May 21, 2025 · May 21, 2025 · May 21, 2025
diff --git a/Lib/test/test_zipfile/test_core.py b/Lib/test/test_zipfile/test_core.py
 self.assertFalse(zipfile.is_zipfile(fp))
 fp.seek(0,0)
 self.assertFalse(zipfile.is_zipfile(fp))
 # - passing non-zipfile with ZIP header elements
 # data created using pyPNG like so:
 #  d = [(ord('P'), ord('K'), 5, 6), (ord('P'), ord('K'), 6, 6)]
 #  w = png.Writer(1,2,alpha=True,compression=0)
 #  f = open('onepix.png', 'wb')
 #  w.write(f, d)
 #  w.close()
 data= (b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00"
 b"\x00\x02\x08\x06\x00\x00\x00\x99\x81\xb6'\x00\x00\x00\x15I"
 b"DATx\x01\x01\n\x00\xf5\xff\x00PK\x05\x06\x00PK\x06\x06\x07"
 b"\xac\x01N\xc6|a\r\x00\x00\x00\x00IEND\xaeB`\x82")
 # - passing a filename
 withopen(TESTFN,"wb")asfp:
 fp.write(data)
 self.assertFalse(zipfile.is_zipfile(TESTFN))
 # - passing a file-like object
 fp=io.BytesIO()
 fp.write(data)
 self.assertFalse(zipfile.is_zipfile(fp))

 deftest_damaged_zipfile(self):
 """Check that zipfiles with missing bytes at the end raise BadZipFile."""
diff --git a/Lib/zipfile/__init__.py b/Lib/zipfile/__init__.py

 def _check_zipfile(fp):
    try:
        if _EndRecData(fp):
            return True         # file has correct magic number
        endrec = _EndRecData(fp)
        if endrec:
            if endrec[_ECD_ENTRIES_TOTAL] == 0 and endrec[_ECD_SIZE] == 0 and endrec[_ECD_OFFSET] == 0:
                return True     # Empty zipfiles are still zipfiles
            elif endrec[_ECD_DISK_NUMBER] == endrec[_ECD_DISK_START]:
                # Central directory is on the same disk
                fp.seek(sum(_handle_prepended_data(endrec)))
                if endrec[_ECD_SIZE] >= sizeCentralDir:
                    data = fp.read(sizeCentralDir)   # CD is where we expect it to be
                    if len(data) == sizeCentralDir:
                        centdir = struct.unpack(structCentralDir, data) # CD is the right size
                        if centdir[_CD_SIGNATURE] == stringCentralDir:
                            return True         # First central directory entry  has correct magic number
    except OSError:
        pass
    return False
        pass
    return result

 def _handle_prepended_data(endrec, debug=0):
    size_cd = endrec[_ECD_SIZE]             # bytes in central directory
    offset_cd = endrec[_ECD_OFFSET]         # offset of central directory

    # "concat" is zero, unless zip was concatenated to another file
    concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
    if endrec[_ECD_SIGNATURE] == stringEndArchive64:
        # If Zip64 extension structures are present, account for them
        concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)

    if debug > 2:
        inferred = concat + offset_cd
        print("given, inferred, offset", offset_cd, inferred, concat)

    return offset_cd, concat

 def _EndRecData64(fpin, offset, endrec):
    """
    Read the ZIP64 end-of-archive records and use that to update endrec
            raise BadZipFile("File is not a zip file")
        if self.debug > 1:
            print(endrec)
        size_cd = endrec[_ECD_SIZE]             # bytes in central directory
        offset_cd = endrec[_ECD_OFFSET]         # offset of central directory
        self._comment = endrec[_ECD_COMMENT]    # archive comment

        # "concat" is zero, unless zip was concatenated to another file
        concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
        if endrec[_ECD_SIGNATURE] == stringEndArchive64:
            # If Zip64 extension structures are present, account for them
            concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
        offset_cd, concat = _handle_prepended_data(endrec, self.debug)

