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gh-135462: Fix quadratic complexity in processing special input in HTMLParser (GH-135464)
End-of-file errors are now handled according to the HTML5 specs -- comments and declarations are automatically closed, tags are ignored.
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-31
lines changed

3 files changed

+111
-31
lines changed

‎Lib/html/parser.py

Lines changed: 30 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -27,6 +27,7 @@
2727
attr_charref = re.compile(r'&(#[0-9]+|#[xX][0-9a-fA-F]+|[a-zA-Z][a-zA-Z0-9]*)[;=]?')
2828

2929
starttagopen = re.compile('<[a-zA-Z]')
30+
endtagopen = re.compile('</[a-zA-Z]')
3031
piclose = re.compile('>')
3132
commentclose = re.compile(r'--\s*>')
3233
# Note:
@@ -195,25 +196,43 @@ def goahead(self, end):
195196
k = self.parse_pi(i)
196197
elif startswith("<!", i):
197198
k = self.parse_html_declaration(i)
198-
elif (i + 1) < n:
199+
elif (i + 1) < n or end:
199200
self.handle_data("<")
200201
k = i + 1
201202
else:
202203
break
203204
if k < 0:
204205
if not end:
205206
break
206-
k = rawdata.find('>', i + 1)
207-
if k < 0:
208-
k = rawdata.find('<', i + 1)
209-
if k < 0:
210-
k = i + 1
211-
else:
212-
k += 1
213-
if self.convert_charrefs and not self.cdata_elem:
214-
self.handle_data(unescape(rawdata[i:k]))
207+
if starttagopen.match(rawdata, i): # < + letter
208+
pass
209+
elif startswith("</", i):
210+
if i + 2 == n:
211+
self.handle_data("</")
212+
elif endtagopen.match(rawdata, i): # </ + letter
213+
pass
214+
else:
215+
# bogus comment
216+
self.handle_comment(rawdata[i+2:])
217+
elif startswith("<!--", i):
218+
j = n
219+
for suffix in ("--!", "--", "-"):
220+
if rawdata.endswith(suffix, i+4):
221+
j -= len(suffix)
222+
break
223+
self.handle_comment(rawdata[i+4:j])
224+
elif startswith("<![CDATA[", i):
225+
self.unknown_decl(rawdata[i+3:])
226+
elif rawdata[i:i+9].lower() == '<!doctype':
227+
self.handle_decl(rawdata[i+2:])
228+
elif startswith("<!", i):
229+
# bogus comment
230+
self.handle_comment(rawdata[i+2:])
231+
elif startswith("<?", i):
232+
self.handle_pi(rawdata[i+2:])
215233
else:
216-
self.handle_data(rawdata[i:k])
234+
raise AssertionError("we should not get here!")
235+
k = n
217236
i = self.updatepos(i, k)
218237
elif startswith("&#", i):
219238
match = charref.match(rawdata, i)

‎Lib/test/test_htmlparser.py

Lines changed: 77 additions & 20 deletions
Original file line numberDiff line numberDiff line change
@@ -5,6 +5,7 @@
55
import unittest
66

77
from unittest.mock import patch
8+
from test import support
89

910

1011
class EventCollector(html.parser.HTMLParser):
@@ -430,28 +431,34 @@ def test_tolerant_parsing(self):
430431
('data', '<'),
431432
('starttag', 'bc<', [('a', None)]),
432433
('endtag', 'html'),
433-
('data', '\n<img src="URL>'),
434-
('comment', '/img'),
435-
('endtag', 'html<')])
434+
('data', '\n')])
436435

437436
def test_starttag_junk_chars(self):
437+
self._run_check("<", [('data', '<')])
438+
self._run_check("<>", [('data', '<>')])
439+
self._run_check("< >", [('data', '< >')])
440+
self._run_check("< ", [('data', '< ')])
438441
self._run_check("</>", [])
442+
self._run_check("<$>", [('data', '<$>')])
439443
self._run_check("</$>", [('comment', '$')])
440444
self._run_check("</", [('data', '</')])
441-
self._run_check("</a", [('data', '</a')])
445+
self._run_check("</a", [])
446+
self._run_check("</ a>", [('endtag', 'a')])
447+
self._run_check("</ a", [('comment', ' a')])
442448
self._run_check("<a<a>", [('starttag', 'a<a', [])])
443449
self._run_check("</a<a>", [('endtag', 'a<a')])
444-
self._run_check("<!", [('data', '<!')])
445-
self._run_check("<a", [('data', '<a')])
446-
self._run_check("<a foo='bar'", [('data', "<a foo='bar'")])
447-
self._run_check("<a foo='bar", [('data', "<a foo='bar")])
448-
self._run_check("<a foo='>'", [('data', "<a foo='>'")])
449-
self._run_check("<a foo='>", [('data', "<a foo='>")])
450+
self._run_check("<!", [('comment', '')])
451+
self._run_check("<a", [])
452+
self._run_check("<a foo='bar'", [])
453+
self._run_check("<a foo='bar", [])
454+
self._run_check("<a foo='>'", [])
455+
self._run_check("<a foo='>", [])
450456
self._run_check("<a$>", [('starttag', 'a$', [])])
451457
self._run_check("<a$b>", [('starttag', 'a$b', [])])
452458
self._run_check("<a$b/>", [('startendtag', 'a$b', [])])
453459
self._run_check("<a$b >", [('starttag', 'a$b', [])])
454460
self._run_check("<a$b />", [('startendtag', 'a$b', [])])
461+
self._run_check("</a$b>", [('endtag', 'a$b')])
455462

