Exim malware scanner issue after upgrade from Jessie to Stretch

Today I finally upgraded by personal root server from Debian Jessie to Stretch, thereby upgrading Exim from 4.84 to 4.89.

After the upgrade, I observed the following errors in mainlog:

2018-05-31 08:02:03 +0000 1fOIX5-0001rg-AM malware acl condition: cmdline  : scanner returned error code: 36096
2018-05-31 08:02:03 +0000 1fOIX5-0001rg-AM H=([IPv6:2a00:6020:1efc:ee20:8857:7824:6a49:8368]) [2a00:6020:1efc:ee20:8857:7824:6a49:8368]:48523 I=[2a01:4f8:141:429::2]:465 Warning: ACL "warn" statement skipped: condition test deferred
2018-05-31 08:02:04 +0000 1fOIX5-0001rg-AM malware acl condition: cmdline  : scanner returned error code: 13
2018-05-31 08:02:04 +0000 1fOIX5-0001rg-AM H=([IPv6:2a00:6020:1efc:ee20:8857:7824:6a49:8368]) [2a00:6020:1efc:ee20:8857:7824:6a49:8368]:48523 I=[2a01:4f8:141:429::2]:465 Warning: ACL "warn" statement skipped: condition test deferred
2018-05-31 08:02:05 +0000 1fOIX5-0001rg-AM malware acl condition: cmdline  : scanner returned error code: 13
2018-05-31 08:02:05 +0000 1fOIX5-0001rg-AM H=([IPv6:2a00:6020:1efc:ee20:8857:7824:6a49:8368]) [2a00:6020:1efc:ee20:8857:7824:6a49:8368]:48523 I=[2a01:4f8:141:429::2]:465 Warning: ACL "warn" statement skipped: condition test deferred

Each of the three cmdline scanners caused an error, as shown above.

It seems there was a change in Exim from upstream, as reported by another user. Somehow it seems that if you define a cmdline scanner that uses a chain of commands, when there was an error return code encountered in the middle of the chain, the whole chain is considered failed.

To “fix” this issue (or rather work-around it), I changed the three ACL clauses as follows:

   warn  message                = This message contains malware ($malware_name)
         set acl_m0      = cmdline:\
-                               /usr/lib/AntiVir/guard/avscan -s --batch --scan-mode=all %s; /bin/echo -e \N"\navira_retval $?"\N:\
+                               /usr/local/bin/avscan_wrapper %s:\
                                \N^avira_retval 1$\N:\
                                \N^.*ALERT::[ \t]+([^;]*)[ \t]+;.*$\N
         malware                = *

I created a “wrapper” that effectively hides error return codes, and forces a return code of 0. The above wrapper looks like this:

#!/bin/bash

ARG="$1"

/usr/lib/AntiVir/guard/avscan -s --batch --scan-mode=all "${ARG}"
/bin/echo -e "\navira_retval $?"

exit 0

To make sure I didn’t break the malware scanning by my changes, I downloaded the EICAR test virus and sent it to myself. Exim caught the “virus” and ditched it.

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