miniupnp before 4c90b87, as used in Bitcoin Core before 0.12 and other products, lacks checks for snprintf return values, leading to a buffer overflow and significant data leak, a different vulnerability than CVE-2019-12107. In Bitcoin Core before 0.12, remote code execution was possible in conjunction with CVE-2015-6031 exploitation.
The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.