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Securing Debian Manual
Chapter 10 - Before the compromise


10.1 Keep your system secure

You should strive to keep your system secure by monitoring its usage and also the vulnerabilities that might affect it, patching them as soon as patches are available. Even though you might have installed a really secure system initially you have to remember that security in a system degrades with time, security vulnerabilities might be found for exposed system services and users might expose the system security either because of lack of understanding (e.g. accessing a system remotely with a clear-text protocol or using easy to guess passwords) or because they are actively trying to subvert the system's security (e.g. install additional services locally on their accounts).


10.1.1 Tracking security vulnerabilities

Although most administrators are aware of security vulnerabilities affecting their systems when they see a patch that is made available you can strive to keep ahead of attacks and introduce temporary countermeasures for security vulnerabilities by detecting when your system is vulnerable. This is specially true when running an exposed system (i.e. connected to the Internet) and providing a service. In such case the system's administrators should take care to monitor known information sources to be the first to know when a vulnerability is detected that might affect a critical service.

This typically includes subscribing to the announcement mailing lists, project websites or bug tracking systems provided by the software developers for a specific piece of code. For example, Apache users should regularly review Apache's lists of security vulnerabilities and subscribe to the Apache Server Announcements mailing list.

In order to track known vulnerabilities affecting the Debian distribution, the Debian Testing Security Team provides a security tracker that lists all the known vulnerabilities which have not been yet fixed in Debian packages. The information in that tracker is obtained through different public channels and includes known vulnerabilities which are available either through security vulnerability databases or Debian's Bug Tracking system. Administrators can search for the known security issues being tracked for stable, oldstable, testing, or unstable.

The tracker has searchable interfaces (by CVE name and package name) and some tools (such as debsecan, see Automatically checking for security issues with debsecan, Section 10.1.2.4) use that database to provide information of vulnerabilities affecting a given system which have not yet been addressed (i.e. those who are pending a fix).

Concious administrators can use that information to determine which security bugs might affect the system they are managing, determine the severity of the bug and apply (if available) temporary countermeasures before a patch is available fixing this issue.

Security issues tracked for releases supported by the Debian Security Team should eventually be handled through Debian Security Advisories (DSA) and will be available for all users (see Continuously update the system, Section 10.1.2). Once security issues are fixed through an advisory they will not be available in the tracker, but you will be able to search security vulnerabilities (by CVE name) using the security cross references table available for published DSAs.

Notice, however, that the information tracked by the Debian Testing Security Team only involves disclosed vulnerabilities (i.e. those already public). In some occasions the Debian Security Team might be handling and preparing DSAs for packages based on undisclosed information provided to them (for example, through closed vendor mailing lists or by upstream maintainers of software). So do not be surprised to find security issues that only show up as an advisory but never get to show up in the security tracker.


10.1.2 Continuously update the system

You should conduct security updates frequently. The vast majority of exploits result from known vulnerabilities that have not been patched in time, as this paper by Bill Arbaugh (presented at the 2001 IEEE Symposium on Security and Privacy) explains. Updates are described under Execute a security update, Section 4.2.


10.1.2.1 Manually checking which security updates are available

Debian does have a specific tool to check if a system needs to be updated but many users will just want to manually check if any security updates are available for their system.

If you have configured your system as described in Execute a security update, Section 4.2 you just need to do:

     # apt-get update
     # apt-get upgrade -s
     [ ... review packages to be upgraded ... ]
     # apt-get upgrade 
     # checkrestart
     [ ... restart services that need to be restarted ... ]

And restart those services whose libraries have been updated if any. Note: Read Execute a security update, Section 4.2 for more information on library (and kernel) upgrades.

