libspf2 -- Integer Underflow Remote Code Execution

Trendmicro ZDI reports:

Integer Underflow Remote Code Execution Vulnerability
The specific flaw exists within the parsing of SPF macros.
When parsing SPF macros, the process does not properly
validate user-supplied data, which can result in an integer
underflow before writing to memory. An attacker can leverage
this vulnerability to execute code in the context of the
service account.