diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 62d22150f50da5..2ea47978ff6f1c 100755
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1231,6 +1231,8 @@ def _proc_pax(self, tarfile):
length, keyword = match.groups()
length = int(length)
+ if length == 0:
+ raise InvalidHeaderError("invalid header")
value = buf[match.end(2) + 1:match.start(1) + length - 1]
# Normally, we could just use "utf-8" as the encoding and "strict"
diff --git a/Lib/test/recursion.tar b/Lib/test/recursion.tar
new file mode 100644
index 00000000000000..b8237251964983
Binary files /dev/null and b/Lib/test/recursion.tar differ
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 4cd7d5370f58d6..573be812eaa179 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -395,6 +395,13 @@ def test_premature_end_of_archive(self):
with self.assertRaisesRegex(tarfile.ReadError, "unexpected end of data"):
tar.extractfile(t).read()
+ def test_length_zero_header(self):
+ # bpo-39017 (CVE-2019-20907): reading a zero-length header should fail
+ # with an exception
+ with self.assertRaisesRegex(tarfile.ReadError, "file could not be opened successfully"):
+ with tarfile.open(support.findfile('recursion.tar')) as tar:
+ pass
+
class MiscReadTestBase(CommonReadTest):
def requires_name_attribute(self):
pass
diff --git a/Misc/NEWS.d/next/Library/2020-07-12-22-16-58.bpo-39017.x3Cg-9.rst b/Misc/NEWS.d/next/Library/2020-07-12-22-16-58.bpo-39017.x3Cg-9.rst
new file mode 100644
index 00000000000000..ad26676f8b8563
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-07-12-22-16-58.bpo-39017.x3Cg-9.rst
@@ -0,0 +1 @@
+Avoid infinite loop when reading specially crafted TAR files using the tarfile module (CVE-2019-20907).
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