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

After reading it again. To support both macros with conditional skips,
the syntax check would need to be duplicated like this:

.ifdef CHECK_DATA_VERIFY_HEADER_SYNTAX
header syntax check here
.elifndef NO_CHECK_DATA_VERIFY_HEADER_SYNTAX
header syntax check here
.endif