struct.error: unpack requires a buffer of 4 bytes

[piskvorky]

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/msoffcrypto/format/xls97.py", line 644, in is_encrypted
    if not workbook.has_record(recordNameNum["FilePass"]):
  File "/usr/local/lib/python3.10/dist-packages/msoffcrypto/format/xls97.py", line 418, in has_record
    num, size = unpack("<HH", h)
struct.error: unpack requires a buffer of 4 bytes

Unfortunately I cannot share the file that triggers this exception.

But looking at xls97.py line 418, the code assumes that self.data.read(4) returns either exactly 4 bytes or nothing:

msoffcrypto-tool/msoffcrypto/format/xls97.py

Lines 411 to 423 in f727b42

	def has_record(self, target):
	pos = self.data.tell()
	while True:
	h = self.data.read(4)
	if not h:
	self.data.seek(pos)
	return False
	num, size = unpack("<HH", h)
	if num == target:
	self.data.seek(pos)
	return True
	else:
	self.data.read(size)

I'm not sure what self.data there is but normally read(n) is guaranteed to return at most 4 bytes, not exactly 4 bytes.

[nolze]

Thank you for reporting! This bug doesn't seem easy, and I can't think of a way to debug it without a sample. However, if I figure something out in the future, I will make fixes. Any information is also welcome.

(A quick possible fix would be changing if not h: to if not h or len(h) < 4:, but I'm not sure if it is consistent with the spec.)

[piskvorky]

Thank you.

Yes, if len(h) < 4: might be a solution. Depending on what happens after that return False.