Email Security by Procmail 1.16
Ahir vaig estar llegint a fons el funcionament d’aquest filtre de correu per
procmail i esta molt bé, avui ha sortit una nova versió a
cotinuació enganxo la pàgina on indica el funcionament generic
La url és
Enhancing E-Mail Security With Procmail
There are four types of attacks on system security that can be carried out
via electronic mail:
- Active Content attacks, which take
advantage of various active HTML and scripting features and bugs.
- Buffer Overflow attacks, where the
attacker sends something that is too large to fit into a fixed-size memory
buffer in the email client, in the hopes that the part that doesn’t fit will
overwrite critical information rather than being safely discarded.
- Trojan Horse attacks, where an
executable program or macro-language script that grants access, causes damage,
or does other unwelcome things is mailed to the victim as a file attachment
labeled as something innocuous, such as a greeting card or screen saver, or
hidden in something the victim is expecting, such as a spreadsheet or important
- Shell Script attacks, where a
fragment of a Unix shell script is included in the message headers in the hopes
that an improperly-configured Unix mail client will execute the commands.
Active Content Attacks, a.k.a. Browser Attacks, Active HTML Attacks or
These attacks are aimed at people who use a web browser or HTML-enabled
email client to read their email, which these days is a very large portion of
the computing community. Typically these attacks attempt to use the scripting
features of HTML or of the email client to retrieve private information from
the victim’s computer or to execute code on the victim’s computer without the
victim’s consent (and possibly without the victim’s knowledge).
Less dangerous forms of these attacks can automatically cause the
recipient’s computer to display some content the attacker wishes, such as
automatically opening an advertising or pornography web page when the message
is opened, or perform a Denial-of-Service attack on the recipient’s computer
through code that freezes or crashes the browser or the entire computer.
The simplest way to completely avoid such attacks is to not use a web
browser or HTML-enabled email client to read your email. Since many of these
attacks do not depend on bugs in the email client software, they cannot be
prevented through patches to the email client. If you use a web browser or
HTML-aware email client, you will be vulnerable to these kinds of
Also, as some of these attack depend on the email client being able to
execute scripted HTML rather than depending on the weaknesses of any particular
operating system, these attacks can be cross-platform. An HTML-enabled email
client on a Macintosh is just as vulnerable to active-HTML email attacks as an
HTML-enabled email client on Windows or Unix. The vulnerabilty will vary from
system to system based on the email client rather than the operating
Switching to a non-HTML-aware email client is not a realistic option for
many people. An alternative is to filter out or alter the offending HTML or
script code before the email client gets a chance to execute it. It may also be
possible to configure your email client to turn off the interpretation of
script code. See your program documentation for details. Turning off scripting
in your email client is strongly recommended. There is
very little reason to support scripted email messages.
Microsoft Outlook users should visit this page that describes tightening
down Outlook’s security settings.
The recently-announced Outlook email worms are an example of this attack.
See the Bugtraq Vulnerability database for more details.
Buffer Overflow Attacks
These attacks can be used as Denial-of-Service attacks, because when a
program’s memory gets randomly overwritten the program will generally crash.
However, by carefully crafting the exact contents of what overflows the buffer,
it is in some cases possible to supply program instructions for the victim’s
computer to execute. The attacker is mailing a program to the victim, and it
will be executed by the victim’s computer.
Note that this is the result of a bug in the program under attack. A
properly written email client will not allow random strangers to run
programs on your computer without your consent. Programs subject to buffer
overflows are incorrectly written and must be patched to permanently correct
Buffer overflows in mail programs occur in handling the message headers and
attachment headers, which is information the email client needs to process in
order to know details about the message and what to do with it. The text in the
body of the message, which is simply displayed on the screen and which is
expected to be a large amount of text, is not used as the vehicle for buffer
The recently-announced overflow bugs in Outlook, Outlook Express and
Netscape Mail are examples of this. Patches for Outlook are available via
the Microsoft security
The message headers and attachment headers can be preprocessed by the mail
server to limit their lengths to safe values. Doing this will prevent them
being used to attack the email client.
