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Commit87d3daa

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bpo-32997: Fix REDOS in fpformat
The regex to decode a number in fpformat is susceptible tocatastrophic backtracking.This is a potential DOS vector if a server is using fpformat onuntrusted number strings.Replace it with an equivalent non-vulnerable regex.The match behavior of the new regex is slightly different.This difference is addressed with a follow-up check.
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‎Lib/fpformat.py‎

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@@ -19,7 +19,7 @@
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__all__= ["fix","sci","NotANumber"]
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# Compiled regular expression to "decode" a number
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decoder=re.compile(r'^([-+]?)0*(\d*)((?:\.\d*)?)(([eE][-+]?\d+)?)$')
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decoder=re.compile(r'^([-+]?)0*([1-9]\d*)?((?:\.\d*)?)(([eE][-+]?\d+)?)$')
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# \0 the whole thing
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# \1 leading sign or empty
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# \2 digits left of decimal point
@@ -41,6 +41,8 @@ def extract(s):
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res=decoder.match(s)
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ifresisNone:raiseNotANumber,s
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sign,intpart,fraction,exppart=res.group(1,2,3,4)
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ifintpartisNone:# ([1-9]\d*)? may match nothing
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intpart=''
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ifsign=='+':sign=''
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iffraction:fraction=fraction[1:]
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ifexppart:expo=int(exppart[1:])

‎Lib/test/test_fpformat.py‎

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@@ -67,6 +67,16 @@ def test_failing_values(self):
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else:
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self.fail("No exception on non-numeric sci")
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deftest_REDOS(self):
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# This attack string will hang on the old decoder pattern.
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attack='+0'+ ('0'*1000000)+'++'
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digs=5# irrelevant
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# fix returns input if it does not decode
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self.assertEqual(fpformat.fix(attack,digs),attack)
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# sci raises NotANumber
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withself.assertRaises(NotANumber):
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fpformat.sci(attack,digs)
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deftest_main():
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run_unittest(FpformatTest)
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A regex in fpformat was vulnerable to catastrophic backtracking. This regex
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was a potential DOS vector (REDOS). Based on typical uses of fpformat the
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risk seems low. The regex has been refactored and is now safe. Patch by
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Jamie Davis.

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