The first line will download the list of packages available from your configured package sources. The -s will do a simulation run, that is, it will not download or install the packages but rather tell you which ones should be downloaded/installed. From the output you can derive which packages have been fixed by Debian and are available as a security update. Sample:

     # apt-get upgrade -s
     Reading Package Lists... Done
     Building Dependency Tree... Done
     2 packages upgraded, 0 newly installed, 0 to remove and 0  not upgraded.
     Inst cvs (1.11.1p1debian-8.1 Debian-Security:3.0/stable)
     Inst libcupsys2 (1.1.14-4.4 Debian-Security:3.0/stable)
     Conf cvs (1.11.1p1debian-8.1 Debian-Security:3.0/stable)
     Conf libcupsys2 (1.1.14-4.4 Debian-Security:3.0/stable)

In this example, you can see that the system needs to be updated with new cvs and cupsys packages which are being retrieved from woody's security update archive. If you want to understand why these packages are needed, you should go to http://security.debian.org and check which recent Debian Security Advisories have been published related to these packages. In this case, the related DSAs are DSA-233 (for cvs) and DSA-232 (for cupsys).

Notice that you will need to reboot your system if there has been a kernel upgrade.


10.1.2.2 Checking for updates at the Desktop

Since Debian 4.0 lenny Debian provides and installs in a default installation update-notifier. This is a GNOME application that will startup when you enter your Desktop and can be used to keep track of updates available for your system and install them. It uses update-manager for this.

In a stable system updates are only available when a security patch is available or at point releases. Consequently, if the system is properly configured to receive security updates as described in Execute a security update, Section 4.2 and you have a cron task running to update the package information you will be notified through an icon in the desktop notifcation area.

The notification is not intrusive and users are not forced to install updates. From the notification icon a desktop user (with the administrator's password) can access a simple GUI to show available updates and install them.

This application works by checking the package database and comparing the system with its contents. If the package database is updated periodically through a cron task then the contents of the database will be newer than the packages installed in the system and the application will notify you.

Apt installs such a task (/etc/cron.d/apt) which will run based on Apt's configuration (more specifically APT::Periodic). In the GNOME environment this configuration value can be adjusted by going to System > Admin > Software origins > Updates, or running /usr/bin/software-properties.

If the system is set to download the packages list daily but not download the packages themselves your /etc/apt/apt.conf.d/10periodic should look like this:

     APT::Periodic::Update-Package-Lists "1";
     APT::Periodic::Download-Upgradeable-Packages "0";

You can use a different cron task, such as the one installed by cron-apt (see Automatically checking for updates with cron-apt, Section 10.1.2.3). You can also just manually check for upgrades using this application.

Users of the KDE desktop environment will probably prefer to install adept and adept-notifier instead which offers a similar functionality but is not part of the standard installation.


10.1.2.3 Automatically checking for updates with cron-apt

Another method for automatic security updates is the use of cron-apt. This package provides a tool to update the system at regular intervals (using a cron job), and can also be configured to send mails to the system administrator using the local mail transport agent. It will just update the package list and download new packages by default but it can be configured to automatically install new updates.

Notice that you might want to check the distribution release, as described in Per distribution release check, Section 7.5.3, if you intend to automatically updated your system (even if only downloading the packages). Otherwise, you cannot be sure that the downloaded packages really come from a trusted source.

More information is available at the Debian Administration site.


10.1.2.4 Automatically checking for security issues with debsecan

The debsecan program evaluates the security status of by reporting both missing security updates and security vulnerabilities. Unlike cron-apt, which only provides information related to security updates available, but this tool obtains information from the security vulnerability database maintained by the Debian Security Team which includes also information on vulnerabilities which are not yet fixed through a security update. Consequently, it is more efficient at helping administrators track security vulnerabilities (as described in Tracking security vulnerabilities, Section 10.1.1).

Upon installing the Debian package debsecan, and if the administrator consents to it, it will generate a cron task that will make it run and send the output to a specific user whenever it finds a vulnerable package. It will also download the information from the Internet. The location of the security database is also part of the questions ask on installation and are later defined /etc/default/debsecan, it can be easily adjusted for systems that do not have Internet access so that they all pull from a local mirror so that there is a single point that access the vulnerability database.

