From ba8d544c7f1a3340dd967fd1a35c237fc1927daa Mon Sep 17 00:00:00 2001
From: Rob Swindell <rob@synchro.net>
Date: Wed, 4 Nov 2020 19:47:56 -0800
Subject: [PATCH] Detect and reject forged "from" fields in submitted msg
headers
For non-authenticated SMTP clients, if the "From:" header field contains a "name" which appears to be an email address (i.e. it has an '@' in it), and that address does not match the *actual* address in the "From:" field, reject the mail with an error message about the forged/mismatched address.
---
src/sbbs3/mailsrvr.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/src/sbbs3/mailsrvr.c b/src/sbbs3/mailsrvr.c
index 5d328a4488..1ec0ed29eb 100644
--- a/src/sbbs3/mailsrvr.c
+++ b/src/sbbs3/mailsrvr.c
@@ -3659,6 +3659,15 @@ static void smtp_thread(void* arg)
subnum=INVALID_SUB;
continue;
}
+ if(relay_user.number == 0
+ && strchr(sender, '@') != NULL
+ && compare_addrs(sender, sender_addr) != 0) {
+ lprintf(LOG_WARNING,"%04d %s %s !FORGED mail header 'FROM' field (%lu total)"
+ ,socket, client.protocol, client_id, ++stats.msgs_refused);
+ sockprintf(socket,client.protocol,session, "554 Mail header contains mismatched 'FROM' field");
+ subnum=INVALID_SUB;
+ continue;
+ }
char sender_info[512];
if(relay_user.number) {
SAFEPRINTF(str,"%u",relay_user.number);
--
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