[Snort-sigs] False positive

Antonio Henrique A. P. Oliveira tat at ...2620...
Wed Jul 14 13:12:47 EDT 2004


I'm having false positives on the rule for which I attach the report.
I hope this helps. Please let me know if you need additional information.

Kind regards,
Antonio Oliveira

-- 

Anto'nio Henrique A. Proenca de Oliveira

"Although we can never go back, like an old sweet song with a strong refrain, memories remain" - (Someone)

Please avoid sending me Word or PowerPoint attachments.
See http://www.fsf.org/philosophy/no-word-attachments.html
$Id: .signature,v 1.2 2004/06/16 00:57:53 tat Exp $
-------------- next part --------------
# This is a template for submitting snort signature descriptions to
# the snort.org website
#
# Ensure that your descriptions are your own
# and not the work of others.  References in the rules themselves
# should be used for linking to other's work. 
#
# If you are unsure of some part of a rule, use that as a commentary
# and someone else perhaps will be able to fix it.
# 
# $Id$
#
# 

Rule: alert tcp $EXTERNAL_NET any -> $HOME_NET 993 (msg:"IMAP PCT Client_Hello overflow attempt"; flow:to_server,established; content:"|01|"; depth:1; offset:2; byte_test:2,>,0,6; byte_test:2,!,0,8; byte_test:2,!,16,8; byte_test:2,>,20,10; content:"|8F|"; depth:1; offset:11; byte_test:2,>,32768,0,relative; reference:bugtraq,10116; reference:cve,2003-0719; reference:url,www.microsoft.com/technet/security/bulletin/MS04-011.mspx; classtype:attempted-admin; sid:2517; rev:10;) 

--
Sid: 2517

--
Summary:  	 This event is generated when an attempt is made to exploit a known vulnerability in the Microsoft implementation of the Private Communications Transport (PCT) protocol.

--
Impact:  Execution of arbitrary code. Unauthorized administrative access to an affected host.

--
Detailed Information:  	 A vulnerability exists in the handling of PCT
requests that
can be manipulated to give an attacker the opportunity to execute
arbitrary code of their choosing leading to a possible remote
administrative compromize of an affected host.

The condition exists because of poor error handling routines in the
Microsoft Secure Sockets Layer (SSL) library.

--
Affected Systems:  	 Microsoft Windows NT, 2000, 2003 and XP systems using PCT

--
Attack Scenarios:  An attcker needs to make a specially crafted PCT request to
an affected system.

--
Ease of Attack:  	 Simple.

--
False Positives:
I'm using on the server end:
Fedora Core 1.0
uname -a
Linux skyscraper.localdomain 2.4.22-1.2188.nptl #1 Wed Apr 21 20:35:41 EDT 2004 i686 athlon i386 GNU/Linux

dovecot imap server 0.99.10.5 (configuration file attached)
Self-signed SSL certificate

snort -V:
-*> Snort! <*-
Version 2.1.2 (Build 25)
By Martin Roesch (roesch at ...435..., www.snort.org)

Direct internet connection (USB ADSL Modem)

On the client end:
Microsoft Windows XP Professional (Portuguese) SP1
Intel Pentium 4 1.7 GHz
(Gateway: Microsoft Windows Small Business Server 2003)
Outlook Express 6.00.2800.1123 (Portuguese)

I've had a few alerts on this rule coming from my client IP address. I could
not determine exactly how and why this is happening but I'm not attacking my
own machine ;) All Microsoft Updates are installed both on the gateway
(SBS2003) and the client machine (XP SP1).

I will try to send a tcpdump of the packets if you require it but that is not
possible at this moment. If additional information is required please let me
know and I'll try to provide it.

