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bpo-45507: Throw EOF error when gzip headers or trailers are truncated #29023

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bpo-45507: Throw EOF error when gzip headers or trailers are truncated #29023

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83e58a2
Throw EOF error when trailer is not present in gzip member
rhpvorderman Oct 18, 2021
773c4f1
Fix bug where missing NAME or COMMENt fields do not trigger an error
rhpvorderman Oct 18, 2021
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Throw EOF error when trailer is not present in gzip member 
This is to keep error compatibility with 3.10 and lower.
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@rhpvorderman
rhpvorderman committed Oct 18, 2021
commit 83e58a2239878d7f3c802ba7ab5f2f78727bf90a
3 changes: 3 additions & 0 deletions Lib/gzip.py
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Expand Up @@ -607,6 +607,9 @@ def decompress(data):
do = zlib.decompressobj(wbits=-zlib.MAX_WBITS)
# Read all the data except the header
decompressed = do.decompress(data[fp.tell():])
if not do.eof or len(do.unused_data) < 8:
raise EOFError("Compressed file ended before the end-of-stream "
"marker was reached")
crc, length = struct.unpack("<II", do.unused_data[:8])
if crc != zlib.crc32(decompressed):
raise BadGzipFile("CRC check failed")
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13 changes: 13 additions & 0 deletions Lib/test/test_gzip.py
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Expand Up @@ -562,6 +562,19 @@ def test_decompress(self):
datac = gzip.compress(data)
self.assertEqual(gzip.decompress(datac), data)

def test_decompress_uncompressed_header(self):
truncated_headers = [
b"\x1f\x8b\x08\x00\x00\x00\x00\x00\x00", # Missing OS byte
b"\x1f\x8b\x08\x02\x00\x00\x00\x00\x00\xff", # FHRC, but no checksum
b"\x1f\x8b\x08\x04\x00\x00\x00\x00\x00\xff", # FEXTRA, but no xlen
b"\x1f\x8b\x08\x04\x00\x00\x00\x00\x00\xff\xaa\x00", # FEXTRA, xlen, but no data
b"\x1f\x8b\x08\x08\x00\x00\x00\x00\x00\xff", # FNAME but no fname
b"\x1f\x8b\x08\x10\x00\x00\x00\x00\x00\xff", # FCOMMENT, but no fcomment
]
for header in truncated_headers:
with self.subTest(header=header):
self.assertRaises(EOFError, gzip.decompress, header)

def test_read_truncated(self):
data = data1*50
# Drop the CRC (4 bytes) and file size (4 bytes).
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2021-10-18-11-20-24.bpo-45507.vWx2yS.rst
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@@ -0,0 +1,2 @@
Add regression tests for errors that are thrown when decompressing with the
``gzip`` module to ensure backwards-compatibility between Python versions.