456463
def test_slashes_in_starttag(self):
457464
self._run_check('<a foo="var"/>', [('startendtag', 'a', [('foo', 'var')])])
@@ -576,21 +583,50 @@ def test_EOF_in_charref(self):
576583
for html, expected in data:
577584
self._run_check(html, expected)
578585

579-
def test_EOF_in_comments_or_decls(self):
586+
def test_eof_in_comments(self):
580587
data = [
581-
('<!', [('data', '<!')]),
582-
('<!-', [('data', '<!-')]),
583-
('<!--', [('data', '<!--')]),
584-
('<![', [('data', '<![')]),
585-
('<![CDATA[', [('data', '<![CDATA[')]),
586-
('<![CDATA[x', [('data', '<![CDATA[x')]),
587-
('<!DOCTYPE', [('data', '<!DOCTYPE')]),
588-
('<!DOCTYPE HTML', [('data', '<!DOCTYPE HTML')]),
588+
('<!--', [('comment', '')]),
589+
('<!---', [('comment', '')]),
590+
('<!----', [('comment', '')]),
591+
('<!-----', [('comment', '-')]),
592+
('<!------', [('comment', '--')]),
593+
('<!----!', [('comment', '')]),
594+
('<!---!', [('comment', '-!')]),
595+
('<!---!>', [('comment', '-!>')]),
596+
('<!--foo', [('comment', 'foo')]),
597+
('<!--foo-', [('comment', 'foo')]),
598+
('<!--foo--', [('comment', 'foo')]),
599+
('<!--foo--!', [('comment', 'foo')]),
600+
('<!--<!--', [('comment', '<!')]),
601+
('<!--<!--!', [('comment', '<!')]),
589602
]
590603
for html, expected in data:
591604
self._run_check(html, expected)
605+
606+
def test_eof_in_declarations(self):
607+
data = [
608+
('<!', [('comment', '')]),
609+
('<!-', [('comment', '-')]),
610+
('<![', [('comment', '[')]),
611+
('<![CDATA[', [('unknown decl', 'CDATA[')]),
612+
('<![CDATA[x', [('unknown decl', 'CDATA[x')]),
613+
('<![CDATA[x]', [('unknown decl', 'CDATA[x]')]),
614+
('<![CDATA[x]]', [('unknown decl', 'CDATA[x]]')]),
615+
('<!DOCTYPE', [('decl', 'DOCTYPE')]),
616+
('<!DOCTYPE ', [('decl', 'DOCTYPE ')]),
617+
('<!DOCTYPE html', [('decl', 'DOCTYPE html')]),
618+
('<!DOCTYPE html ', [('decl', 'DOCTYPE html ')]),
619+
('<!DOCTYPE html PUBLIC', [('decl', 'DOCTYPE html PUBLIC')]),
620+
('<!DOCTYPE html PUBLIC "foo', [('decl', 'DOCTYPE html PUBLIC "foo')]),
621+
('<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "foo',
622+
[('decl', 'DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "foo')]),
623+
]
624+
for html, expected in data:
625+
self._run_check(html, expected)
626+
592627
def test_bogus_comments(self):
593-
html = ('<! not really a comment >'
628+
html = ('<!ELEMENT br EMPTY>'
629+
'<! not really a comment >'
594630
'<! not a comment either -->'
595631
'<! -- close enough -->'
596632
'<!><!<-- this was an empty comment>'
@@ -604,6 +640,7 @@ def test_bogus_comments(self):
604640
'<![CDATA]]>' # required '[' after CDATA
605641
)
606642
expected = [
643+
('comment', 'ELEMENT br EMPTY'),
607644
('comment', ' not really a comment '),
608645
('comment', ' not a comment either --'),
609646
('comment', ' -- close enough --'),
@@ -684,6 +721,26 @@ def test_convert_charrefs_dropped_text(self):
684721
('endtag', 'a'), ('data', ' bar & baz')]
685722
)
686723

724+
@support.requires_resource('cpu')
725+
def test_eof_no_quadratic_complexity(self):
726+
# Each of these examples used to take about an hour.
727+
# Now they take a fraction of a second.
728+
def check(source):
729+
parser = html.parser.HTMLParser()
730+
parser.feed(source)
731+
parser.close()
732+
n = 120_000
733+
check("<a " * n)
734+
check("<a a=" * n)
735+
check("</a " * 14 * n)
736+
check("</a a=" * 11 * n)
737+
check("<!--" * 4 * n)
738+
check("<!" * 60 * n)
739+
check("<?" * 19 * n)
740+
check("</$" * 15 * n)
741+
check("<![CDATA[" * 9 * n)
742+
check("<!doctype" * 35 * n)
743+
687744

688745
class AttributesTestCase(TestCaseBase):
689746

‎Misc/NEWS.d/next/Security/2025-06-13-15-55-22.gh-issue-135462.KBeJpc.rst

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,4 @@
1+
Fix quadratic complexity in processing specially crafted input in
2+
:class:`html.parser.HTMLParser`. End-of-file errors are now handled according
3+
to the HTML5 specs -- comments and declarations are automatically closed,
4+
tags are ignored.

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