Trojan Horse Attacks
These attacks are usually used to breach security by getting a trusted user
to run a program that grants access to an untrusted user, or to cause damage
such as attempting to erase all of the files on the victim’s hard disk. Trojan
Horses can also act to steal information or resources or implement a
distributed attack, such as by distributing a program that attempts to steal
passwords or other security information, or a program that mails itself around
(a “worm”) and also mailbombs a target (a worm with an attitude :).
For this attack to succeed the victim must take action to run the program
that they’ve received. The attacker can use various “social engineering”
methods to convince the victim to run the program; for example, the program may
be disguised as a love letter or joke list, with the filename specially
constructed to take advantage of Windows’ propensity for hiding important
information from the user.
This attack can be avoided simply by not running programs that have been
received in email until they have been checked over, even if the program seems
to be harmless and especially if it comes from someone you don’t know
well and trust.
Bugs in the email client or poor design may allow the attack message to
automatically execute the Trojan Horse attachment without user intervention,
through either the use of active HTML, scripting or buffer overflow exploits.
This is an extremely dangerous scenario and HAS BEEN
DEMONSTRATED in a public computer security forum.
In an attempt to prevent this, the names of executable file attachments can
be changed in such a way that the operating system no longer thinks they are
executable (for example, by changing “EXPLOIT.EXE” to
“EXPLOIT.DEFANGED-EXE“). This will force the user to save and rename
the file before it can be executed (giving them a chance to think about whether
it should be executed), and it reduces the possibility that other
exploits in the same message will be able to find and execute the Trojan Horse
program automatically (since the name has changed).
In addition, for known Trojan Horse programs the attachment format itself
can be mangled in such a way that the email client no longer sees the
attachment as an attachment. This will force the user to contact technical
support to retrieve the attachment, and gives the system administrator a chance
to examine it.
Unmangling a mangled attachment is fairly straightforward for the
administrator. In mangling the attachment the original MIME attachment header
is shifted down and an attack warning attachment header is inserted. No
information is deleted or altered. To reattach the attachment simply edit the
mailbox file with a text editor and delete the attack warning header, returning
the original attachment header to its original location.
Here is a list of recent Trojan Horse executables and documents, gleaned
from bugtraq and Usenet newsgroup warnings and antivirus vendor advisories:
*.[a-z0-9][a-z0-9][a-z0-9].[a-z0-9]+ (to catch “double-extension”
Another channel for Trojan Horse attacks is via a data file for a program
that provides a macro (programming) language, for example, modern high-powered
word processors, spreadsheets, and user database tools.
If you cannot simply discard attachments that may put you at risk, it is
recommended that you install anti-virus software (which detects and disables
macro-language Trojan Horses) and that you always open data file
attachments in the program’s “do not automatically execute macros” mode (for
example, by holding down the [SHIFT] key when double-clicking the
Also: if your system administrator (or someone claiming to be your system
administrator) emails you a program and asks you to run it, immediately become
very suspicious and verify the origin of the email by contacting your
Shell Script Attacks
Many programs running under Unix and similar operating systems support the
ability to embed short shell scripts (sequences of commands similar to batch
files under DOS) in their configuration files. This is a common way to allow
the flexible extension of their capabilities.
Some mail-processing programs improperly extend this support for embedded
shell commands to the messages they are processing. Generally this capability
is included by mistake, by calling a shell script taken from the configuration
file to process the text of some headers. If the header is specially-formatted
and contains shell commands, it is possible that those shell commands will get
executed as well. This can be prevented by the program scanning the header text
for the special formatting and changing that formatting before it gets passed
to the shell for further processing.
Since the formatting needed to embed a shell script in an email header is
fairly special, it’s fairly easy to detect and alter.
Filtering Email for Security
(Finally! The meat!)
Procmail is a
program that processes email messages looking for particular information in the
headers or body of each message, and takes actions based on what it finds. If
you’re familiar with the concept of “rules” as provided in many major user mail
clients (such as the cc:Mail client), then you are already familiar with the
concept of automatically
processing email messages based on their content.
This page provides you with a procmail ruleset specifically designed to
“sanitize” your email against these attacks.
