tar: add a note about -C and symlink-in-tarball attack

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

archival/tar.c

@@ -23,6 +23,25 @@
  * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  */
 
+/* TODO: security with -C DESTDIR option can be enhanced.
+ * Consider tar file created via:
+ * $ tar cvf bug.tar anything.txt
+ * $ ln -s /tmp symlink
+ * $ tar --append -f bug.tar symlink
+ * $ rm symlink
+ * $ mkdir symlink
+ * $ tar --append -f bug.tar symlink/evil.py
+ *
+ * This will result in an archive which contains:
+ * $ tar --list -f bug.tar
+ * anything.txt
+ * symlink
+ * symlink/evil.py
+ *
+ * Untarring it puts evil.py in '/tmp' even if the -C DESTDIR is given.
+ * This doesn't feel right, and IIRC GNU tar doesn't do that.
+ */
+
 #include <fnmatch.h>
 #include "libbb.h"
 #include "archive.h"