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SpamAssassin / spamc+BSMTP remote buffer overflow

Well, I was going to wait until 2.50 release, but it seems to be taking and
this likely affects only few installations. Besides, it's been in their
public bugzilla for over a month. So:

Attacker may be able to execute arbitrary code by sending a specially
crafted e-mail to a system using SpamAssassin's spamc program in BSMTP mode
(-B option). Versions from 2.40 to 2.43 are affected.

Exim users especially should check if they're affected, the -B option is
used in several Exim+SpamAssassin HOWTOs.

The problem is with escaping '.' characters at the beginning of lines.
Off-by-one bounds checking error allows writing '.' character past a
buffer, overwriting the stack frame address. Depending on system this may
be exploitable. Pre-built Debian unstable/x86 package wasn't vulnerable, my
self compiled was.


diff -ru spamassassin-2.43-old/spamd/libspamc.c spamassassin-2.43/spamd/libspamc.c
--- spamassassin-2.43-old/spamd/libspamc.c	2002-10-15 18:22:49.000000000 +0300
+++ spamassassin-2.43/spamd/libspamc.c	2002-12-27 20:19:36.000000000 +0200
@@ -309,7 +309,7 @@
       case MESSAGE_BSMTP:
         total=full_write(fd, m->pre, m->pre_len);
         for(i=0; i<m->out_len; ){
-            for(j=0; i<m->out_len && j<sizeof(buffer)/sizeof(*buffer)-1; ){
+            for(j=0; i<m->out_len && j<sizeof(buffer)/sizeof(*buffer)-2; ){
                 if(i+1<m->out_len && m->out[i]=='\n' && m->out[i+1]=='.'){