CVE-2021-33913

libspf2 before 1.2.11 has a heap-based buffer overflow that might allow
remote attackers to execute arbitrary code (via an unauthenticated e-mail
message from anywhere on the Internet) with a crafted SPF DNS record,
because of SPF_record_expand_data in spf_expand.c. The amount of overflowed
data depends on the relationship between the length of an entire domain
name and the length of its leftmost label. The vulnerable code may be part
of the supply chain of a site’s e-mail infrastructure (e.g., with
additional configuration, Exim can use libspf2; the Postfix web site links
to unofficial patches for use of libspf2 with Postfix; older versions of
spfquery relied on libspf2) but most often is not.