a996106eee
The RFCs are very clear that in DATA contents: > CR and LF MUST only occur together as CRLF; they MUST NOT appear > independently in the body. https://www.rfc-editor.org/rfc/rfc5322#section-2.3 https://www.rfc-editor.org/rfc/rfc5321#section-2.3.8 Allowing "independent" CR and LF can cause a number of problems. In particular, there is a new "SMTP smuggling attack" published recently that involves the server incorrectly parsing the end of DATA marker `\r\n.\r\n`, which an attacker can exploit to impersonate a server when email is transmitted server-to-server. https://www.postfix.org/smtp-smuggling.html https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ Currently, chasquid is vulnerable to this attack, because Go's standard libraries net/textproto and net/mail do not enforce CRLF strictly. This patch fixes the problem by introducing a new "dot reader" function that strictly enforces CRLF when reading dot-terminated data, used in the DATA input processing. When an invalid newline terminator is found, the connection is aborted immediately because we cannot safely recover from that state. We still keep the internal representation as LF-terminated for convenience and simplicity. However, the MDA courier is changed to pass CRLF-terminated lines, since that is an external program which could be strict when receiving email messages. See https://github.com/albertito/chasquid/issues/47 for more details and discussion.
29 lines
564 B
Plaintext
29 lines
564 B
Plaintext
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c tcp_connect localhost:1025
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c <~ 220
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c -> EHLO localhost
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c <... 250 HELP
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c -> MAIL FROM: <>
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c <~ 250
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c -> RCPT TO: user@testserver
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c <~ 250
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c -> DATA
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c <~ 354
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c -> Subject: Message too big
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c ->
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# Max message size is 1 MiB. Note this includes line endings but converted to
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# \n (as per textproto.DotReader), and excluding the final ".".
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# We already sent (in the header) 26.
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# Send lines of len 900 to stay under the limit.
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# (1024 * 1024 - 26) - (900 * 1166) = -850
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c ~> ('a' * 899 + '\r\n') * 1166
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c -> .
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c <~ 552 5.3.4 Message too big
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c -> QUIT
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c <~ 221
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