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bpo-35603: Escape table header of make_table output that can cause potential XSS #11341

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Dec 29, 2018
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bpo-35603: Escape table header of make_table output that can cause potential XSS #11341

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9c29a77
Escape table header output of HtmlDiff.make_table
tirkarthi Dec 28, 2018
c096b77
Add news entry
tirkarthi Dec 28, 2018
2f6be82
Fix test to use make_table
tirkarthi Dec 28, 2018
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4 changes: 4 additions & 0 deletions Lib/difflib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2036,6 +2036,10 @@ def make_table(self,fromlines,tolines,fromdesc='',todesc='',context=False,
s.append( fmt % (next_id[i],next_href[i],fromlist[i],
next_href[i],tolist[i]))
if fromdesc or todesc:
fromdesc = fromdesc.replace("&", "&").replace(">", ">") \
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Why not use html.escape()?

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@tirkarthi tirkarthi Dec 28, 2018 •
edited
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I thought about the same but text which was encoded was using the logic inlined and there was some discussion in the issue about just inlining the logic to avoid dependency at https://bugs.python.org/issue914575#msg45518 which I realize now is about xml.sax and not html.

html module was also importing entities at

cpython/Lib/html/__init__.py

Line 6 in c465682

from html.entities import html5 as _html5
that seemed little import heavy for me. But if it's okay to import html then I think it's good to use html.escape across the module to avoid duplicate code.

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Well, for a bug fix we can keep an inlined code and consider using html.escape() in a separate issue.

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Sure, thanks for the review.

.replace("<", "&lt;")
todesc = todesc.replace("&", "&amp;").replace(">", "&gt;") \
.replace("<", "&lt;")
header_row = '<thead><tr>%s%s%s%s</tr></thead>' % (
'<th class="diff_next"><br /></th>',
'<th colspan="2" class="diff_header">%s</th>' % fromdesc,
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9 changes: 9 additions & 0 deletions Lib/test/test_difflib.py
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Expand Up @@ -238,6 +238,15 @@ def test_html_diff(self):
with open(findfile('test_difflib_expect.html')) as fp:
self.assertEqual(actual, fp.read())

def test_make_table_escape_table_header(self):
html_diff = difflib.HtmlDiff()
output = html_diff.make_table(patch914575_from1.splitlines(),
patch914575_to1.splitlines(),
fromdesc='<from>',
todesc='<to>')
self.assertIn('&lt;from&gt;', output)
self.assertIn('&lt;to&gt;', output)

def test_recursion_limit(self):
# Check if the problem described in patch #1413711 exists.
limit = sys.getrecursionlimit()
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2018-12-28-14-53-22.bpo-35603.rVCZAE.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Escape table header output of :meth:`difflib.HtmlDiff.make_table`.
Patch by Karthikeyan Singaravelan.