Bug#1004323: exim4-config: CHECK_DATA_VERIFY_HEADER_SYNTAX prevents header syntax verification

Mon Jan 24 22:37:45 GMT 2022

Package: exim4-config
Version: 4.93-13ubuntu1.5
Severity: normal
Tags: patch

Dear Maintainer,

   * What led up to the situation?

I noticed that exim accepted an email with invalid header syntax, yet I
had previously configured an exim macro to work with debian's exim
config to reject those emails.

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

I was confused, because the header syntax check had become the default
in a package upgrade, yet it still wasn't working. I eventually figured
out the cause.


The issue is in the following commit, from https://salsa.debian.org/exim-team/exim4:

commit b561c99ba7edd94891bfc66257823f79178ece62 (tag: experimental/4.91--RC1-1)
Author: Andreas Metzler <ametzler at bebt.de>
Date:   Sat Mar 17 17:40:50 2018 +0100

    verify = header_syntax by default

    Upstream now enables verify = header_syntax check in default config,
    mirror this change in Debian, introduce
    NO_CHECK_DATA_VERIFY_HEADER_SYNTAX macro to override this.

diff --git a/debian/changelog b/debian/changelog
index 7a8ebc40..0f7126ca 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -3,13 +3,15 @@ exim4 (4.91~RC1-1) experimental; urgency=medium
   * Point watchfile to test subdirectory.
   * New upstream version:
     + Drop debian/patches/75_*.
-    + Update example.conf.md5. Upstream now enables verify = header_syntax
-      check in default config.
+    + Update example.conf.md5.
+      Upstream now enables verify = header_syntax check in default config,
+      mirror this change in Debian, introduce
+      NO_CHECK_DATA_VERIFY_HEADER_SYNTAX macro to override this.
   * Build with newly available (well, for GnuTLS) DANE support.
   * Pull 75_01-Fix-heavy-pipeline-SMTP-command-input-corruption.-Bu.patch from
     upstream master, fixing https://bugs.exim.org/show_bug.cgi?id=2250.

- -- Andreas Metzler <ametzler at debian.org>  Sat, 17 Mar 2018 16:09:34 +0100
+ -- Andreas Metzler <ametzler at debian.org>  Sat, 17 Mar 2018 17:41:51 +0100

 exim4 (4.90.1-3) unstable; urgency=medium

diff --git a/debian/debconf/conf.d/acl/40_exim4-config_check_data b/debian/debconf/conf.d/acl/40_exim4-con
fig_check_data
index abfa1643..07a949db 100644
--- a/debian/debconf/conf.d/acl/40_exim4-config_check_data
+++ b/debian/debconf/conf.d/acl/40_exim4-config_check_data
@@ -17,14 +17,14 @@ acl_check_data:
           condition  = ${if > {$max_received_linelength}{998}}
   .endif

-  # Deny unless the address list headers are syntactically correct.
+  # Deny if the headers contain badly-formed addresses.
   #
-  # If you enable this, you might reject legitimate mail.
-  .ifdef CHECK_DATA_VERIFY_HEADER_SYNTAX
+  .ifndef NO_CHECK_DATA_VERIFY_HEADER_SYNTAX
   deny
-    message = Message headers fail syntax check
     !acl = acl_local_deny_exceptions
     !verify = header_syntax
+    message = header syntax
+    log_message = header syntax ($acl_verify_message)
   .endif

END COMMIT

The problem is that if you had a line in your exim config like:

CHECK_DATA_VERIFY_HEADER_SYNTAX = true

then after this change,

.ifndef NO_CHECK_DATA_VERIFY_HEADER_SYNTAX

gets expanded to

.ifndef NO_true

which exim sees as "yes, defined" and the .ifndef becomes false, and so
header syntax checking is removed from the config. Thus, people like me
who opted in to header syntax checking, suddenly got it turned off after
an exim package upgrade. That was clearly not what was intended. The new
macro name should have not included the old as a substring. Exim spec
6.5 warns about macro substrings.

The ideal solution is to support both the old and new macro names. I
think that can be done like so:

.ifndef CHECK_DATA_VERIFY_HEADER_SYNTAX
.ifdef NO_CHECK_DATA_VERIFY_HEADER_SYNTAX
.else
header syntax check here
.endif
.endif

-- Package-specific info:
Exim version 4.93 #3 built 28-Apr-2021 13:19:17
Copyright (c) University of Cambridge, 1995 - 2018
(c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007 - 2018
Berkeley DB: Berkeley DB 5.3.28: (September  9, 2013)
Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc GnuTLS move_frozen_messages Content_Scanning DANE DKIM DNSSEC Event I18N OCSP PRDR PROXY SOCKS TCP_Fast_Open
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmjz dbmnz dnsdb dsearch ldap ldapdn ldapm mysql nis nis0 passwd pgsql sqlite
Authenticators: cram_md5 cyrus_sasl dovecot plaintext spa tls
Routers: accept dnslookup ipliteral iplookup manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
Malware: f-protd f-prot6d drweb fsecure sophie clamd avast sock cmdline
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Configuration file search path is /etc/exim4/exim4.conf:/var/lib/exim4/config.autogenerated
Configuration file is /var/lib/exim4/config.autogenerated

-- System Information:
Debian Release: bullseye/sid
Architecture: amd64 (x86_64)

Kernel: Linux 5.8.0-63-generic (SMP w/16 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages exim4-config depends on:
ii  adduser                3.118ubuntu2
ii  debconf [debconf-2.0]  1.5.73

exim4-config recommends no packages.

exim4-config suggests no packages.

-- debconf information excluded

--
Ian Kelling | Senior Systems Administrator, Free Software Foundation
GPG Key: B125 F60B 7B28 7FF6 A2B7  DF8F 170A F0E2 9542 95DF
https://fsf.org | https://gnu.org

