The war over spam has
erupted recently in a new arena: weblog comments.
The parallels to the battles that have been fought on the email spam
front are considerable, but unlike email spam, weblog spam is targeted
at Google (and other search engines that use number of links to derive
page rankings) to increase the visibility of the sites that are being
advertised via spam. Comment spam seems to be on the rise with weblog
owners noticing a large increase in the number of incidents over the last
month or two.
Weblogs are sites that allow the owner to post articles and essays of
whatever happens to strike their fancy that day and most weblog software
enables readers to post comments on the stories. LWN's comment system provides
the same feature for this site but, unlike LWN comments, many weblogs allow
(and even encourage) anonymous comments. That openness, like the lack
of sender authentication for email, provides an avenue for abuse. Requiring
registration before allowing comments does not eliminate the problem
entirely (LWN has had a small amount of comment spam), but it does increase
the amount of work the spammer must do.
The basic mode of attack uses a program to automatically post comments
on multiple articles throughout the weblog. These unwanted messages include
the URL of a website that
will give you the opportunity to buy one or more of the usual items:
diplomas, prescription drugs, porn,
etc. The program then moves on to other sites using the same software,
aided, no doubt, by the various directories of weblogs using a particular
software package that are available. Eventually, Google and other search
engines visit the weblog sites; thereafter, the
spammer's site gains a high ranking due to all of the links to it that are
found.
One of the more popular (though not entirely free) packages for running a
weblog is
Movable Type; its user
community has been the most active so far in combating comment spam.
For example,
one set of tips
(described by Yoz Grahame)
attempts to thwart the way the current spam programs work by changing
the default behavior of the software. Something as simple as changing the
"post a comment" link can be sufficient to confuse most automated comment
posting scripts. These techniques will only help until
enough people implement them and it makes it worth the effort for a
spammer to write more adaptable code to circumvent them.
Many of the other comment spam handling techniques will seem very familiar
to anyone who has been dealing with the deluge of email spam:
bayesian filtering
and
blacklisting
based on the URLs in the comment and/or user profile are two of the more
popular techniques.
Bayesian filtering uses the frequency of words in
a message and a database of word counts
from previous messages that have been categorized as spam or non-spam
(often called "ham") to determine a probability that the new message is
spam. If the probability is too high, the message is rejected.
The blacklisting patch collects the URLs that are advertised in the offending
messages and rejects any comments that refer to any of those URLs.
Both of these techniques can be worked around by a spammer with enough
incentive, but it does make it much more difficult.
Another technique that is becoming more popular is email and web-based
challenge-response systems which generate a blurry graphic that is (presumably)
only readable by humans. Such systems require that the text in the graphic be typed
into a form to ensure that a human, and not a program, is initiating the
action. This technique, too, has made its way into the arsenal of webloggers
via
this plug-in
for Movable Type.
This scheme does have a number of downsides because it requires a graphical
browser to post messages and may be unusable by the visually impaired.
Other weblogging software developers may have run into this problem and come up
with their own sets of fixes, but the Movable Type community appears to be
the at the forefront of this particular battle. Perhaps the spammers have
yet to target other systems
in an automated way. If (or more likely when) they do, newly targeted weblogging software can
use one or more of the techniques above to combat the spam.
Both weblog comment and email spam fighters are running into the same issues
and producing similar solutions in many cases and cooperation between the
two groups will lead to better spam fighting.
One of the future plans for Jay Allen's blacklist
(above) is to create a distributed list of URLs that are being advertised
via spam and with proper controls one can imagine that list being useful
to the email spam fighting crowd. A filter using the rules for email
message bodies in
SpamAssassin might be useful
for folks confronting spam in their weblog comments as well.
Another buffer overflow in Apache 2.0.47 and earlier in mod_cgid's
mishandling of CGI redirect paths could result in CGI output going to the
wrong client when a threaded MPM is used.
CAN-2003-0789.
libnids (a NIDS plugin which emulates the Linux 2.0 IP stack) contains a buffer overflow vulnerability which can be exploited remotely. Version 1.18 fixes the problem.