Notice, however, that the Security Team tracks many vulnerabilities including low-risk issues which might not be fixed through a security update and some vulnerabilities initially reported as affecting Debian might, later on, upon investigation, be dismissed. Debsecan will report on all the vulnerabilities, which makes it a quite more verbose than the other tools described above.

More information is available at the author's siste.


10.1.2.5 Other methods for security updates

There is also the apticron, which, similarly to cron-apt will check for updates and send mails to the administrator. More information on apticron is available at the Debian Administration site.

You might also want to take a look at secpack which is an unofficial program to do security updates from security.debian.org with signature checking written by Fruhwirth Clemens. Or to the Nagios Plugin check_debian_updates.sh written by Dean Wilson.


10.1.3 Avoid using the unstable branch

Unless you want to dedicate time to patch packages yourself when a vulnerability arises, you should not use Debian's unstable branch for production-level systems. The main reason for this is that there are no security updates for unstable (see How is security handled for testing and unstable?, Section 12.3.8).

The fact is that some security issues might appear in unstable and not in the stable distribution. This is due to new functionality constantly being added to the applications provided there, as well as new applications being included which might not yet have been thoroughly tested.

In order to do security upgrades in the unstable branch, you might have to do full upgrades to new versions (which might update much more than just the affected package). Although there have been some exceptions, security patches are usually only back ported into the stable branch. The main idea being that between updates, no new code should be added, just fixes for important issues.

Notice, however, that you can use the security tracker (as described in Tracking security vulnerabilities, Section 10.1.1) to track known security vulnerabilities affecting this branch.


10.1.4 Security support for the testing branch

If you are using the testing branch, there are some issues that you must take into account regarding the availability of security updates:

This behavior might change based on the release state of the distribution. When a release is almost imminent, the Security Team or package maintainers might provide updates directly to testing.

Additionally, the Debian Testing Security Team can issue Debian Testing Security Advisories (DTSAs) for packages in the testing branch if there is an inmediate need to fix a security issue in that branch and cannot wait for the normal procedure (or the normal procedure is being blocked by some other packages).

Users willing to take advantage of this support should add the following lines to their /etc/apt/sources.list (instead of the lines described in Execute a security update, Section 4.2):

         deb http://security.debian.org testing/updates main contrib non-free
     # This line makes it possible to donwload source packages too
         deb-src  http://security.debian.org testing/updates main contrib non-free

For additional information on this support please read the announcement. This support officially started in September 2005 in a separate repository and was later integrated into the main security archive.


10.1.5 Automatic updates in a Debian GNU/Linux system

First of all, automatic updates are not fully recommended, since administrators should review the DSAs and understand the impact of any given security update.

If you want to update your system automatically you should:

A safer alternative might be to use the -d (or --download-only) option, which will download but not install the necessary packages. Then if the cron execution shows that the system needs to be updated, it can be done manually.

In order to accomplish any of these tasks, the system must be properly configured to download security updates as discussed in Execute a security update, Section 4.2.

However, this is not recommended for unstable without careful analysis, since you might bring your system into an unusable state if some serious bug creeps into an important package and gets installed in your system. Testing is slightly more secure with regard to this issue, since serious bugs have a better chance of being detected before the package is moved into the testing branch (although, you may have no security updates available whatsoever).

If you have a mixed distribution, that is, a stable installation with some packages updated to testing or unstable, you can fiddle with the pinning preferences as well as the --target-release option in apt-get to update only those packages that you have updated.[71]


10.2 Do periodic integrity checks

Based on the baseline information you generated after installation (i.e. the snapshot described in Taking a snapshot of the system, Section 4.19), you should be able to do an integrity check from time to time. An integrity check will be able to detect filesystem modifications made by an intruder or due to a system administrators mistake.

Integrity checks should be, if possible, done offline.[72] That is, without using the operating system of the system to review, in order to avoid a false sense of security (i.e. false negatives) produced by, for example, installed rootkits. The integrity database that the system is checked against should also be used from read-only media.