        # self.start_dir:  Position of start of central directory
        self.start_dir = offset_cd + concat

        # store the offset to the beginning of data for the
        # .data_offset property
        self._data_offset = concat

        if self.debug > 2:
            inferred = concat + offset_cd
            print("given, inferred, offset", offset_cd, inferred, concat)
        # self.start_dir:  Position of start of central directory
        self.start_dir = offset_cd + concat
        if self.start_dir < 0:
            raise BadZipFile("Bad offset for central directory")
        fp.seek(self.start_dir, 0)
        size_cd = endrec[_ECD_SIZE]
        data = fp.read(size_cd)
        fp = io.BytesIO(data)
        total = 0
diff --git a/Misc/NEWS.d/next/Library/2017-12-30-18-21-00.bpo-28494.Dt_Wks.rst b/Misc/NEWS.d/next/Library/2017-12-30-18-21-00.bpo-28494.Dt_Wks.rst
 Improve Zip file validation false positive rate in :func:`zipfile.is_zipfile`.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -1991,6 +1991,25 @@ def test_is_zip_erroneous_file(self):
		self.assertFalse(zipfile.is_zipfile(fp))
		fp.seek(0,0)
		self.assertFalse(zipfile.is_zipfile(fp))
		# - passing non-zipfile with ZIP header elements
		# data created using pyPNG like so:
		# d = [(ord('P'), ord('K'), 5, 6), (ord('P'), ord('K'), 6, 6)]
		# w = png.Writer(1,2,alpha=True,compression=0)
		# f = open('onepix.png', 'wb')
		# w.write(f, d)
		# w.close()
		data= (b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00"
		b"\x00\x02\x08\x06\x00\x00\x00\x99\x81\xb6'\x00\x00\x00\x15I"
		b"DATx\x01\x01\n\x00\xf5\xff\x00PK\x05\x06\x00PK\x06\x06\x07"
		b"\xac\x01N\xc6\|a\r\x00\x00\x00\x00IEND\xaeB`\x82")
		# - passing a filename
		withopen(TESTFN,"wb")asfp:
		fp.write(data)
		self.assertFalse(zipfile.is_zipfile(TESTFN))
		# - passing a file-like object
		fp=io.BytesIO()
		fp.write(data)
		self.assertFalse(zipfile.is_zipfile(fp))

		deftest_damaged_zipfile(self):
		"""Check that zipfiles with missing bytes at the end raise BadZipFile."""
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -234,8 +234,19 @@ def strip(cls, data, xids):

		def _check_zipfile(fp):
		try:
		if _EndRecData(fp):
		return True # file has correct magic number
		endrec = _EndRecData(fp)
		if endrec:
		if endrec[_ECD_ENTRIES_TOTAL] == 0 and endrec[_ECD_SIZE] == 0 and endrec[_ECD_OFFSET] == 0:
		return True # Empty zipfiles are still zipfiles
		elif endrec[_ECD_DISK_NUMBER] == endrec[_ECD_DISK_START]:
		# Central directory is on the same disk
		fp.seek(sum(_handle_prepended_data(endrec)))
		if endrec[_ECD_SIZE] >= sizeCentralDir:
		data = fp.read(sizeCentralDir) # CD is where we expect it to be
		if len(data) == sizeCentralDir:
		centdir = struct.unpack(structCentralDir, data) # CD is the right size
		if centdir[_CD_SIGNATURE] == stringCentralDir:
		return True # First central directory entry has correct magic number
		except OSError:
		pass
		return False
Expand All		@@ -258,6 +269,22 @@ def is_zipfile(filename):
		pass
		return result

		def _handle_prepended_data(endrec, debug=0):
		size_cd = endrec[_ECD_SIZE] # bytes in central directory
		offset_cd = endrec[_ECD_OFFSET] # offset of central directory

		# "concat" is zero, unless zip was concatenated to another file
		concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
		if endrec[_ECD_SIGNATURE] == stringEndArchive64:
		# If Zip64 extension structures are present, account for them
		concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)

		if debug > 2:
		inferred = concat + offset_cd
		print("given, inferred, offset", offset_cd, inferred, concat)

		return offset_cd, concat

		def _EndRecData64(fpin, offset, endrec):
		"""
		Read the ZIP64 end-of-archive records and use that to update endrec
Expand DownExpand Up		@@ -1501,28 +1528,21 @@ def _RealGetContents(self):
		raise BadZipFile("File is not a zip file")
		if self.debug > 1:
		print(endrec)
		size_cd = endrec[_ECD_SIZE] # bytes in central directory
		offset_cd = endrec[_ECD_OFFSET] # offset of central directory
		self._comment = endrec[_ECD_COMMENT] # archive comment

		# "concat" is zero, unless zip was concatenated to another file
		concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
		if endrec[_ECD_SIGNATURE] == stringEndArchive64:
		# If Zip64 extension structures are present, account for them
		concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
		offset_cd, concat = _handle_prepended_data(endrec, self.debug)

		# self.start_dir: Position of start of central directory
		self.start_dir = offset_cd + concat

		# store the offset to the beginning of data for the
		# .data_offset property
		self._data_offset = concat

		if self.debug > 2:
		inferred = concat + offset_cd
		print("given, inferred, offset", offset_cd, inferred, concat)
		# self.start_dir: Position of start of central directory
		self.start_dir = offset_cd + concat
		if self.start_dir < 0:
		raise BadZipFile("Bad offset for central directory")
		fp.seek(self.start_dir, 0)
		size_cd = endrec[_ECD_SIZE]
		data = fp.read(size_cd)
		fp = io.BytesIO(data)
		total = 0
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Improve Zip file validation false positive rate in :func:`zipfile.is_zipfile`.