TL;DR: procmail is a security liability and has been abandoned upstream for the last two decades. If you are still using it, you should probably drop everything and at least remove its SUID flag. There are plenty of alternatives to chose from, and conversion is a one-time, acceptable trade-off.

Procmail is unmaintained
Procmail is insecure
Procmail has better alternatives
Conclusion
Appendix

Procmail is unmaintained

procmail is unmaintained. The "Final release", according to Wikipedia, dates back to September 10, 2001 (3.22). That release was shipped in Debian since then, all the way back from Debian 3.0 "woody", twenty years ago.

Debian also ships 25 uploads on top of this, with 3.22-21 shipping the "3.23pre" release that has been rumored since at least the November 2001, according to debian/changelog at least:

procmail (3.22-1) unstable; urgency=low

  * New upstream release, which uses the `standard' format for Maildir
    filenames and retries on name collision. It also contains some
    bug fixes from the 3.23pre snapshot dated 2001-09-13.
  * Removed `sendmail' from the Recommends field, since we already
    have `exim' (the default Debian MTA) and `mail-transport-agent'.
  * Removed suidmanager support. Conflicts: suidmanager (<< 0.50).
  * Added support for DEB_BUILD_OPTIONS in the source package.
  * README.Maildir: Do not use locking on the example recipe,
    since it's wrong to do so in this case.

 -- Santiago Vila <sanvila@debian.org>  Wed, 21 Nov 2001 09:40:20 +0100

All Debian suites from buster onwards ship the 3.22-26 release, although the maintainer just pushed a 3.22-27 release to fix a seven year old null pointer dereference, after this article was drafted.

Procmail is also shipped in all major distributions: Fedora and its derivatives, Debian derivatives, Gentoo, Arch, FreeBSD, OpenBSD. We all seem to be ignoring this problem.

The upstream website (http://procmail.org/) has been down since about 2015, according to Debian bug #805864, with no change since.

In effect, every distribution is currently maintaining its fork of this dead program.

Note that, after filing a bug to keep Debian from shipping procmail in a stable release again, I was told that the Debian maintainer is apparently in contact with the upstream. And, surprise! they still plan to release that fabled 3.23 release, which has been now in "pre-release" for all those twenty years.

In fact, it turns out that 3.23 is considered released already, and that the procmail author actually pushed a 3.24 release, codenamed "Two decades of fixes". That amounts to 25 commits since 3.23pre some of which address serious security issues, but none of which address fundamental issues with the code base.

Procmail is insecure

By default, procmail is installed SUID root:mail in Debian. There's no debconf or pre-seed setting that can change this. There has been two bug reports against the Debian to make this configurable (298058, 264011), but both were closed to say that, basically, you should use dpkg-statoverride to change the permissions on the binary.

So if anything, you should immediately run this command on any host that you have procmail installed on:

dpkg-statoverride --update --add root root 0755 /usr/bin/procmail

Note that this might break email delivery. It might also not work at all, thanks to usrmerge. Not sure. Yes, everything is on fire. This is fine.

In my opinion, even assuming we keep procmail in Debian, that default should be reversed. It should be up to people installing procmail to assign it those dangerous permissions, after careful consideration of the risk involved.

The last maintainer of procmail explicitly advised us (in that null pointer dereference bug) and other projects (e.g. OpenBSD, in [2]) to stop shipping it, back in 2014. Quote:

Executive summary: delete the procmail port; the code is not safe and should not be used as a basis for any further work.

I just read some of the code again this morning, after the original author claimed that procmail was active again. It's still littered with bizarre macros like:

#define bit_set(name,which,value) \
  (value?(name[bit_index(which)]|=bit_mask(which)):\
  (name[bit_index(which)]&=~bit_mask(which)))

... from regexp.c, line 66 (yes, that's a custom regex engine). Or this one:

#define jj  (aleps.au.sopc)

It uses insecure functions like strcpy extensively. malloc() is thrown around gotos like it's 1984 all over again. (To be fair, it has been feeling like 1984 a lot lately, but that's another matter entirely.)

That null pointer deref bug? It's fixed upstream now, in this commit merged a few hours ago, which I presume might be in response to my request to remove procmail from Debian.

So while that's nice, this is the just tip of the iceberg. I speculate that one could easily find an exploitable crash in procmail if only by running it through a fuzzer. But I don't need to speculate: procmail had, for years, serious security issues that could possibly lead to root privilege escalation, remotely exploitable if procmail is (as it's designed to do) exposed to the network.

