Logcheck devel - Nov 2017 - Bug#860052: [PATCH] Let mime-construct pick an encoding (closes: #860052)

Package: logcheck
Version: 1.3.18
Severity: important
Tags: security

[Note: I've tagged this with security because of the DoS potential,
where admins relying on logcheck can have their logs "lost" if someone
generates a long log message. Your choice whether you think it's
legitimate or not, but I am obligated to point it out.]

If a log line is generated that is >998 characters, logcheck will
generate an illegal (MUST in SMTP RFC is violated) e-mail message that
exim (and possibly other mailers) will choke on.

This was caused because we force mime-encode to use an encoding of 7bit
on the resultant logfiles.

That's fundamentally a mistake, as mime-encode is smart enough to
recognize that the encoding needs to be changed in cases where lines
have non-ascii characters or are too long, and will re-encode as
quoted-printable.

I can imagine, in days past, that this was a deliberate choice because
so many sysadmin types were using non-mime-compliant MUAs and wanted to
be able to simply cut and paste out of /bin/mailx output, but we don't
live in that universe and haven't for a couple of decades.

Dropping all log messages because of one overly-long-line is highly
problematic.

Fix:

Any place in `/usr/sbin/logcheck` where there is --enconding "7bit"
can simply be removed.  Let mime-encode do its job, it knows better than
logcheck what to do with the input data.

-- System Information:
Debian Release: 9.0
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.32-042stab120.16 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages logcheck depends on:
ii  adduser                                    3.115
ii  cron [cron-daemon]                         3.0pl1-128+b1
ii  exim4-daemon-light [mail-transport-agent]  4.88-5
ii  lockfile-progs                             0.1.17+b1
ii  logtail                                    1.3.18
ii  mime-construct                             1.11+nmu2
ii  rsyslog [system-log-daemon]                8.24.0-1

Versions of packages logcheck recommends:
ii  logcheck-database  1.3.18

Versions of packages logcheck suggests:
pn  syslog-summary  <none>

-- Configuration Files:
/etc/logcheck/logcheck.conf [Errno 13] Permission denied:
'/etc/logcheck/logcheck.conf'
/etc/logcheck/logcheck.logfiles [Errno 13] Permission denied:
'/etc/logcheck/logcheck.logfiles'

-- no debconf information

tags 860052 + patch
thanks

Note that this uses arrays, which are of course a bashism, but the top 
of the script explicitly calls for bash, so I presume bashisms are OK. 
Unfortunately, because of the "eval", it doesn't fully protect
from
weirdness in the config file. Of course, it looks like that eval could 
fairly easily be removed...


 From 86c8dc10a6452c011b993569ea0994bf280fb40f Mon Sep 17 00:00:00 2001
From: Anthony DeRobertis
Date: Fri, 17 Nov 2017 12:43:01 -0500
Subject: [PATCH] Let mime-construct pick an encoding (closes: #860052)

Forcing 7bit is nice for MUAs and other tools that don't fully support
MIME encoding messages, but has the downside of sometimes generating
non-compliant messages. In particular, overly long lines get the
logcheck message rejected by Debian's default Exim configuration.

Add a configuration option to allow this to be overridden if required to
work with some MIME-unaware tool.
---
  etc/logcheck.conf | 11 +++++++++++
  src/logcheck      | 16 ++++++++++++----
  2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/etc/logcheck.conf b/etc/logcheck.conf
index 4653c11..1699fae 100644
--- a/etc/logcheck.conf
+++ b/etc/logcheck.conf
@@ -71,6 +71,17 @@ FQDN=1
  
  #ADDTAG="no"
  
+# Previous versions of logcheck always sent messages in 7bit encoding,
+# even if that resulted in RFC-violating messages. For example, really
+# long syslog lines would generate too-long SMTP lines, which are
+# rejected at least by Debian's default exim configuration. The new
+# default is to let mime-construct pick an appropriate encoding, but you
+# can override it by setting the below (to any of the encodings
+# supported by mime-construct). You may need to do this if you have
+# tools handling logcheck emails that don't understand MIME encoding.
+
+#MIMEENCODING="7bit"
+
  # Set a different location for temporary files than /tmp
  # this is useful if your /tmp is small and you are getting
  # errors such as:
diff --git a/src/logcheck b/src/logcheck
index a8c58ea..48082f3 100755
--- a/src/logcheck
+++ b/src/logcheck
@@ -84,6 +84,7 @@ INTRO=1
  LOGCHECKDEBUG=0
  MAILOUT=0
  MAILASATTACH=0
+MIMEENCODING  NOCLEANUP=0
  REBOOT=0
  FQDN=0
@@ -176,7 +177,7 @@ Also verify that the logcheck user can read all files
referenced in
  $(export)
  EOF
  	} | eval mime-construct $MIMECONSTRUCTARGS \
-	    --subject "'Logcheck: $HOSTNAME $DATE exiting due to
errors'" --encoding "7bit" \
+	    --subject "'Logcheck: $HOSTNAME $DATE exiting due to
errors'" "${ENCODING[@]}" \
          --file - --to "$SENDMAILTO"
  
      elif [ "$MAILOUT" -eq 1 ]; then
@@ -306,14 +307,14 @@ sendreport() {
  	debug "Sending report: '$subject' to $SENDMAILTO"
          if [ "$MAILASATTACH" -eq 1 ]; then
            debug "Sending report as attachment"
-          eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" --encoding "7bit" --string
"'Report attached'" --to "$SENDMAILTO" --attachment
"logcheck_report" --encoding "7bit" --file
"$TMPDIR/report"
+          eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" "${ENCODING[@]}" --string
"'Report attached'" --to "$SENDMAILTO" --attachment
"logcheck_report" "${ENCODING[@]}" --file
"$TMPDIR/report"
            return $?
          elif [ "$MAILASATTACH" -eq 2 ]; then
            debug "Sending report as gzip attachment"
-          eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" --encoding "7bit" --string
"'Report attached'" --to "$SENDMAILTO" --type
"application/x-gzip" --attachment "logcheck_report.gz"
--file "'gzip -c $TMPDIR/report |'"
+          eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" "${ENCODING[@]}" --string
"'Report attached'" --to "$SENDMAILTO" --type
"application/x-gzip" --attachment "logcheck_report.gz"
--file "'gzip -c $TMPDIR/report |'"
            return $?
          fi
-        eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" --to "$SENDMAILTO" --encoding
"7bit" --file "$TMPDIR/report"
+        eval mime-construct $MIMECONSTRUCTARGS --subject
"'$subject'" --to "$SENDMAILTO"
"${ENCODING[@]}" --file "$TMPDIR/report"
      fi
  }
  
@@ -536,6 +537,13 @@ else
      SORT="sort -k 1,3 -s"
  fi
  
+# Set the forced mime encoding if not left blank (auto)
+if [ -n "$MIMEENCODING" ]; then
+	ENCODING=(--encoding "$MIMEENCODING")
+else
+	ENCODING=()
+fi
+
  # Hostname either fully qualified or not.
  if [ "$FQDN" -eq 1 ]; then
          HOSTNAME="$(hostname --fqdn 2>/dev/null)"
-- 
2.15.0