From 70074d5b0d3fd9efeed57381044024f295167765 Mon Sep 17 00:00:00 2001
From: Rishi <rishi_devan@mail.com>
Date: Wed, 15 Jul 2020 13:51:00 +0200
Subject: [PATCH] bpo-39017: Avoid infinite loop in the tarfile module

 (GH-21454)

Avoid infinite loop when reading specially crafted TAR files using the tarfile module
(CVE-2019-20907).

---
 Lib/tarfile.py           | 2 ++
 Lib/test/test_tarfile.py | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index adf91d5..574a6bb 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1400,6 +1400,8 @@ class TarInfo(object):
 
             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]
 
             keyword = keyword.decode("utf8")
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 89bd738..c61d02b 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -321,6 +321,13 @@ class CommonReadTest(ReadTest):
                 with self.assertRaisesRegexp(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 MiscReadTest(CommonReadTest):
     taropen = tarfile.TarFile.taropen