--
False Negatives:

--
Corrective Action:
Apply the appropriate vendor supplied patches

Disable the use of PCT
--
Contributors:
Sourcefire Research Team
Matt Watchinski <mwatchinski at ...435...>
Nigel Houghton <nigel.houghton at ...435...>

-- 
Additional References:
url: www.microsoft.com/technet/security/bulletin/MS04-011.mspx
cve: 2003-0719
bugtraq: 10116
-------------- next part --------------
## Dovecot 1.0 configuration file

# Default values are shown after each value, it's not required to uncomment
# any of the lines. Exception to this are paths, they're just examples
# with real defaults being based on configure options. The paths listed here
# are for configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var
# --with-ssldir=/usr/share/ssl

# Base directory where to store runtime data.
#base_dir = /var/run/dovecot/

# Protocols we want to be serving:
#  imap imaps pop3 pop3s
protocols = imap imaps

# IP or host address where to listen in for connections. It's not currently
# possible to specify multiple addresses. "*" listens in all IPv4 interfaces.
# "[::]" listens in all IPv6 interfaces, but may also listen in all IPv4
# interfaces depending on the operating system. You can specify ports with
# "host:port".
#imap_listen = *
#pop3_listen = *

# IP or host address where to listen in for SSL connections. Defaults
# to above non-SSL equilevants if not specified.
#imaps_listen = 
#pop3s_listen = 

# Disable SSL/TLS support.
#ssl_disable = no

# PEM encoded X.509 SSL/TLS certificate and private key. They're opened before
# dropping root privileges, so keep the key file unreadable by anyone but
# root. Included doc/mkcert.sh can be used to easily generate self-signed
# certificate, just make sure to update the domains in dovecot-openssl.cnf
ssl_cert_file = /usr/share/ssl/certs/formiga.cer
ssl_key_file = /usr/share/ssl/certs/formiga.key
#ssl_cipher_list = HIGH:MEDIUM

# SSL parameter file. Master process generates this file for login processes.
# It contains Diffie Hellman and RSA parameters.
#ssl_parameters_file = /var/run/dovecot/ssl-parameters.dat

# How often to regenerate the SSL parameters file. Generation is quite CPU
# intensive operation. The value is in hours, 0 disables regeneration
# entirely.
#ssl_parameters_regenerate = 24

# Disable LOGIN command and all other plaintext authentications unless
# SSL/TLS is used (LOGINDISABLED capability)
#disable_plaintext_auth = no

# Use this logfile instead of syslog(). /dev/stderr can be used if you want to
# use stderr for logging (ONLY /dev/stderr - otherwise it is closed).
#log_path = 

# For informational messages, use this logfile instead of the default
#info_log_path = 

# Prefix for each line written to log file. % codes are in strftime(3)
# format. Note the extra space at the end of line.
#log_timestamp = %b %d %H:%M:%S 

##
## Login processes
##

# Directory where authentication process places authentication UNIX sockets
# which login needs to be able to connect to. The sockets are created when
# running as root, so you don't have to worry about permissions.
login_dir = /var/run/dovecot-login

# chroot login process to the login_dir. Only reason not to do this is if you
# wish to run the whole Dovecot without roots.
#login_chroot = yes


##
## IMAP login process
##

login = imap

# Executable location.
#login_executable = /usr/libexec/dovecot/imap-login

# User to use for the login process. Create a completely new user for this,
# and don't use it anywhere else. The user must also belong to a group where
# only it has access, it's used to control access for authentication process.
login_user = dovecot

# Set max. process size in megabytes. If you don't use
# login_process_per_connection you might need to grow this.
#login_process_size = 16

# Should each login be processed in it's own process (yes), or should one
# login process be allowed to process multiple connections (no)? Yes is more
# secure, espcially with SSL/TLS enabled. No is faster since there's no need
# to create processes all the time.
#login_process_per_connection = yes

# Number of login processes to create. If login_process_per_user is
# yes, this is the number of extra processes waiting for users to log in.
#login_processes_count = 3

# Maximum number of extra login processes to create. The extra process count
# usually stays at login_processes_count, but when multiple users start logging
# in at the same time more extra processes are created. To prevent fork-bombing
# we check only once in a second if new processes should be created - if all
# of them are used at the time, we double their amount until limit set by this
# setting is reached. This setting is used only if login_process_per_use is yes.
#login_max_processes_count = 128

# Maximum number of connections allowed in login state. When this limit is
# reached, the oldest connections are dropped. If login_process_per_user
# is no, this is a per-process value, so the absolute maximum number of users
# logging in actually login_processes_count * max_logging_users.
#login_max_logging_users = 256

