Let's specify in main.cf:

header_checks = pcre:/etc/postfix/header_checks.pcre

In /etc/postfix/header_checks.pcre add:

/^Subject: (.*?)RandomStringToKeepEverythingFunctioning(.*?)$/ REJECT

Then send an email where the subject contains "RandomStringToKeepEverythingFunctioningMoreText", it will get rejected (correctly)

Now let's send another email where the subject contains "RandomStringToKeepEverythingFunctioning`MoreText", will pass the filter, the left single quotation mark (U+2018) will make the email bypass all regex/pcre filters.

With regards,

Michiel Hazelhof.

Attachment: OpenPGP_0xEECB9B96355B5EBF.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature.asc
Description: OpenPGP digital signature

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