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CVE-2026-28348, CVE-2026-28350 (bsc#1259378, bsc#1259379) #2

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nkrapp wants to merge 1 commits from nkrapp/python-lxml_html_clean:leap-16.1 into leap-16.1
pull from: nkrapp/python-lxml_html_clean:leap-16.1
merge into: pool:leap-16.1
Conversation 4 Commits 1 Files Changed 4 +215
4 changed files with 215 additions and 0 deletions
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110
CVE-2026-28348.patch Normal file
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@@ -0,0 +1,110 @@
From 2ef732667ddbc74ea59847bcf24b75809aaeed3b Mon Sep 17 00:00:00 2001
From: Lumir Balhar <lbalhar@redhat.com>
Date: Wed, 25 Feb 2026 22:35:58 +0100
Subject: [PATCH] Implement unicode escape decoding
Unicode escapes in CSS were not properly decoded before security
checks. This prevents attackers from bypassing filters using
escape sequences.
---
CHANGES.rst | 7 ++++++
lxml_html_clean/clean.py | 22 +++++++++++++++++-
tests/test_clean.py | 48 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 76 insertions(+), 1 deletion(-)
diff --git a/lxml_html_clean/clean.py b/lxml_html_clean/clean.py
index 3eeda47..5424d9f 100644
--- a/lxml_html_clean/clean.py
+++ b/lxml_html_clean/clean.py
@@ -578,6 +578,26 @@ def _remove_javascript_link(self, link):
_comments_re = re.compile(r'/\*.*?\*/', re.S)
_find_comments = _comments_re.finditer
_substitute_comments = _comments_re.sub
+ _css_unicode_escape_re = re.compile(r'\\([0-9a-fA-F]{1,6})\s?')
+
+ def _decode_css_unicode_escapes(self, style):
+ """
+ Decode CSS Unicode escape sequences like \\69 or \\000069 to their
+ actual character values. This prevents bypassing security checks
+ using CSS escape sequences.
+
+ CSS escape syntax: backslash followed by 1-6 hex digits,
+ optionally followed by a whitespace character.
+ """
+ def replace_escape(match):
+ hex_value = match.group(1)
+ try:
+ return chr(int(hex_value, 16))
+ except (ValueError, OverflowError):
+ # Invalid unicode codepoint, keep original
+ return match.group(0)
+
+ return self._css_unicode_escape_re.sub(replace_escape, style)
def _has_sneaky_javascript(self, style):
"""
@@ -591,7 +611,7 @@ def _has_sneaky_javascript(self, style):
more sneaky attempts.
"""
style = self._substitute_comments('', style)
- style = style.replace('\\', '')
+ style = self._decode_css_unicode_escapes(style)
style = _substitute_whitespace('', style)
style = style.lower()
if _has_javascript_scheme(style):
diff --git a/tests/test_clean.py b/tests/test_clean.py
index 64ad52d..d1ebcb1 100644
--- a/tests/test_clean.py
+++ b/tests/test_clean.py
@@ -393,3 +393,51 @@ def test_possibly_invalid_url_without_whitelist(self):
self.assertEqual(len(w), 0)
self.assertNotIn("google.com", result)
self.assertNotIn("example.com", result)
+
+ def test_unicode_escape_in_style(self):
+ # Test that CSS Unicode escapes are properly decoded before security checks
+ # This prevents attackers from bypassing filters using escape sequences
+ # CSS escape syntax: \HHHHHH where H is a hex digit (1-6 digits)
+