The thttpd web server has a pair of vulnerabilities which can lead to information disclosure and arbitrary code execution; both are remotely exploitable.
Several security issues have been discovered affecting the Linux kernel:
CAN-2003-0461: /proc/tty/driver/serial reveals the exact character
counts for serial links. This could be used by a local attacker to infer
password lengths and inter-keystroke timings during password entry.
CAN-2003-0462: Paul Starzetz discovered a file read race condition
existing in the execve() system call, which could cause a local crash.
CAN-2003-0464: A recent change in the RPC code set the reuse flag on
newly-created sockets. Olaf Kirch noticed that his could allow normal
users to bind to UDP ports used for services such as nfsd.
CAN-2003-0476: The execve system call in Linux 2.4.x records the file
descriptor of the executable process in the file table of the calling
process, allowing local users to gain read access to restricted file
descriptors.
CAN-2003-0501: The /proc filesystem in Linux allows local users to
obtain sensitive information by opening various entries in /proc/self
before executing a setuid program. This causes the program to fail to
change the ownership and permissions of already opened entries.
CAN-2003-0550: The STP protocol is known to have no security, which
could allow attackers to alter the bridge topology. STP is now turned
off by default.
CAN-2003-0551: STP input processing was lax in its length checking,
which could lead to a denial of service.
CAN-2003-0552: Jerry Kreuscher discovered that the Forwarding table
could be spoofed by sending forged packets with bogus source addresses
the same as the local host.
A problem was discovered in Apache2 where CGI scripts that write more than
4k to the standard error stream will hang the script's execution. This problem can lead to a
denial of service situation. See this bug
report for additional details.
Several security problems have been found in Ethereal
0.9.12. "It may be possible to make Ethereal crash or run
arbitrary code by injecting a purposefully malformed packet onto the wire,
or by convincing someone to read a malformed packet trace file."
"fam" (file alteration monitor) watches files and directories for changes and lets interested applications know when something happens. This package has a flaw in its group handling that blocks some legitimate operations while, at the same time, exposing the names of files that should otherwise be invisible.
There is, it seems, an integer overflow vulnerability in "ls" which can be exploited via wu-ftpd to create a denial of service situation. See this advisory from Georgi Guninski for details.
The GNU C library contains a buffer overflow in the getgrouplist() function. If the user belongs to more groups than the calling application expects, the allocated storage will be overrun.
DNS stub resolvers from multiple vendors contain a buffer overflow
vulnerability. The impact of this vulnerability appears to be limited to
denial of service. (See CERT Vulnerability Note
VU#738331)
The BIND 4 and BIND 8.2.x stub resolver libraries, and other libraries such
as glibc 2.2.5 and earlier, libc, and libresolv, uses the maximum buffer
size instead of the actual size when processing a DNS response, which
causes the stub resolvers to read past the actual boundary ("read buffer
overflow"), allowing remote attackers to cause a denial of service
(crash).
A key validation bug was discovered in the GNU Privacy Guard (GPG) which
would cause keys with more then one user ID to trust all user ID's with the
amount of trust given to the most-valid user ID.
GtkHTML is the HTML rendering widget used by the Evolution mail reader.
GtkHTML supplied with versions of Evolution prior to 1.2.4 contain a bug
when handling HTML messages. Alan Cox discovered that certain malformed
messages could cause the Evolution mail component to crash.
Piotr Kucharski reported a buffer
overflow vulnerability that may allow an attacker to crash the ircd server,
thus causing a denial of service condition. The Common Vulnerabilities and
Exposures project (cve.mitre.org) has assigned the name
CAN-2003-0864 to this issue.
The kernel-utils package contains several utilities that can be used to
control the kernel or machine hardware. In Red Hat Linux 8.0 this package
contains user mode linux (UML) utilities.
The uml_net utility in kernel-utils packages with Red Hat Linux 8.0 was
incorrectly shipped setuid root. This could allow local users to control
certain network interfaces, add and remove arp entries and routes, and put
interfaces in and out of promiscuous mode.
