This concerns a remote buffer overflow vulnerability in OpenVPN. It has been fixed in OpenVPN 2.4.4 and 2.3.18. It is suspected that only a small number of users is vulnerable to this issue, because it requires having explicitly enabled the outdated ‘key method 1’.
The advisory can be found here: https://community.openvpn.net/openvpn/wiki/CVE-2017-12166
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In ssl.c, key_method_1_read() calls read_key() which doesn’t perform adequate
bounds checks. cipher_length and hmac_length are specified by the
peer:
1643 uint8_t cipher_length;
1644 uint8_t hmac_length;
1645
1646 CLEAR(*key);
1647 if (!buf_read(buf, &cipher_length, 1))
1648 {
1649 goto read_err;
1650 }
1651 if (!buf_read(buf, &hmac_length, 1))
1652 {
1653 goto read_err;
1654 }
And this many bytes of data are then read into key->cipher and key->hmac:
1656 if (!buf_read(buf, key->cipher, cipher_length))
1657 {
1658 goto read_err;
1659 }
1660 if (!buf_read(buf, key->hmac, hmac_length))
1661 {
1662 goto read_err;
1663 }
In other words, it’s a classic example of lack of a bounds check resulting in a buffer overflow.
Guido Vranken 
hi，Guido，when I reproduce this vulnerability, I found that, while the read_key() has this potential stack overflow, however, the cipher_length and hmac_length is read from buf which is received from the peer who use write_key. I found in the write_key, there is a ASSERT(kt->cipher_lengthhmac_length<=MAX_YYY), this means it will not cause the receiver's cipher_length more than MAX_XXX and MAX_YYY, is it right???
Hope your response, thank you very much!
Hi,
Yes, read_key() on the server reads data that passed to write_key() on the client. In order to trigger the bug, you need to modify the client OpenVPN code to send a wrong key size.