Pre-Auth MySQL remote DOS (Integer Overflow)（CVE-2017-3599）

MySQL server is affected by a remote DoS attack, which could be exploited by a remote unauthenticated attacker to cause a loss of availability on the targeted service. The issue has been verified to affect 5.6.X branch up to 5.6.35 and 5.7.X branch up to 5.7.17\. It is strongly recommended that MySQL servers are updated to the [latest version](https://dev.mysql.com/downloads/mysql/). Upon connection from a client, the server sends a greeting message and the client continues the communication by starting the authentication process. The authentication packet sent by the client contains a wealth of information including the client capabilities, username, password, etc. The packet is received by the server, and parsed by `parse_client_handshake_packet()` function, in /sql/auth/sql_authentication.cc. In particular, the following code retrieves the password from the packet: ``` passwd= get_length_encoded_string(&end, &bytes_remaining_in_packet, &passwd_len); ``` `_get_length_encoded_string_` in turn calls `_get_56_lenc_string_` function. The password field in the packet is defined by two values: the length of the password (1 byte), followed by the actual password field content. This length is calculated by calling the `net_field_length_ll()` function. ``` my_ulonglong net_field_length_ll(uchar **packet) { uchar *pos= *packet; if (*pos < 251) { (*packet)++; return (my_ulonglong) *pos; } if (*pos == 251) { (*packet)++; return (my_ulonglong) NULL_LENGTH; } if (*pos == 252) { (*packet)+=3; return (my_ulonglong) uint2korr(pos+1); } if (*pos == 253) { (*packet)+=4; return (my_ulonglong) uint3korr(pos+1); } (*packet)+=9; /* Must be 254 when here */ return (my_ulonglong) uint8korr(pos+1); } ``` As shown in the code above, if the buffer sent to the MySQL server points to a length of \xFF, the pointer to the packet would be increased by 9 bytes. If an attacker sends an authentication packet with a password length of \xFE or \xFF with less than 9 bytes following that value, the `net_field_length_ll` function will position the pointer to the packet outside the boundaries of the variable. The _get_56_lenc_string_ function will continue the execution calculating the number of bytes remaining in the packet. The instruction `max_bytes_available -= len_len` will provoke an integer overflow where the value of max_bytes_available will become a very large unsigned integer. The next time `get_string` function is called, it will make _memchr_ read out of boundaries and exit with a segmentation fault. A [Proof of Concept](https://github.com/SECFORCE/CVE-2017-3599) code for this vulnerability has been released. #### References: [Oracle Critical Patch Update](http://www.oracle.com/technetwork/security-advisory/cpuapr2017-3236618.html)