+ # Test inline style attributes (requires safe_attrs_only=False)
+ cleaner = Cleaner(safe_attrs_only=False)
+ inline_style_cases = [
+ # \6a\61\76\61\73\63\72\69\70\74 = "javascript"
+ ('<div style="background: url(\\6a\\61\\76\\61\\73\\63\\72\\69\\70\\74:alert(1))">test</div>', '<div>test</div>'),
+ # \69 = 'i', so \69mport = "import"
+ ('<div style="@\\69mport url(evil.css)">test</div>', '<div>test</div>'),
+ # \69 with space after = 'i', space consumed as part of escape
+ ('<div style="@\\69 mport url(evil.css)">test</div>', '<div>test</div>'),
+ # \65\78\70\72\65\73\73\69\6f\6e = "expression"
+ ('<div style="\\65\\78\\70\\72\\65\\73\\73\\69\\6f\\6e(alert(1))">test</div>', '<div>test</div>'),
+ ]
+
+ for html, expected in inline_style_cases:
+ with self.subTest(html=html):
+ cleaned = cleaner.clean_html(html)
+ self.assertEqual(expected, cleaned)
+
+ # Test <style> tag content (uses default clean_html)
+ style_tag_cases = [
+ # Unicode-escaped "javascript:" in url()
+ '<style>url(\\6a\\61\\76\\61\\73\\63\\72\\69\\70\\74:alert(1))</style>',
+ # Unicode-escaped "javascript:" without url()
+ '<style>\\6a\\61\\76\\61\\73\\63\\72\\69\\70\\74:alert(1)</style>',
+ # Unicode-escaped "expression"
+ '<style>\\65\\78\\70\\72\\65\\73\\73\\69\\6f\\6e(alert(1))</style>',
+ # Unicode-escaped @import with 'i'
+ '<style>@\\69mport url(evil.css)</style>',
+ # Unicode-escaped "data:" scheme
+ '<style>url(\\64\\61\\74\\61:image/svg+xml;base64,PHN2ZyBvbmxvYWQ9YWxlcnQoMSk+)</style>',
+ # Space after escape is consumed: \69 mport = "import"
+ '<style>@\\69 mport url(evil.css)</style>',
+ # 6-digit escape: \000069 = 'i'
+ '<style>@\\000069mport url(evil.css)</style>',
+ # 6-digit escape with space
+ '<style>@\\000069 mport url(evil.css)</style>',
+ ]
+
+ for html in style_tag_cases:
+ with self.subTest(html=html):
+ cleaned = clean_html(html)
+ self.assertEqual('<div><style>/* deleted */</style></div>', cleaned)

91
CVE-2026-28350.patch Normal file
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@@ -0,0 +1,91 @@
From 9c5612ca33b941eec4178abf8a5294b103403f34 Mon Sep 17 00:00:00 2001
From: Lumir Balhar <lbalhar@redhat.com>
Date: Wed, 25 Feb 2026 22:57:28 +0100
Subject: [PATCH] Remove <base> tags to prevent URL hijacking attacks
<base> tags are now automatically removed whenever <head> is removed to
prevent URL hijacking attacks. According to HTML spec, <base> must be in
<head>, but browsers may interpret misplaced <base> tags, allowing
attackers to redirect all relative URLs to malicious servers.
---
CHANGES.rst | 5 +++++
lxml_html_clean/clean.py | 6 +++++
tests/test_clean.py | 48 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 59 insertions(+)
diff --git a/lxml_html_clean/clean.py b/lxml_html_clean/clean.py
index 5424d9f..6f95b26 100644
--- a/lxml_html_clean/clean.py
+++ b/lxml_html_clean/clean.py
@@ -422,6 +422,12 @@ def __call__(self, doc):
if self.annoying_tags:
remove_tags.update(('blink', 'marquee'))
+ # Remove <base> tags whenever <head> is being removed.
+ # According to HTML spec, <base> must be in <head>, but browsers
+ # may interpret it even when misplaced, allowing URL hijacking attacks.
