This vulnerability allows network-adjacent attackers to execute arbitrary code
on affected installations of Exim libspf2. Authentication is not required to
exploit this 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.

06/06/22 – ZDI requested a PSIRT contact.

06/14/22 – ZDI reported the vulnerability to the vendor.

04/25/23 – ZDI asked for an update.

04/25/23 – The vendor asked us to re-send the reports.

05/10/23 – ZDI sent the vulnerability to the vendor.

09/25/23 – ZDI asked for an update and informed the vendor that we intend to
publish the case as a zero-day advisory on 09/27/23.

-- Mitigation: Given the nature of the vulnerability, the only salient
mitigation strategy is to restrict interaction with the application.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2023-42118