You can consider doing integrity checks online using any of the filesystem integrity tools available (described in Checking file system integrity, Section 4.17.3) if taking offline the system is not an option. However, precaution should be taken to use a read-only integrity database and also assure that the integrity checking tool (and the operating system kernel) has not been tampered with.

Some of the tools mentioned in the integrity tools section, such as aide, integrit or samhain are already prepared to do periodic reviews (through the crontab in the first two cases and through a standalone daemon in samhain) and can warn the administrator through different channels (usually e-mail, but samhain can also send pages, SNMP traps or syslog alerts) when the filesystem changes.

Of course, if you execute a security update of the system, the snapshot taken for the system should be re-taken to accommodate the changes done by the security update.


10.3 Set up Intrusion Detection

Debian GNU/Linux includes tools for intrusion detection, which is the practice of detecting inappropriate or malicious activity on your local system, or other systems in your private network. This kind of defense is important if the system is very critical or you are truly paranoid. The most common approaches to intrusion detection are statistical anomaly detection and pattern-matching detection.

Always be aware that in order to really improve the system's security with the introduction of any of these tools, you need to have an alert+response mechanism in place. Intrusion detection is a waste of time if you are not going to alert anyone.

When a particular attack has been detected, most intrusion detection tools will either log the event with syslogd or send e-mail to the root user (the mail recipient is usually configurable). An administrator has to properly configure the tools so that false positives do not trigger alerts. Alerts may also indicate an ongoing attack and might not be useful, say, one day later, since the attack might have already succeeded. So be sure that there is a proper policy on handling alerts and that the technical mechanisms to implement this policy are in place.

An interesting source of information is CERT's Intrusion Detection Checklist


10.3.1 Network based intrusion detection

Network based intrusion detection tools monitor the traffic on a network segment and use this information as a data source. Specifically, the packets on the network are examined, and they are checked to see if they match a certain signature.

snort is a flexible packet sniffer or logger that detects attacks using an attack signature dictionary. It detects a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, and much more. snort also has real-time alerting capability. You can use snort for a range of hosts on your network as well as for your own host. This is a tool which should be installed on every router to keep an eye on your network. Just install it with apt-get install snort, follow the questions, and watch it log. For a little broader security framework, see Prelude.

Debian's snort package has many security checks enabled by default. However, you should customize the setup to take into account the particular services you run on your system. You may also want to seek additional checks specific to these services.

There are other, simpler tools that can be used to detect network attacks. portsentry is an interesting package that can tip you off to port scans against your hosts. Other tools like ippl or iplogger will also detect some IP (TCP and ICMP) attacks, even if they do not provide the kind of advanced techniques snort does.

You can test any of these tools with the Debian package idswakeup, a shell script which generates false alarms, and includes many common attack signatures.


10.3.2 Host based intrusion detection

Host based intrusion detection involves loading software on the system to be monitored which uses log files and/or the systems auditing programs as a data source. It looks for suspicious processes, monitors host access, and may even monitor changes to critical system files.

tiger is an older intrusion detection tool which has been ported to Debian since the Woody branch. tiger provides checks of common issues related to security break-ins, like password strength, file system problems, communicating processes, and other ways root might be compromised. This package includes new Debian-specific security checks including: MD5sums checks of installed files, locations of files not belonging to packages, and analysis of local listening processes. The default installation sets up tiger to run each day, generating a report that is sent to the superuser about possible compromises of the system.

Log analysis tools, such as logcheck can also be used to detect intrusion attempts. See Using and customizing logcheck, Section 4.13.1.

In addition, packages which monitor file system integrity (see Checking file system integrity, Section 4.17.3) can be quite useful in detecting anomalies in a secured environment. It is most likely that an effective intrusion will modify some files in the local file system in order to circumvent local security policy, install Trojans, or create users. Such events can be detected with file system integrity checkers.