##
## POP3 login process
##

# Settings default to same as above, so you don't have to set anything
# unless you want to override them.

login = pop3

# Exception to above rule being the executable location.
#login_executable = /usr/libexec/dovecot/pop3-login

##
## Mail processes
##

# Maximum number of running mail processes. When this limit is reached,
# new users aren't allowed to log in.
#max_mail_processes = 1024

# Show more verbose process titles (in ps). Currently shows user name and
# IP address. Useful for seeing who are actually using the IMAP processes
# (eg. shared mailboxes or if same uid is used for multiple accounts).
#verbose_proctitle = no

# Show protocol level SSL errors.
#verbose_ssl = no

# Valid UID range for users, defaults to 500 and above. This is mostly
# to make sure that users can't log in as daemons or other system users.
# Note that denying root logins is hardcoded to dovecot binary and can't
# be done even if first_valid_uid is set to 0.
first_valid_uid = 100
last_valid_uid = 200

# Valid GID range for users, defaults to non-root/wheel. Users having
# non-valid GID as primary group ID aren't allowed to log in. If user
# belongs to supplementary groups with non-valid GIDs, those groups are
# not set.
first_valid_gid = 100
last_valid_gid = 100

# ':' separated list of directories under which chrooting is allowed for mail
# processes (ie. /var/mail will allow chrooting to /var/mail/foo/bar too).
# This setting doesn't affect login_chroot or auth_chroot variables.
# WARNING: Never add directories here which local users can modify, that
# may lead to root exploit. Usually this should be done only if you don't
# allow shell access for users. See doc/configuration.txt for more information.
#valid_chroot_dirs = 

# Default MAIL environment to use when it's not set. By leaving this empty
# dovecot tries to do some automatic detection as described in
# doc/mail-storages.txt. There's a few special variables you can use:
#
#   %u - username
#   %n - user part in user at ...2622..., same as %u if there's no domain
#   %d - domain part in user at ...2622..., empty if user there's no domain
#   %h - home directory
#
# You can also limit a width of string by giving the number of max. characters
# after the '%' character. For example %1u gives the first character of
# username. Some examples:
#
#   maildir:/var/mail/%1u/%u/Maildir
#   mbox:~/mail/:INBOX=/var/mail/%u
#   mbox:/var/mail/%d/%n/:INDEX=/var/indexes/%d/%n
#
#default_mail_env =mbox:~/Mail/:INBOX=/var/spool/mail/%u 
default_mail_env = 
# Space-separated list of fields to cache for all mails. Currently these
# fields are allowed followed by a list of commands they speed up:
#
#  Envelope      - FETCH ENVELOPE and SEARCH FROM, TO, CC, BCC, SUBJECT,
#                  SENTBEFORE, SENTON, SENTSINCE, HEADER MESSAGE-ID,
#                  HEADER IN-REPLY-TO
#  Body          - FETCH BODY
#  Bodystructure - FETCH BODY, BODYSTRUCTURE
#  MessagePart   - FETCH BODY[1.2.3] (ie. body parts), RFC822.SIZE,
#                  SEARCH SMALLER, LARGER, also speeds up BODY/BODYSTRUCTURE
#                  generation. This is always set with mbox mailboxes, and
#                  also default with Maildir.
#
# Different IMAP clients work in different ways, that's why Dovecot by default
# only caches MessagePart which speeds up most operations. Whenever client
# does something where caching could be used, the field is automatically marked
# to be cached later. For example after FETCH BODY the BODY will be cached
# for all new messages. Normally you should leave this alone, unless you know
# what most of your IMAP clients are. Caching more fields than needed makes
# the index files larger and generate useless I/O.
#
# With maildir there's one extra optimization - if nothing is cached, indexing
# the maildir becomes much faster since it's not opening any of the mail files.
# This could be useful if your IMAP clients access only new mails.