All users of the kernel-utils package should update to these packages that
contain a version of uml_net that is not setuid root.
Alternatively, as a work-around to this vulnerability issue the following
command as root:
Glenn Randers-Pehrson discovered a problem in connection with 16-bit
samples from libpng, an interface for reading and writing PNG
(Portable Network Graphics) format files. The starting offsets for
the loops are calculated incorrectly which causes a buffer overrun
beyond the beginning of the row buffer.
A remotely exploitable buffer overflow vulnerability was found in
MPlayer. A malicious host can craft a harmful ASX header, and trick MPlayer
into executing arbitrary code upon parsing that header. Read the full advisory
for details.
Some some vulnerabilities exsist in the Nessus NASL scripting engine. To
exploit these flaws, an attacker would need to have a valid Nessus account
as well as the ability to upload arbitrary Nessus plugins in the Nessus
server (this option is disabled by default) or he/she would need to trick a
user somehow into running a specially crafted nasl script. Read the full
advisory for additional information.
Linux NFS utils package contains remotely exploitable off-by-one bug.
A local or remote attacker could exploit this vulnerability by sending
specially crafted request to rpc.mountd daemon. See this BugTraq post for more details.
From the advisory:
"During a pen-test we stumbled across a nasty bug in OpenSSH-portable
with PAM support enabled (via the --with-pam configure script switch). This
bug allows a remote attacker to identify valid users on vulnerable systems,
through a simple timing attack. The vulnerability is easy to exploit and
may have high severity, if combined with poor password policies and other
security problems that allow local privilege escalation."
Vulnerabilities have been found in OpenSSL ASN.1 code. This advisory contains details of 4 separate
problems in versions of OpenSSL up to and including 0.9.6j and 0.9.7b and
all versions of SSLeay.
An attack against other applications that use OpenSSL could result in a
Denial of Service. See
CAN-2003-0543 and
CAN-2003-0544.
It may be possible for an attacker to exploit this issue to execute
arbitrary code. See
CAN-2003-0545.
The postfix MTA, versions through 1.1.12 (but not 2.0) is subject to two remotely exploitable denial of service vulnerabilities; see this advisory from Michal Zalewski for details.
A new set of buffer overflows has been discovered in PostgreSQL 7.2.2; they affect the circle_poly(), path_encode(), and path_addr() functions. Exploiting these overflows requires that the attacker first obtain a connection to the PostgreSQL server.
The ASCII translation mechanism in ProFTPD 1.2.8 contains a vulnerability which will provide a remote attacker with a root shell - if the attacker is able to download a specially-crafted file. See this ISS advisory for more information.
usePerl has a
description of a vulnerability in the Safe.pm Perl module. It seems
that if a Safe compartment is used more than once, it ceases to be safe.
The problem is fixed in Safe 2.08.
Alexander Hvostov, Julien Blache and Aurelien Jarno discovered several
security-related problems in the sane-backends package, which contains
an API library for scanners including a scanning daemon (in the
package libsane) that can be remotely exploited. These problems allow
a remote attacker to cause a segfault fault and/or consume arbitrary
amounts of memory. The attack is successful, even if the attacker's
computer isn't listed in saned.conf.
You are only vulnerable if you actually run saned e.g. in xinetd or
inetd. If the entries in the configuration file of xinetd or inetd
respectively are commented out or do not exist, you are safe.
Try "telnet localhost 6566" on the server that may run saned. If you
get "connection refused" saned is not running and you are safe.
The Common Vulnerabilities and Exposures project identifies the
following problems:
CAN-2003-0773: saned checks the identity (IP address) of the remote
host only after the first communication took place (SANE_NET_INIT). So
everyone can send that RPC, even if the remote host is not allowed to
scan (not listed in saned.conf).
CAN-2003-0774: saned lacks error checking nearly everywhere in the
code. So connection drops are detected very late. If the drop of the
connection isn't detected, the access to the internal wire buffer leaves
the limits of the allocated memory. So random memory "after" the wire
buffer is read which will be followed by a segmentation fault.