Please see my Procmail Kit page for details
about how to use procmail, and a kit of rules that includes the sanitizing
ruleset as well as anti-spam filters. If you just want to sanitize your email
and don’t care about spam, then you can download and install just the
sanitizing ruleset. Note that the full kit has an older version of the
sanitizing filter, and is not currently being maintained.
The email security procmail ruleset is available at:
Mirror 1 (USA: UT) | FTP Mirror 2 (Europe:
NL) | HTTP
Mirror 1 (USA: WA) ]
A tarball of the ruleset and some other files is available at:
Mirror 1 (USA: UT) | FTP Mirror 2
(Europe: NL) | HTTP Mirror 1
(USA: WA) ]
If you are downloading this on a Windows system for use on a Unix or Linux
system, make sure that you take care of text-file conversion – the
script will not run properly with DOS end-of-line characters in it. One way to
do this is to open the sanitizer script in vi and type:
If you’re an administrator and you wish to sanitize all of your users’ email
automatically, here’s how to do it:
- Your email must be received by a system that runs or can run procmail. See
the procmail link above for information about obtaining and installing procmail
for your platform.
- If you are running sendmail then it
must be set up to use procmail as the local delivery agent. Look for the
following line in your /etc/sendmail.cf file:
Mlocal, P=/usr/bin/procmail, F=lsDFMAw5:/|@qSPfhn9, S=10/30, R=20/40, A=procmail -Y -a $h -d $u
- If you want to install the sanitizer on a sendmail relay (for example, to
protect a Microsoft Exchange system that is not directly connected to the
Internet), follow these directions:
Mirror 1 (USA: UT) | FTP Mirror 2
(Europe: NL) | HTTP Mirror 1
(USA: WA) ]
- You must have perl installed.
- If you wish to scan attachments, you must have mimencode
(which is a part of the metamail package) and mktemp
installed. I have decided that Mime::Base64 from CPAN is better for
this, so sometime soon the sanitizer will use that instead of mimencode.
Installation under *nix and workalikes (Linux, *BSD, etc.)
- Create a directory /etc/procmail owner and group root,
- Download the sanitizing ruleset and save it in that directory, owner and
group root, permissions rw-r--r--. If you are using Lynx,
highlight the above link and press “D” to download the file – don’t view it and
save it, it’ll be corrupted.
- Create /etc/procmailrc owner and group root, permissions
rw-r--r-- and put the following into it:
Of course, if you’ve already got an /etc/procmailrc file
you’ll have to incorporate the
INCLUDERC=/etc/procmail/html-trap.procmail call into what’s already
Notes about local policy:
This will create a file named procmail.log in the home directory of
any user who receives mail. Your users should be instructed to periodically
review and delete this file, or an administrative daemon (e.g. logrotate)
should be set up to periodically collect statistics and delete the files or
warn the user to do so. If you don’t care to log the sanitizing messages and
possible exploit warnings, then change the logfile line to:
Note that the DROPPRIVS at the beginning means the log file
must be writable by the recipient.
You may override the default list of executable extensions by setting
MANGLE_EXTENSIONS. The default list is:
Note that what you specify completely replaces the default list, so,
for example, if you don’t want to mangle .EXE files for some reason,
This can also be used to customize the sanitizer on a by-sender or by-recipient
basis. For example, if has a legitimate reason to send you .EXE
attachments, you might do something like this before calling the sanitizer:
This sort of thing can become arbitratrily complex.
If you don’t want to mangle document filenames, try this:
They will still be scanned for poisonous macros and quarantined if appropriate,
but they will not be checked against the poisoned executables list
Note that the MANGLE_EXTENSIONS settings you enter must
all be on one line.
If you wish to block specific executable and document attachments, create a
text file containing one filename or filespec per line, with no comments or
leading spaces, permissions rw-r--r--, and set the variable
POISONED_EXECUTABLES to point at that file before calling
html-trap.procmail. See above for a recommended list of filenames of
known trojans. The names in this list are not case-sensitive
and wildcards may be used. For example:
Note that only extensions in the MANGLE_EXTENSIONS list are compared
against the poisoned executables file. Omitting .EXE from
MANGLE_EXTENSIONS means you will not be able to poison any
.EXE attachments. This is a design weakness in the current sanitizer.
