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gh-123067: Denial of Service Vulnerability in http.cookies._unquote() #123066

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gh-123067: Denial of Service Vulnerability in http.cookies._unquote() #123066

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50 changes: 14 additions & 36 deletions Lib/http/cookies.py
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Original file line number Diff line number Diff line change
Expand Up @@ -184,51 +184,29 @@ def _quote(str):
return '"' + str.translate(_Translator) + '"'


_OctalPatt = re.compile(r"\\[0-3][0-7][0-7]")
_QuotePatt = re.compile(r"[\\].")
_OctalPatt = re.compile(r"\\([0-3][0-7][0-7])")
_QuotePatt = re.compile(r'\\"')

def _unquote(str):
# If there aren't any doublequotes,
# then there can't be any special characters. See RFC 2109.
if str is None or len(str) < 2:
return str
if str[0] != '"' or str[-1] != '"':
return str

# We have to assume that we must decode this string.
# Down to work.

# Remove the "s
# Remove the surrounding quotes
str = str[1:-1]

# Check for special sequences. Examples:
# \012 --> \n
# \" --> "
#
i = 0
n = len(str)
res = []
while 0 <= i < n:
o_match = _OctalPatt.search(str, i)
q_match = _QuotePatt.search(str, i)
if not o_match and not q_match: # Neither matched
res.append(str[i:])
break
# else:
j = k = -1
if o_match:
j = o_match.start(0)
if q_match:
k = q_match.start(0)
if q_match and (not o_match or k < j): # QuotePatt matched
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This appears to be removing all backslashes from the cookie, where the new behavior is only removing backslashes in front of quotes. Is that what we're expecting to change with this PR?

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From reading the RFC, I think we should preserve the original behavior of unquoting all single-character preceded by a backslash. If you agree, can we add a test case that ensures this behavior is preserved.

res.append(str[i:k])
res.append(str[k+1])
i = k + 2
else: # OctalPatt matched
res.append(str[i:j])
res.append(chr(int(str[j+1:j+4], 8)))
i = j + 4
return _nulljoin(res)
# Function to replace octal patterns
def replace_octal(match):
octal_value = match.group(1)
return chr(int(octal_value, 8))

# Replace octal escape sequences
str = _OctalPatt.sub(replace_octal, str)
# Replace escaped quotes
str = _QuotePatt.sub('"', str)

return str

# The _getdate() routine is used to set the expiration time in the cookie's HTTP
# header. By default, _getdate() returns the current time in the appropriate
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