#mail_cache_fields = MessagePart

# Space-separated list of fields that Dovecot should never set to be cached.
# Useful if you want to save disk space at the cost of more I/O when the fields
# needed.
#mail_never_cache_fields = 

# Workarounds for various client bugs:
#   oe6-fetch-no-newmail:
#     Never send EXISTS/RECENT when replying to FETCH command. Outlook Express
#     seems to think they are FETCH replies and gives user "Message no longer
#     in server" error. Note that OE6 still breaks even with this workaround
#     if synchronization is set to "Headers Only".
#   outlook-idle:
#     Outlook and Outlook Express never abort IDLE command, so if no mail
#     arrives in half a hour, Dovecot closes the connection. This is still
#     fine, except Outlook doesn't connect back so you don't see if new mail
#     arrives.
#client_workarounds = 

# Dovecot can notify client of new mail in selected mailbox soon after it's
# received. This setting specifies the minimum interval in seconds between
# new mail notifications to client - internally they may be checked more or
# less often. Setting this to 0 disables the checking.
# NOTE: Evolution client breaks with this option when it's trying to APPEND.
#mailbox_check_interval = 0

# Like mailbox_check_interval, but used for IDLE command.
#mailbox_idle_check_interval = 30

# Allow full filesystem access to clients. There's no access checks other than
# what the operating system does for the active UID/GID. It works with both
# maildir and mboxes, allowing you to prefix mailboxes names with eg. /path/
# or ~user/.
#mail_full_filesystem_access = no

# Maximum allowed length for custom flag name. It's only forced when trying
# to create new flags.
#mail_max_flag_length = 50

# Save mails with CR+LF instead of plain LF. This makes sending those mails
# take less CPU, especially with sendfile() syscall with Linux and FreeBSD.
# But it also creates a bit more disk I/O which may just make it slower.
#mail_save_crlf = no

# Use mmap() instead of read() to read mail files. read() seems to be a bit
# faster with my Linux/x86 and it's better with NFS, so that's the default.
#mail_read_mmaped = no

# Copy mail to another folders using hard links. This is much faster than
# actually copying the file. This is problematic only if something modifies
# the mail in one folder but doesn't want it modified in the others. I don't
# know any MUA which would modify mail files directly. IMAP protocol also
# requires that the mails don't change, so it would be problematic in any case.
# If you care about performance, enable it.
#maildir_copy_with_hardlinks = no

# Check if mails' content has been changed by external programs. This slows
# down things as extra stat() needs to be called for each file. If changes are
# noticed, the message is treated as a new message, since IMAP protocol
# specifies that existing message are immutable.
#maildir_check_content_changes = no

# Which locking methods to use for locking mbox. There's three available:
#  dotlock: Create <mailbox>.lock file. This is the oldest and most NFS-safe
#           solution. If you want to use /var/mail/ like directory, the users
#           will need write access to that directory.
#  fcntl  : Use this if possible. Works with NFS too if lockd is used.
#  flock  : May not exist in all systems. Doesn't work with NFS.
#
# You can use both fcntl and flock too; if you do the order they're declared
# with is important to avoid deadlocks if other MTAs/MUAs are using both fcntl
# and flock. Some operating systems don't allow using both of them
# simultaneously, eg. BSDs. If dotlock is used, it's always created first.
mbox_locks = fcntl

# Should we create dotlock file even when we want only a read-lock? Setting
# this to yes hurts the performance when the mailbox is accessed simultaneously
# by multiple processes, but it's needed for reliable reading if no other
# locking methods are available.
#mbox_read_dotlock = no

# Maximum time in seconds to wait for lock (all of them) before aborting.
#mbox_lock_timeout = 300

# If dotlock exists but the mailbox isn't modified in any way, override the
# lock file after this many seconds.
#mbox_dotlock_change_timeout = 30

# umask to use for mail files and directories
#umask = 0077

# Drop all privileges before exec()ing the mail process. This is mostly
# meant for debugging, otherwise you don't get core dumps. Note that setting
# this to yes means that log file is opened as the logged in user, which
# might not work. It could also be a small security risk if you use single UID
# for multiple users, as the users could ptrace() each others processes then.
#mail_drop_priv_before_exec = no

##
## IMAP process
##

# Executable location
#imap_executable = /usr/libexec/dovecot/imap

# Set max. process size in megabytes. Most of the memory goes to mmap()ing
# files, so it shouldn't harm much even if this limit is set pretty high.
#imap_process_size = 256