If you wish to be notified when the filter traps poisoned attachments, set
SECURITY_NOTIFY and/or SECURITY_NOTIFY_VERBOSE to a
comma-delimited list of email addresses to notify. The
SECURITY_NOTIFY_VERBOSE recipients will receive a full copy of the
message, the SECURITY_NOTIFY recipents will only receive the
If SECURITY_NOTIFY or SECURITY_NOTIFY_VERBOSE are set,
then the message sender may also be notified of the message being trapped. To
do this, set SECURITY_NOTIFY_SENDER to any value. If you wish to
override the simple default message, set SECURITY_NOTIFY_SENDER to
point at a text file and that text file will be included as the body of the
message – I recommend you explain site policy there if you are blocking
attachments by wildcard. If SECURITY_NOTIFY_SENDER is set but not
pointing at a file, a simple default message will be sent. NOTE: Be
careful that you set it to something that is NOT a file if you don’t want to
override the default message. “SECURITY_NOTIFY_SENDER=/etc/passwd“,
for example, would be bad. “SECURITY_NOTIFY_SENDER=YES” is probably
safe. If you do not want to send a notification reply, you may want to set it
explicitly to empty (“”) to avoid any inherited value.
If you wish to quarantine poisoned messages, set
SECURITY_QUARANTINE to the file you wish to save the message in. This
will prevent delivery to the original addressees. It is suggested that
you set SECURITY_NOTIFY if you set SECURITY_QUARANTINE as
this will remind you to check the quarantine file. If for any reason the
message cannot be quarantined it will be bounced and the
SECURITY_NOTIFY list will be notified. If you want the (mangled)
message to be delivered instead of bounced, set
SECURITY_QUARANTINE_OPTIONAL to any value. The
SECURITY_QUARANTINE file must already exist – the sanitizer cannot
reliably create it on-the-fly.
Note that the DROPPRIVS at the beginning means that the security
quarantine file must be writable by users: access permissions on the
quarantine file must be -rw--w--w-. If this is a problem, you may want
to try setting the quarantine filename to /dev/null and rely on
SECURITY_NOTIFY_VERBOSE to keep a copy of the original message.
The sanitizer implements a scanner which checks Microsoft document and
worksheet attachments for macros that appear to be trying to do dangerous
things. Depending on what the macros in a document or worksheet try to do, you
may see false positives (safe documents being marked as dangerous). The score
at which an attachment is considered “poisoned” may be set via
POISONED_SCORE. If not given, it defaults to 25. The minimum
POISONED_SCORE is 5.
It appears that many virus scanners do not actually delete evil macros;
instead, they are mangled enough to disable them and are left in place. The
sanitizer will very likely detect enough of these fragments to consider the
document as still infected. If this happens, suggest to the sender of the
document that they save the document in a format that does not support macros
at all, such as Rich Text (RTF).
If you wish to completely disable macro scanning, set
DISABLE_MACRO_CHECK to any value. If you wish to scan and save the
scores to do profiling but not mark any attachments as “poisoned”, you can
either set SCORE_ONLY to any value (not recommended), or set
POISONED_SCORE to a very high value (100-200 is recommended – this
will trap currently known exploits while giving you a chance to profile). Set
SCORE_HISTORY to the name of the file to save scores in (with the same
DROPPRIVS caveats as the other log files).
All of these site policy customizations must be made in the procmail script
that calls the sanitizer, before the sanitizer is called, or they will have no
effect on the sanitizer’s behavior.
Suggestions for what to consider dangerous in a macro are welcome, as are
samples of infected documents.
News & Notes
version of the html-trap.procmail ruleset is:
It is recommended you update your copy if your version is older, as bugfixes
and filtering for newer exploits will have been added.
An announce list for email security issues has been set up. It will
This page is now being tracked by the Daily Diffs service.
08/04/00 Okay, don’t trigger the Excessively Long Header trap until the
header exceeds 250 characters. Added asd to the default
MANGLE_EXTENSIONS. If you are overriding the default list you should add it to
your custom list. Fixed a problem where it was possible for the sanitizer to
overlook every other attachment in a series of document attachments. Added
clearing of the MIME content type if the attachment filename gets mangled, to
prevent the mail program from figuring out what program to run even though the
filename is mangled. For the same reason, drop x-mac-* clauses that Eudora uses
to indicate the file type and restore the filename extension.