Maybe I'm overreacting. Maybe the procmail author will go through the code base and do a proper rewrite. But I don't think that's what is in the cards right now. What I expect will happen next is that people will start fuzzing procmail, throw an uncountable number of bug reports at it which will get fixed in a trickle while never fixing the underlying, serious design flaws behind procmail.

Procmail has better alternatives

The reason this is so frustrating is that there are plenty of modern alternatives to procmail which do not suffer from those problems.

Alternatives to procmail(1) itself are typically part of mail servers. For example, Dovecot has its own LDA which implements the standard Sieve language (RFC 5228). (Interestingly, Sieve was published as RFC 3028 in 2001, before procmail was formally abandoned.)

Courier also has "maildrop" which has its own filtering mechanism, and there is fdm (2007) which is a fetchmail and procmail replacement. Update: there's also mailprocessing, which is not an LDA, but processing an existing folder. It was, however, specifically designed to replace complex Procmail rules.

But procmail, of course, doesn't just ship procmail; that would just be too easy. It ships mailstat(1) which we could probably ignore because it only parses procmail log files. But more importantly, it also ships:

lockfile(1) - conditional semaphore-file creator
formail(1) - mail (re)formatter

lockfile(1) already has a somewhat acceptable replacement in the form of flock(1), part of util-linux (which is Essential, so installed on any normal Debian system). It might not be a direct drop-in replacement, but it should be close enough.

formail(1) is similar: the courier maildrop package ships reformail(1) which is, presumably, a rewrite of formail. It's unclear if it's a drop-in replacement, but it should probably possible to port uses of formail to it easily.

Update: the maildrop package ships a SUID root binary (two, even). So if you want only reformail(1), you might want to disable that with:
dpkg-statoverride --update --add root root 0755 /usr/bin/lockmail.maildrop 
dpkg-statoverride --update --add root root 0755 /usr/bin/maildrop
It would be perhaps better to have reformail(1) as a separate package, see bug 1006903 for that discussion.

The real challenge is, of course, migrating those old .procmailrc recipes to Sieve (basically). I added a few examples in the appendix below. You might notice the Sieve examples are easier to read, which is a nice added bonus.

Conclusion

There is really, absolutely, no reason to keep procmail in Debian, nor should it be used anywhere at this point.

It's a great part of our computing history. May it be kept forever in our museums and historical archives, but not in Debian, and certainly not in actual release.

It's just a bomb waiting to go off. It is irresponsible for distributions to keep shipping obsolete and insecure software like this for unsuspecting users.

Note that I am grateful to the author, I really am: I used procmail for decades and it served me well. But now, it's time to move, not bring it back from the dead.

Appendix

Previous work

It's really weird to have to write this blog post. Back in 2016, I rebuilt my mail setup at home and, to my horror, discovered that procmail had been abandoned for 15 years at that point, thanks to that LWN article from 2010. I would have thought that I was the only weirdo still running procmail after all those years and felt kind of embarrassed to only "now" switch to the more modern (and, honestly, awesome) Sieve language.

But no. Since then, Debian shipped three major releases (stretch, buster, and bullseye), all with the same vulnerable procmail release.

Then, in early 2022, I found that, at work, we actually had procmail installed everywhere, possibly because userdir-ldap was using it for lockfile until 2019. I sent a patch to fix that and scrambled to remove get rid of procmail everywhere. That took about a day.

But many other sites are now in that situation, possibly not imagining they have this glaring security hole in their infrastructure.

Procmail to Sieve recipes

I'll collect a few Sieve equivalents to procmail recipes here. If you have any additions, do contact me.

All Sieve examples below assume you drop the file in ~/.dovecot.sieve.

deliver mail to "plus" extension folder

Say you want to deliver user+foo@example.com to the folder foo. You might write something like this in procmail:

MAILDIR=$HOME/Maildir/
DEFAULT=$MAILDIR
LOGFILE=$HOME/.procmail.log
VERBOSE=off
EXTENSION=$1            # Need to rename it - ?? does not like $1 nor 1

:0
* EXTENSION ?? [a-zA-Z0-9]+
        .$EXTENSION/

That, in sieve language, would be:

require ["variables", "envelope", "fileinto", "subaddress"];

