[3.15] gh-157190: Follow symlinks when extracting tarfile hard links (GH-157191) (GH-157262)

(cherry picked from commit b38be2e)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Lib/tarfile.py

@@ -2823,7 +2823,11 @@ def makelink_with_filter(self, tarinfo, targetpath,
                     if os.path.lexists(targetpath):
                         # Avoid FileExistsError on following os.link.
                         os.unlink(targetpath)
-                    os.link(tarinfo._link_target, targetpath)
+                    # Resolve the target so the hard link points to the file
+                    # itself. Otherwise os.link() may duplicate a symlink to a
+                    # shallower location, where its relative target escapes the
+                    # destination directory. (CVE-2026-82049)
+                    os.link(os.path.realpath(tarinfo._link_target), targetpath)
                     return
         except symlink_exception:
             keyerror_to_extracterror = True

Lib/test/test_tarfile.py

@@ -4547,6 +4547,24 @@ def test_sneaky_hardlink_fallback_deep(self):
                     self.expect_file("a/b/s", symlink_to=os.path.join('..', 'escape'))
                     self.expect_file("s", symlink_to=os.path.join('..', 'escape'))
 
+    @symlink_test
+    @os_helper.skip_unless_hardlink
+    def test_sneaky_hardlink_relocation(self):
+        with ArchiveMaker() as arc:
+            arc.add("a/escape", content="decoy")
+            arc.add("a/b/s", symlink_to=os.path.join("..", "escape"))
+            arc.add("s", hardlink_to=os.path.join("a", "b", "s"))
+
+        for filter in 'data', 'tar':
+            with self.subTest(filter), self.check_context(arc.open(), filter):
+                self.expect_file("a/escape", content="decoy")
+                if os_helper.can_symlink():
+                    self.expect_file("a/b/s", symlink_to=os.path.join('..', 'escape'))
+                else:
+                    self.expect_file("a/b/s", content="decoy")
+                self.expect_file("s", content="decoy")
+                self.assertFalse((self.destdir / "s").is_symlink())
+
     @symlink_test
     def test_exfiltration_via_symlink(self):
         # (CVE-2025-4138)