ares_parse: handle ADMD-less Authentication-Results headers (closes #73)#372
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Office 365 generates Authentication-Results headers that omit the authserv-id (ADMD), jumping straight to method=result tokens. This is non-compliant with RFC 8601 but common enough in practice that the hard parse failure causes log noise and, downstream, can cause OpenDMARC to report authentication failures for otherwise valid mail. When state 1 encounters '=' instead of ';' or a version digit, the accumulated host token is actually a method name. Recover by leaving ares_host empty and continuing from the result value. ares_host will be empty, signalling no authserv-id was present. Closes trusteddomainproject#73
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Summary
Office 365 generates
Authentication-Resultsheaders that omit the authserv-id (ADMD), jumping straight tomethod=resulttokens:RFC 8601 requires the authserv-id before the semicolon, so these are technically non-compliant. However they are common enough in practice that the current hard parse failure has real consequences: log noise on every O365-originated message, and downstream propagation to OpenDMARC which can interpret the unparsed header as an authentication failure.
Fix
In state 1 of
ares_parse(), when=is seen instead of;or a version digit, the token we read as authserv-id is actually the first method name. The fix recovers gracefully:ares_hostis left empty (signalling no authserv-id present), the token is slotted as the method, and parsing continues from the result value.The same fix has been applied to OpenDMARC (see trusteddomainproject/OpenDMARC#329) and should be applied to OpenARC as well.
Test plan
Authentication-Results: spf=pass smtp.mailfrom=example.comparses without error,ares_hostis empty, method isspf, result ispassAuthentication-Results: compauth=pass reason=000parses without error, method isunknownmake checkpasses