# Support for dynamically loadable modules.
#imap_use_modules = no
#imap_modules = /usr/lib/dovecot/imap

##
## POP3 process
##

# Executable location
#pop3_executable = /usr/libexec/dovecot/pop3

# Set max. process size in megabytes. Most of the memory goes to mmap()ing
# files, so it shouldn't harm much even if this limit is set pretty high.
#pop3_process_size = 256

# Support for dynamically loadable modules.
#pop3_use_modules = no
#pop3_modules = /usr/lib/dovecot/pop3

##
## Authentication processes
##

# You can have multiple processes; each time "auth = xx" is seen, a new
# process definition is started. The point of multiple processes is to be
# able to set stricter permissions to others. For example, plain/PAM
# authentication requires roots, but if you also use digest-md5 authentication
# for some users, you can authenticate them without any privileges in a
# separate auth process. Just remember that only one auth process is asked
# for the password, so you can't have different passwords with different
# processes (unless they have different auth methods, and you're ok with
# having different password for each method).

# Authentication process name.
auth = default

# Space separated list of wanted authentication mechanisms:
#   plain digest-md5 anonymous
auth_mechanisms = plain

# Space separated list of realms for SASL authentication mechanisms that need
# them. You can leave it empty if you don't want to support multiple realms.
# Many clients simply use the first one listed here, so keep the default realm
# first.
#auth_realms =

# Default realm to use if none was specified.
#auth_default_realm = 

# Where user database is kept:
#   passwd: /etc/passwd or similiar, using getpwnam()
#   passwd-file <path>: passwd-like file with specified location
#   static uid=<uid> gid=<gid> home=<dir template>: static settings
#   vpopmail: vpopmail library
#   ldap <config path>: LDAP, see doc/dovecot-ldap.conf
#   pgsql <config path>: a PostgreSQL database, see doc/dovecot-pgsql.conf
auth_userdb = passwd

# Where password database is kept:
#   passwd: /etc/passwd or similiar, using getpwnam()
#   shadow: /etc/shadow or similiar, using getspnam()
#   pam [<service> | *]: PAM authentication
#   passwd-file <path>: passwd-like file with specified location
#   vpopmail: vpopmail authentication
#   ldap <config path>: LDAP, see doc/dovecot-ldap.conf
#   pgsql <config path>: a PostgreSQL database, see doc/dovecot-pgsql.conf
auth_passdb = pam

# Executable location
#auth_executable = /usr/libexec/dovecot/dovecot-auth

# Set max. process size in megabytes.
#auth_process_size = 256

# User to use for the process. This user needs access to only user and
# password databases, nothing else. Only shadow and pam authentication
# requires roots, so use something else if possible.
auth_user = root

# Directory where to chroot the process. Most authentication backends don't
# work if this is set, and there's no point chrooting if auth_user is root.
#auth_chroot = 

# Number of authentication processes to create
#auth_count = 1

# List of allowed characters in username. If the user-given username contains
# a character not listed in here, the login automatically fails. This is just
# an extra check to make sure user can't exploit any potential quote escaping
# vulnerabilities with SQL/LDAP databases. If you want to allow all characters,
# set this value to empty.
#auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@

# Username to use for users logging in with ANONYMOUS SASL mechanism
#auth_anonymous_username = anonymous

# More verbose logging. Useful for figuring out why authentication isn't
# working.
#auth_verbose = no

# digest-md5 authentication process. It requires special MD5 passwords which
# /etc/shadow and PAM doesn't support, so we never need roots to handle it.
# Note that the passwd-file is opened before chrooting and dropping root
# privileges, so it may be 0600-root owned file.

#auth = digest_md5
#auth_methods = digest-md5
#auth_realms = 
#auth_userdb = passwd-file /etc/passwd.imap
#auth_passdb = passwd-file /etc/passwd.imap
#auth_user = imapauth
#auth_chroot = 

# if you plan to use only passwd-file, you don't need the two auth processes,
# simply set "auth_methods = plain digest-md5"


More information about the Snort-sigs mailing list