) > safe_image_urls
 
 _substitute_whitespace = re.compile(r'[\s\x00-\x08\x0B\x0C\x0E-\x19]+').sub
 
@@ -521,7 +519,7 @@ class Cleaner(object):
     def _remove_javascript_link(self, link):
         # links like "j a v a s c r i p t:" might be interpreted in IE
         new = _substitute_whitespace('', unquote_plus(link))
-        if _is_javascript_scheme(new):
+        if _has_javascript_scheme(new):
             # FIXME: should this be None to delete?
             return ''
         return link
@@ -543,7 +541,7 @@ class Cleaner(object):
         style = style.replace('\\', '')
         style = _substitute_whitespace('', style)
         style = style.lower()
-        if 'javascript:' in style:
+        if _has_javascript_scheme(style):
             return True
         if 'expression(' in style:
             return True
diff --git a/src/lxml/html/tests/test_clean.py b/src/lxml/html/tests/test_clean.py
index a05d967..aec87cd 100644
--- a/src/lxml/html/tests/test_clean.py
+++ b/src/lxml/html/tests/test_clean.py
@@ -126,7 +126,7 @@ class CleanerTest(unittest.TestCase):
             lxml.html.tostring(clean_html(s)))
 
     def test_sneaky_import_in_style(self):
-        # Prevent "@@importimport" -> "@import" replacement.
+        # Prevent "@@importimport" -> "@import" replacement etc.
         style_codes = [
             "@@importimport(extstyle.css)",
             "@ @  import import(extstyle.css)",
@@ -134,6 +134,11 @@ class CleanerTest(unittest.TestCase):
             "@@  import import(extstyle.css)",
             "@ @import import(extstyle.css)",
             "@@importimport()",
+            "@@importimport()  ()",
+            "@/* ... */import()",
+            "@im/* ... */port()",
+            "@ @import/* ... */import()",
+            "@    /* ... */      import()",
         ]
         for style_code in style_codes:
             html = '<style>%s</style>' % style_code
@@ -145,6 +150,41 @@ class CleanerTest(unittest.TestCase):
                 cleaned,
                 "%s  ->  %s" % (style_code, cleaned))
 
+    def test_sneaky_schemes_in_style(self):
+        style_codes = [
+            "javasjavascript:cript:",
+            "javascriptjavascript::",
+            "javascriptjavascript:: :",
+            "vbjavascript:cript:",
+        ]
+        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_sneaky_urls_in_style(self):
+        style_codes = [
+            "url(data:image/svg+xml;base64,...)",
+            "url(javasjavascript:cript:)",
+            "url(javasjavascript:cript: ::)",
+            "url(vbjavascript:cript:)",
+            "url(vbjavascript:cript: :)",
+        ]
+        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>url()</style>',
+                cleaned,
+                "%s  ->  %s" % (style_code, cleaned))
+
     def test_svg_data_links(self):
         # Remove SVG images with potentially insecure content.
         svg = b'<svg onload="alert(123)" />'
@@ -188,6 +228,29 @@ class CleanerTest(unittest.TestCase):
                 cleaned,
                 "%s  ->  %s" % (url, cleaned))
 
+    def test_image_data_links_in_style(self):
+        data = b'123'
+        data_b64 = base64.b64encode(data).decode('ASCII')
+        urls = [
+            "data:image/jpeg;base64," + data_b64,
+            "data:image/apng;base64," + data_b64,
+            "data:image/png;base64," + data_b64,
+            "data:image/gif;base64," + data_b64,
+            "data:image/webp;base64," + data_b64,
+            "data:image/bmp;base64," + data_b64,
+            "data:image/tiff;base64," + data_b64,
+            "data:image/x-icon;base64," + data_b64,
+        ]
+        for url in urls:
+            html = '<style> url(%s) </style>' % url
+            s = lxml.html.fragment_fromstring(html)
+
+            cleaned = lxml.html.tostring(clean_html(s))
+            self.assertEqual(
+                html.encode("UTF-8"),
+                cleaned,
+                "%s  ->  %s" % (url, 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
-- 
2.13.7