07/26/00 Bugfix in NOTIFY SENDER.
07/23/00 Added checks for certain excessively long standard headers, to
address the MS Outlook header buffer-overflow bug; previously only MIME-related
headers were length-limited, and only in MIME messages. Disabled sanitizing of
encrypted/signed messages; changing the body of such a message breaks the
signature, so there’s no good way to sanitize it. Moved DROPPRIVS=YES
into the sanitizer itself to avoid configuration errors – this may break
gateway use, watch it closely. Enabled scanning of PowerPoint files, which
weren’t being scanned due to an oversight (D’oh!). Improved handling of RFC822
comments embedded in unquoted attachment filenames. Improved handling of
filenames containing international characters. Added a debugging mode – if you
want to see the poisoned filespecs it is comparing attachment names to, define
$DEBUG to be anything. Improved loop-prevention in notification messages; if
you want to secure your system against someone forging the X-Loop: headers in
an attempt to suppress attack notification messages, define $SECRET to be a
short string of random text.
Given the severity of the Outlook BO bug, you probably want to install the
updated sanitizer right away.
05/18/00 (Announcement here delayed, sorry) Okay, it’s happened. A
working demonstration attack that uses a combination of active-scripted HTML
and a scriptable attachment (in the form of a Microsoft Compiled Help file) to
automatically save and execute an arbitrary program remotely via email
without the user having to double-click on an
attachment has been posted to Bugtraq. This means that, for
example, someone could email you a copy of Back Orifice that would install
itself on your computer the moment you simply previewed the message in your
Make sure that chm appears in your MANGLE_EXTENSIONS
list and that *.chm is in your poisoned executables list. You should
also visit this page
that describes tightening down Outlook’s security settings.
05/22/00 Added some new executable extensions to MANGLE_EXTENSIONS. See
above for the new default. Fixed a bug that prevented macro scanning if
document attachments were in MANGLE_EXTENSIONS. Dynamically set LINEBUF so that
we’re no longer vulnerable to extremely long To: headers.
05/14/00 Fixed a bug in notification. Added error logging on failure to open
poisoned spec file.
05/13/00 Made sender notification optional. Added ability to specify
executable extensions list in configuration file. No more script updates for
new executables! Site-customized executable mangling!
05/12/00 Improved sender notification. Added quarantine reliability
assurance (i.e. bounce if quarantine fails).
05/10/00 Added “.vbe” to the executable extensions list. You should add
“*.vbe” to your poisoned executables list. Fixed a problem where a message that
was *only* a poisoned executable (e.g. no text body at all) wouldn’t be
05/06/00 Added “.wsf” and “.wsh” to the executable extensions list. Fixed
another DoS bug in header fixups. Fixed a missing executable extension in the
UUE checker. Added notification of the message sender on hits.
03/26/00 Added “.eml” to the executable extensions list. Dynamic
configuration of this soon…
02/01/00 Improved handling of quotes in tag arguments.
01/22/00 Sanitizer now deals with attempted obscuration of tag options with
and % escapes.
01/14/00 Fixed another DoS bug in certain quoted strings, and generally
improved quoted string and wrapped-header handling.
If you get “Word too long” errors, try adding
“SHELL=/bin/sh” or “SHELL=/bin/ksh” to
/etc/procmailrc just before the call to html-trap.procmail –
csh can’t handle a
command-line argument the size of the Perl script that’s in the filter.
Do not put html-trap.procmail into
/etc/procmailrcs/ as implied by the procmail
man page. You’ll get security errors from Perl about -e and setuid scripts if
you do this. You may also have problems with filtering mail sent to root for
It looks like this perl script can be a bit of a memory hog on some systems.
If you start getting “Out of memory” errors in your procmail log file, try
ulimit -d 15000;
just before the perl -p -e in the MIME-sanitizing rule:
| ulimit -d 15000; perl -p -e ' #
You might also have to increase the hard memory limit originally set for
I can be contacted at – you could
also visit my home page.