[3.15] gh-156002: Bound zipfile decompression for bzip2/LZMA/Zstandard (

GH-156003) (GH-156362) Patch by @tonghuaroot. zipfile.ZipExtFile._read1() bounds the output of each decompress() call for DEFLATE members by passing a max_length to zlib, but for bzip2, LZMA, and Zstandard members it called decompress() with no bound. A whole compressed chunk was therefore expanded into a single allocation before the data[:self._left] clip ran, so a consumer that deliberately reads in small chunks to limit memory (for example zf.open(name).read(8192)) was silently unprotected for non-DEFLATE members. A small, spec-conformant archive member declaring a large uncompressed size could drive multi-GB peak memory. _read1() now passes a per-call bound to the non-DEFLATE decompress() (mirroring the DEFLATE branch) and drains the decompressor's internal buffer across calls by checking needs_input before reading more compressed input. zipfile's LZMADecompressor wrapper forwards max_length and exposes needs_input so the bound also holds for LZMA members. (cherry picked from commit f897dbf) (cherry picked from commit 1b424c0) Co-authored-by: Petr Viktorin <encukou@gmail.com> Co-authored-by: tonghuaroot <tonghuaroot@gmail.com>
Yhg1s · Sep 30, 2026 · Aug 31, 2026 · Sep 3, 2026 · Sep 8, 2026 · Sep 16, 2026
commit 6f598fd50f4db1e1d5e0bebd2b6a64161ae2b28b
diff --git a/Lib/test/test_zipfile/test_core.py b/Lib/test/test_zipfile/test_core.py
@@ -2590,6 +2590,48 @@ def tearDown(self):
         unlink(TESTFN2)
 
 
+class AbstractBoundedDecompressTests:
+    # ZipExtFile._read1() bounds the output of each decompress() call so that a
+    # small member declaring a large uncompressed size cannot expand into one
+    # unbounded read.
+    def test_read1_output_is_bounded(self):
+        buf = io.BytesIO()
+        with zipfile.ZipFile(buf, "w", compression=self.compression) as zf:
+            zf.writestr("big", b"\0" * (4 * 1024 * 1024))
+        with zipfile.ZipFile(io.BytesIO(buf.getvalue())) as zf:
+            with zf.open("big") as f:
+                self.assertLessEqual(len(f._read1(100)), f.MIN_READ_SIZE)
+
+
+class StoredBoundedDecompressTests(AbstractBoundedDecompressTests,
+                                   unittest.TestCase):
+    compression = zipfile.ZIP_STORED
+
+
+@requires_zlib()
+class DeflateBoundedDecompressTests(AbstractBoundedDecompressTests,
+                                    unittest.TestCase):
+    compression = zipfile.ZIP_DEFLATED
+
+
+@requires_bz2()
+class Bzip2BoundedDecompressTests(AbstractBoundedDecompressTests,
+                                  unittest.TestCase):
+    compression = zipfile.ZIP_BZIP2
+
+
+@requires_lzma()
+class LzmaBoundedDecompressTests(AbstractBoundedDecompressTests,
+                                 unittest.TestCase):
+    compression = zipfile.ZIP_LZMA
+
+
+@requires_zstd()
+class ZstdBoundedDecompressTests(AbstractBoundedDecompressTests,
+                                 unittest.TestCase):
+    compression = zipfile.ZIP_ZSTANDARD
+
+
 class AbstractBadCrcTests:
     def test_testzip_with_bad_crc(self):
         """Tests that files with bad CRCs return their name from testzip."""

diff --git a/Lib/zipfile/__init__.py b/Lib/zipfile/__init__.py
@@ -698,7 +698,16 @@ def __init__(self):
         self._unconsumed = b''
         self.eof = False
 
-    def decompress(self, data):
+    @property
+    def _needs_input(self):
+        # While the LZMA properties header is still being buffered, more input
+        # is required; afterwards defer to the wrapped decompressor so a bounded
+        # decompress() call can be drained across reads.
+        if self._decomp is None:
+            return True
+        return self._decomp.needs_input
+
+    def decompress(self, data, max_length=-1):
         if self._decomp is None:
             self._unconsumed += data
             if len(self._unconsumed) <= 4:
@@ -714,7 +723,7 @@ def decompress(self, data):
             data = self._unconsumed[4 + psize:]
             del self._unconsumed
 
-        result = self._decomp.decompress(data)
+        result = self._decomp.decompress(data, max_length)
         self.eof = self._decomp.eof
         return result
 
@@ -774,6 +783,13 @@ def _get_compressor(compress_type, compresslevel=None):
         return None
 
 
+def _decompressor_needs_input(decompressor):
+    # bz2/zstd expose the stdlib decompressor's public needs_input; the LZMA
+    # wrapper keeps it private (_needs_input) to avoid adding public API.
+    needs_input = getattr(decompressor, "needs_input", None)
+    return decompressor._needs_input if needs_input is None else needs_input
+
+
 def _get_decompressor(compress_type):
     _check_compression(compress_type)
     if compress_type == ZIP_STORED:
@@ -1074,8 +1090,15 @@ def _read1(self, n):
             data = self._decompressor.unconsumed_tail
             if n > len(data):
                 data += self._read2(n - len(data))
-        else:
+        elif self._compress_type == ZIP_STORED:
             data = self._read2(n)
+        else:
+            # bzip2/lzma/zstd: a bounded decompress() call may leave input
+            # buffered inside the decompressor; drain that before reading more.
+            if _decompressor_needs_input(self._decompressor):
+                data = self._read2(n)
+            else:
+                data = b''
 
         if self._compress_type == ZIP_STORED:
             self._eof = self._compress_left <= 0
@@ -1088,8 +1111,13 @@ def _read1(self, n):
             if self._eof:
                 data += self._decompressor.flush()
         else:
-            data = self._decompressor.decompress(data)
-            self._eof = self._decompressor.eof or self._compress_left <= 0
+            # Bound the output of a single decompress() call (mirroring the
+            # DEFLATE path above) so that a small compressed member cannot
+            # expand into one unbounded read.
+            data = self._decompressor.decompress(data, max(n, self.MIN_READ_SIZE))
+            self._eof = (self._decompressor.eof or
+                         self._compress_left <= 0 and
+                         _decompressor_needs_input(self._decompressor))
 
         data = data[:self._left]
         self._left -= len(data)

diff --git a/Misc/NEWS.d/next/Security/2026-08-18-13-54-05.gh-issue-156002.CcWXPP.rst b/Misc/NEWS.d/next/Security/2026-08-18-13-54-05.gh-issue-156002.CcWXPP.rst
@@ -0,0 +1,4 @@
+Bound the amount of data :mod:`zipfile` decompresses per read for members
+compressed with bzip2, LZMA, or Zstandard, matching the existing limit for
+deflate. A small archive member could previously expand into an unbounded
+allocation even when read in small chunks.