An integer overflow at an unserialize_uep memory allocation site would occur for vim before patch 8.0.0378, if it does not properly validate values for tree length when reading a corrupted undo file, which may lead to resultant buffer overflows.
https://usn.ubuntu.com/4309-1/
https://security.gentoo.org/glsa/201706-26
https://groups.google.com/forum/#%21topic/vim_dev/QPZc0CY9j3Y
https://groups.google.com/forum/#%21topic/vim_dev/L_dOHOOiQ5Q
https://github.com/vim/vim/commit/0c8485f0e4931463c0f7986e1ea84a7d79f10c75