Home > Articles > Security > Network Security

  • Print
  • + Share This
Like this article? We recommend

Looking for the Invisible

After using the RPM trick, let's say that files like netstat and ls actually were modified. The question that followed is fairly obvious: "What now?"

You have a fair number of options. Depending on the importance of the system, I will usually recommend taking a backup of the user directories, password, and other critical system files, and rebuilding the system without these files, using the backup as a reference for the new system. I won't just copy those files back. Our cracker may have hidden things in legitimate places, and we don't want to let him back in quite that easily.

You can also leave the system alone, tie down the host access with TCP wrappers, shutting down nonessential services, and replacing affected packages. Starting clean is important, but we don't always have that luxury—not immediately, anyway. If you discover that your procps or net-tools package has been modified by a cracker, the first thing to do is to reinstall the package. Since that package may have been the hole through which your cracker entered, it is usually a good idea to get the latest build from your vendor (Red Hat, Caldera, Debian, and so on). For the truly paranoid, the fact is that once a cracker has access to your system, they can replace anything, including the very files we use to track down the damage. Like the Shaolin priests in the old TV series "Kung-Fu," the cracker succeeds by being invisible.

Now, let's have a look at those invisible things.

Here is a real-life example. After a cracker attack, the machine was tied down, TCP wrappers were installed, and all affected packages were replaced. It was time to scope out the damage while keeping a close eye on the logs for repeated attempts at break-in. Looking at the /etc/passwd file, I noticed a user that did not belong on the system, jon. It looked like a normal passwd entry and did not have root privs. With several users on this machine, our cracker hid nicely in the passwd list.

When I went to his home directory (/home/jon) and did an ls -l, all I got was this:

.  ..  ..  .bashrc  .bash_history .screenrc   emech.tar.gz

Other than a file called emech.tar.gz, things did not look that strange. Could that be all that was wrong? With a closer look, though, you'll notice that there are two .. directories (pointers to the previous directory in your filesystem hierarchy). That's strange. However, if I change directory to .. with cd .., I just wind up in the /home directory. What's up?

What's up is that there is an extra space after the second dot-dot. I can find this out like this:

  # cd /home/jon
  # echo .* | cat -v

. .. .. .bashrc .bash_history .screenrc emech.tar.gz

Look very closely. Notice how each item is separated by only one space. Now look between the second dot-dot and .bashrc. There are actually two spaces, which means the directory is actually "dot-dot-space." To get into that directory and have a look around, I do this:

# cd ".. "

Now an ls shows me all this fun stuff:

randfiles mech.set mech.pid checkmech cpu.memory
mech.help mech.usage   mech mech.levels  emech.users

That's interesting. Let's see if jon has any more files hidden around the disk. Using the find command again, I specify a search for files belonging only to this user ID.

# find / -user jon -print

Aside from what is in the /home/jon directory, I get this partial list:


Looking a bit more interesting, isn't it? Sniffers. Port scanners. Our cracker was making quite a home for himself. Furthermore, we discovered two other users coming from different hosts with their own files. Our cracker was either operating from different locations with different IDs or he had friends.

In doing this search, there were even files belonging to this cracker in legitimate user directories, including one very scary file, something called tcp.log. This file was several hundred lines long and contained every telnet and ftp login that had come to and from the machine. Every one! Aside from telling the person whose machine had been broken into that they should rebuild the whole thing from scratch, I also told them to change each and every password, not only on this system but on every system they have access to.

Here's the scoop. Part of the information your cracker collects is a list of logins and passwords you use on other systems. Why? So they have an easier time breaking into someone else's system. Every system you have been accessing while your cracker has had access to your system is at risk. You should contact the system administrators of those other systems and inform them of the risk they face. The flip side is that someone logging into your system on a regular basis whose system had been hacked may have give the cracker a valid login and password on your system. Spooky, huh?

Here are a few examples to help you search for the hidden and dangerous. For starters, check the user directories for suid or guid files. These are programs that have an s instead of an x when you do an ls. For instance, an ls -l on /usr/bin/passwd returns this information:

-r-s--x--x  1 root   root    10704 Apr 14 1999 /usr/bin/passwd

The s in the fourth position means that the passwd program acts as root when it is being executed. This is necessary in order to allow users to change their passwords. The second x is simply an x, but an s in this position would mean that any user in that group would act as that group. Programs that can act as a specific user or group are not a bad thing—usually. That said, for the most part, no regular (nonadministrative) user needs to have root-suid files in their home directories. Look for them this way. The command assumes that your users are created in the /home directory.

# find /home -perm -4000 -o -perm -2000 -print

What else can we do? Since we want to speed up the process of finding programs and files left behind by our cracker, a quick way to look for hidden directories would be good. This command will show you those. It will also show you things like .kde and so on, but you'll also find things like dot-dot-space and dot-dot-dot, perfect hidey-holes for your cracker.

# find / -type d -name ".*" -print

The -type d option means to list directories only. This can be a big list, but it is certainly a smaller one than you would get if you just walked through every file and directory on the system. What's nice here is that your proper dot and dot-dot directories (. being the current directory and .. being the parent directory) do not show up in this list. If you see a dot-dot, it will have some other hidden character following it.

Let's sum up. Blowing away everything on your cracked system and starting over is a quick-and-dirty approach that lets you create a properly secure system right from scratch. Eventually, this is what you should probably do anyhow. If your system must be up, using a new box and making that your new production system is probably the next best bet, but providing a brand new system while you investigate the damage to the old one can be costly. PCs are inexpensive, but not everybody is ready to shell out a few thousand to bring another system online. The catch is this—your cracker has left a wealth of information behind, information you may need. Getting rid of that information is a bit like getting rid of the evidence. It's tough to do an investigation without evidence. Weigh the costs of either decision, and then act. But do act.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020