+ if 'head' in kill_tags or 'head' in remove_tags:
+ kill_tags.add('base')
+
_remove = deque()
_kill = deque()
for el in doc.iter():
diff --git a/tests/test_clean.py b/tests/test_clean.py
index d1ebcb1..93f6da1 100644
--- a/tests/test_clean.py
+++ b/tests/test_clean.py
@@ -394,6 +394,54 @@ def test_possibly_invalid_url_without_whitelist(self):
self.assertNotIn("google.com", result)
self.assertNotIn("example.com", result)
+ def test_base_tag_removed_with_page_structure(self):
+ # Test that <base> tags are removed when page_structure=True (default)
+ # This prevents URL hijacking attacks where <base> redirects all relative URLs
+
+ test_cases = [
+ # <base> in proper location (inside <head>)
+ '<html><head><base href="http://evil.com/"></head><body><a href="page.html">link</a></body></html>',
+ # <base> outside <head>
+ '<div><base href="http://evil.com/"><a href="page.html">link</a></div>',
+ # Multiple <base> tags
+ '<base href="http://evil.com/"><div><base href="http://evil2.com/"></div>',
+ # <base> with target attribute
+ '<base target="_blank"><div>content</div>',
+ # <base> at various positions
+ '<html><base href="http://evil.com/"><body>test</body></html>',
+ ]
+
+ for html in test_cases:
+ with self.subTest(html=html):
+ cleaned = clean_html(html)
+ # Verify <base> tag is completely removed
+ self.assertNotIn('base', cleaned.lower())
+ self.assertNotIn('evil.com', cleaned)
+ self.assertNotIn('evil2.com', cleaned)
+
+ def test_base_tag_kept_when_page_structure_false(self):
+ # When page_structure=False and head is not removed, <base> should be kept
+ cleaner = Cleaner(page_structure=False)
+ html = '<html><head><base href="http://example.com/"></head><body>test</body></html>'
+ cleaned = cleaner.clean_html(html)
+ self.assertIn('<base href="http://example.com/">', cleaned)
+
+ def test_base_tag_removed_when_head_in_remove_tags(self):
+ # Even with page_structure=False, <base> should be removed if head is manually removed
+ cleaner = Cleaner(page_structure=False, remove_tags=['head'])
+ html = '<html><head><base href="http://evil.com/"></head><body>test</body></html>'
+ cleaned = cleaner.clean_html(html)
+ self.assertNotIn('base', cleaned.lower())
+ self.assertNotIn('evil.com', cleaned)
+
+ def test_base_tag_removed_when_head_in_kill_tags(self):
+ # Even with page_structure=False, <base> should be removed if head is in kill_tags
+ cleaner = Cleaner(page_structure=False, kill_tags=['head'])
+ html = '<html><head><base href="http://evil.com/"></head><body>test</body></html>'
+ cleaned = cleaner.clean_html(html)
+ self.assertNotIn('base', cleaned.lower())
+ self.assertNotIn('evil.com', cleaned)
+
def test_unicode_escape_in_style(self):
# Test that CSS Unicode escapes are properly decoded before security checks
# This prevents attackers from bypassing filters using escape sequences

10
python-lxml_html_clean.changes
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@@ -1,3 +1,13 @@
-------------------------------------------------------------------
Tue Mar 10 12:59:11 UTC 2026 - Nico Krapp <nico.krapp@suse.com>
- CVE-2026-28348: improper keywords checking can allow external CSS loading
(bsc#1259378)
* added CVE-2026-28348.patch
- CVE-2026-28350: lack of base tag handling can allow the hijacking of the
resolution of relative URLs (bsc#1259379)
* added CVE-2026-28350.patch
-------------------------------------------------------------------
Fri Apr 11 20:57:19 UTC 2025 - Dirk Müller <dmueller@suse.com>

4
python-lxml_html_clean.spec
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@@ -26,6 +26,10 @@ License: BSD-3-Clause
Group: Development/Languages/Python
URL: https://github.com/fedora-python/lxml_html_clean/
Source: https://files.pythonhosted.org/packages/source/l/lxml-html-clean/lxml_html_clean-%{version}.tar.gz
# PATCH-FIX-UPSTREAM CVE-2026-28348.patch bsc#1259378 gh#fedora-python/lxml_html_clean@2ef7326
Patch1: CVE-2026-28348.patch
# PATCH-FIX-UPSTREAM CVE-2026-28350.patch bsc#1259379 gh#fedora-python/lxml_html_clean@9c5612c
Patch2: CVE-2026-28350.patch
BuildRequires: %{python_module base >= 3.6}
BuildRequires: %{python_module pip}
BuildRequires: %{python_module setuptools >= 61.0}