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An attacker may use an infinite number of SPF referrals in his/her SPF
setting and can send an email to your mail server which would make
your SMTP server make a lot of DNS queries. By exploiting this
vulnerability, an attacker can block your SMTP queue, flood the
associated recursive resolver, or any DNS authoritative server.
According to RFC recommendations
(https://datatracker.ietf.org/doc/html/rfc7208#section-4.6), a few DNS
lookup limits exist that an SMTP server needs to maintain while
resolving an SPF record. That is, SPF implementations MUST limit the
total number of query-causing terms to 10 and the number of void
lookups to 2 to avoid unreasonable load on the DNS.
from:
Taejoong “Tijay” Chung (tijay@vt.edu)
Ishtiaq Ashiq (iashiq5@vt.edu)
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