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[3.9] gh-136063: fix quadratic-complexity parsing in email.message._parseparam (GH-136072) #140832

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Merged
ambv merged 1 commit into from
Oct 31, 2025
Merged

[3.9] gh-136063: fix quadratic-complexity parsing in email.message._parseparam (GH-136072) #140832

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gh-136063: fix quadratic-complexity parsing in `email.message._parsep… 
…aram` (GH-136072)

(cherry picked from commit 680a5d0)

Co-authored-by: Bénédikt Tran <[email protected]>
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@picnixz @miss-islington
picnixz authored and miss-islington committed Oct 31, 2025
commit bbebf3b1b9993129d02ba6269dc28eb8379a255d
26 changes: 16 additions & 10 deletions Lib/email/message.py
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Original file line number Diff line number Diff line change
Expand Up @@ -74,19 +74,25 @@ def _parseparam(s):
# RDM This might be a Header, so for now stringify it.
s = ';' + str(s)
plist = []
while s[:1] == ';':
s = s[1:]
end = s.find(';')
while end > 0 and (s.count('"', 0, end) - s.count('\\"', 0, end)) % 2:
end = s.find(';', end + 1)
start = 0
while s.find(';', start) == start:
start += 1
end = s.find(';', start)
ind, diff = start, 0
while end > 0:
diff += s.count('"', ind, end) - s.count('\\"', ind, end)
if diff % 2 == 0:
break
end, ind = ind, s.find(';', end + 1)
if end < 0:
end = len(s)
f = s[:end]
if '=' in f:
i = f.index('=')
f = f[:i].strip().lower() + '=' + f[i+1:].strip()
i = s.find('=', start, end)
if i == -1:
f = s[start:end]
else:
f = s[start:i].rstrip().lower() + '=' + s[i+1:end].lstrip()
plist.append(f.strip())
s = s[end:]
start = end
return plist


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21 changes: 21 additions & 0 deletions Lib/test/test_email/test_email.py
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Expand Up @@ -445,6 +445,27 @@ def test_get_param_with_quotes(self):
"Content-Type: foo; bar*0=\"baz\\\"foobar\"; bar*1=\"\\\"baz\"")
self.assertEqual(msg.get_param('bar'), 'baz"foobar"baz')

def test_get_param_linear_complexity(self):
# Ensure that email.message._parseparam() is fast.
# See https://github.com/python/cpython/issues/136063.
N = 100_000
for s, r in [
("", ""),
("foo=bar", "foo=bar"),
(" FOO = bar ", "foo=bar"),
]:
with self.subTest(s=s, r=r, N=N):
src = f'{s};' * (N - 1) + s
res = email.message._parseparam(src)
self.assertEqual(len(res), N)
self.assertEqual(len(set(res)), 1)
self.assertEqual(res[0], r)

# This will be considered as a single parameter.
malformed = 's="' + ';' * (N - 1)
res = email.message._parseparam(malformed)
self.assertEqual(res, [malformed])

def test_field_containment(self):
msg = email.message_from_string('Header: exists')
self.assertIn('header', msg)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Security/2025-06-28-13-23-53.gh-issue-136063.aGk0Jv.rst
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@@ -0,0 +1,2 @@
:mod:`email.message`: ensure linear complexity for legacy HTTP parameters
parsing. Patch by Bénédikt Tran.
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