10.4 Avoiding root-kits


10.4.1 Loadable Kernel Modules (LKM)

Loadable kernel modules are files containing dynamically loadable kernel components used to expand the functionality of the kernel. The main benefit of using modules is the ability to add additional devices, like an Ethernet or sound card, without patching the kernel source and recompiling the entire kernel. However, crackers are now using LKMs for root-kits (knark and adore), opening up back doors in GNU/Linux systems.

LKM back doors are more sophisticated and less detectable than traditional root-kits. They can hide processes, files, directories and even connections without modifying the source code of binaries. For example, a malicious LKM can force the kernel into hiding specific processes from procfs, so that even a known good copy of the binary ps would not list accurate information about the current processes on the system.


10.4.2 Detecting root-kits

There are two approaches to defending your system against LKM root-kits, a proactive defense and a reactive defense. The detection work can be simple and painless, or difficult and tiring, depending on the approach taken.


10.4.2.1 Proactive defense

The advantage of this kind of defense is that it prevents damage to the system in the first place. One such strategy is getting there first, that is, loading an LKM designed to protect the system from other malicious LKMs. A second strategy is to remove capabilities from the kernel itself. For example, you can remove the capability of loadable kernel modules entirely. Note, however, that there are rootkits which might work even in this case, there are some that tamper with /dev/kmem (kernel memory) directly to make themselves undetectable.

Debian GNU/Linux has a few packages that can be used to mount a proactive defense:

If you don't really need many kernel features on your GNU/Linux system, you may want to disable loadable modules support during kernel configuration. To disable loadable module support, just set CONFIG_MODULES=n during the configuration stage of building your kernel, or in the .config file. This will prevent LKM root-kits, but you lose this powerful feature of the Linux kernel. Also, disabling loadable modules can sometimes overload the kernel, making loadable support necessary.


10.4.2.2 Reactive defense

The advantage of a reactive defense is that it does not overload system resources. It works by comparing the system call table with a known clean copy in a disk file, System.map. Of course, a reactive defense will only notify the system administrator after the system has already been compromised.

Detection of some root-kits in Debian can be accomplished with the chkrootkit package. The Chkrootkit program checks for signs of several known root-kits on the target system, but is not a definitive test.


10.5 Genius/Paranoia Ideas — what you could do

This is probably the most unstable and funny section, since I hope that some of the "duh, that sounds crazy" ideas might be realized. The following are just some ideas for increasing security — maybe genius, paranoid, crazy or inspired depending on your point of view.


10.5.1 Building a honeypot

A honeypot is a system designed to teach system administrators how crackers probe for and exploit a system. It is a system setup with the expectation and goal that the system will be probed, attacked and potentially exploited. By learning the tools and methods employed by the cracker, a system administrator can learn to better protect their own systems and network.

Debian GNU/Linux systems can easily be used to setup a honeynet, if you dedicate the time to implement and monitor it. You can easily setup the fake honeypot server as well as the firewall[75] that controls the honeynet and some sort of network intrusion detector, put it on the Internet, and wait. Do take care that if the system is exploited, you are alerted in time (see The importance of logs and alerts, Section 4.13) so that you can take appropriate measures and terminate the compromise when you've seen enough. Here are some of the packages and issues to consider when setting up your honeypot:

If you cannot use spare systems to build up the honeypots and the network systems to protect and control it you can use the virtualisation technology available in xen or uml (User-Mode-Linux). If you take this route you will need to patch your kernel with either kernel-patch-xen or kernel-patch-uml.

You can read more about building honeypots in Lanze Spitzner's excellent article To Build a Honeypot (from the Know your Enemy series). Also, the Honeynet Project provides valuable information about building honeypots and auditing the attacks made on them.


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Securing Debian Manual

Version: 3.16, built on Sun, 08 Apr 2012 02:48:09 +0000

Javier Fernández-Sanguino Peña jfs@debian.org
Authors, Section 1.1