CAN-2003-0775: If saned expects strings, it mallocs the memory
necessary to store the complete string after it receives the size of the
string. If the connection was dropped before transmitting the size,
malloc will reserve an arbitrary size of memory. Depending on that size
and the amount of memory available either malloc fails (->saned quits
nicely) or a huge amount of memory is allocated. Swapping and OOM
measures may occur depending on the kernel.
CAN-2003-0776: saned doesn't check the validity of the RPC numbers
it gets before getting the parameters.
CAN-2003-0777: If debug messages are enabled and a connection is
dropped, non-null-terminated strings may be printed and segmentation
faults may occur.
CAN-2003-0778: It's possible to allocate an arbitrary amount of
memory on the server running saned even if the connection isn't dropped.
At the moment this can not easily be fixed according to the author.
Better limit the total amount of memory saned may use (ulimit).
Michal Zalewski has reported a buffer overflow in sendmail. This overflow, apparently, may be exploited remotely, but only in certain (non-default) configurations. Sendmail 8.12.10 has the fix.
Stunnel is a wrapper for network connections. It can be used to tunnel an
unencrypted network connection over a secure connection (encrypted using
SSL or TLS) or to provide a secure means of connecting to services that do
not natively support encryption.
When configured to listen for incoming connections (instead of being
invoked by xinetd), stunnel can be configured to either start a thread or a
child process to handle each new connection. If Stunnel is configured to
start a new child process to handle each connection, it will receive a
SIGCHLD signal when that child exits.
Stunnel versions prior to 4.04 would perform tasks in the SIGCHLD signal
handler which, if interrupted by another SIGCHLD signal, could be unsafe.
This could lead to a denial of service.
The tar utility does not properly filter file names containing
"../", meaning that a hostile archive can, if unpacked by an
unsuspecting user, overwrite any file that is writable by that user. GNU
tar versions 1.13.19 and earlier are vulnerable; unzip through version 5.42
has the same vulnerability.
This vulnerability,
originally thought to be confined to BSD-derived systems, was first covered
in the July 26th Security
Summary. It is now known that Linux telnet daemons are vulnerable as
well.
A vulnerabilitiy in unzip version 5.50 and earlier allows attackers to
overwrite arbitrary files during archive extraction by placing invalid
(non-printable) characters between two "." characters. These non-printable
characters are filtered, resulting in a ".." sequence. See the full
advisory for further information.
VIM allows a user to set the modeline differently for each edited text file
by placing special comments in the files. Georgi Guninski found that these
comments can be carefully crafted in order to call external programs. This
could allow an attacker to create a text file such that when it is opened
arbitrary commands are executed.
miniserv.pl in the webmin package does not properly handle
metacharacters, such as line feeds and carriage returns, in
Base64-encoded strings used in Basic authentication. This
vulnerability allows remote attackers to spoof a session ID, and
thereby gain root privileges.
Several vulnerabilities were discovered by blexim(at)hush.com in the font
libraries of XFree86 version 4.3.0 and earlier. These bugs could
potentially lead to execution of arbitrary code or a DoS by a remote user
in any way that calls these functions, which are related to the transfer
and enumeration of fonts from font servers to clients. See the
advisory for additional details.
Xinetd is a 'master server' that is used to to accept service connection
requests and start the appropriate servers.
Because of a programming error, memory was allocated and never freed if a
connection was refused for any reason. An attacker could exploit this flaw
to crash the xinetd server, rendering all services it controls unavailable.
In addition, other flaws in xinetd could cause incorrect operation in
certain unusual server configurations.
All users of xinetd are advised to update to xinetd-2.3.11 which is not
vulnerable to these issues.
LinuxQuestions.org interviews
Brian Hatch, author of Hacking Linux Exposed. "So true,
not everyone can read and understand the code that they end up running, and
not anyone can read all of the code that they end up running. There's a
level of trust, and that's no different than when you run proprietary
software. The big difference is the number of individuals who do view that
code."