########################################################################
# wildcard +extension
# https://doc.dovecot.org/configuration_manual/sieve/examples/#plus-addressed-mail-filtering
if envelope :matches :detail "to" "*" {
  # Save name in ${name} in all lowercase
  set :lower "name" "${1}";
  fileinto "${name}";
  stop;
}

Subject into folder

This would file all mails with a Subject: line having FreshPorts in it into the freshports folder, and mails from alternc.org mailing lists into the alternc folder:

:0
## mailing list freshports
* ^Subject.*FreshPorts.*
.freshports/

:0
## mailing list alternc
* ^List-Post.*mailto:.*@alternc.org.*
.alternc/

Equivalent Sieve:

if header :contains "subject" "FreshPorts" {
    fileinto "freshports";
} elsif header :contains "List-Id" "alternc.org" {
    fileinto "alternc";
}

Mail sent to root to a reports folder

This double rule:

:0
* ^Subject: Cron
* ^From: .*root@
.rapports/

Would look something like this in Sieve:

if header :comparator "i;octet" :contains "Subject" "Cron" {
  if header :regex :comparator "i;octet"  "From" ".*root@" {
        fileinto "rapports";
  }
}

Note that this is what the automated converted does (below). It's not very readable, but it works.

Bulk email

I didn't have an equivalent of this in procmail, but that's something I did in Sieve:

if header :contains "Precedence" "bulk" {
    fileinto "bulk";
}

Any mailing list

This is another rule I didn't have in procmail but I found handy and easy to do in Sieve:

if exists "List-Id" {
    fileinto "lists";
}

This or that

I wouldn't remember how to do this in procmail either, but that's an easy one in Sieve:

if anyof (header :contains "from" "example.com",
           header :contains ["to", "cc"] "anarcat@example.com") {
    fileinto "example";
}

You can even pile up a bunch of options together to have one big rule with multiple patterns:

if anyof (exists "X-Cron-Env",
          header :contains ["subject"] ["security run output",
                                        "monthly run output",
                                        "daily run output",
                                        "weekly run output",
                                        "Debian Package Updates",
                                        "Debian package update",
                                        "daily mail stats",
                                        "Anacron job",
                                        "nagios",
                                        "changes report",
                                        "run output",
                                        "[Systraq]",
                                        "Undelivered mail",
                                        "Postfix SMTP server: errors from",
                                        "backupninja",
                                        "DenyHosts report",
                                        "Debian security status",
                                        "apt-listchanges"
                                        ],
           header :contains "Auto-Submitted" "auto-generated",
           envelope :contains "from" ["nagios@",
                                      "logcheck@",
                                      "root@"])
    {
    fileinto "rapports";
}

Automated script

There is a procmail2sieve.pl script floating around, and mentioned in the dovecot documentation. It didn't work very well for me: I could use it for small things, but I mostly wrote the sieve file from scratch.

Progressive migration

Enrico Zini has progressively migrated his procmail setup to Sieve using a clever way: he hooked procmail inside sieve so that he could deliver to the Dovecot LDA and progressively migrate rules one by one, without having a "flag day".

See this explanatory blog post for the details, which also shows how to configure Dovecot as an LMTP server with Postfix.

Other examples

The Dovecot sieve examples are numerous and also quite useful. At the time of writing, they include virus scanning and spam filtering, vacation auto-replies, includes, archival, and flags.

Harmful considered harmful

I am aware that the "considered harmful" title has a long and controversial history, being considered harmful in itself (by some people who are obviously not afraid of contradictions).

I have nevertheless deliberately chosen that title, partly to make sure this article gets maximum visibility, but more specifically because I do not have doubts at this moment that procmail is, clearly, a bad idea at this moment in history.

Developing story

I must also add that, incredibly, this story has changed while writing it. This article is derived from this bug I filed in Debian to, quite frankly, kick procmail out of Debian. But filing the bug had the interesting effect of pushing the upstream into action: as mentioned above, they have apparently made a new release and merged a bunch of patches in a new git repository.

This doesn't change much of the above, at this moment. If anything significant comes out of this effort, I will try to update this article to reflect the situation. I am actually happy to retract the claims in this article if it turns out that procmail is a stellar example of defensive programming and survives fuzzing attacks. But at this moment, I'm pretty confident that will not happen, at least not in scope of the next Debian release cycle.

Comments on this page are closed.

Created 2022-03-02 13:16. Edited 2022-03-07 17:04.