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Chapter: Security in Computing : Protection in General-Purpose Operating Systems

Attacks on Passwords

How secure are passwords themselves? Passwords are somewhat limited as protection devices because of the relatively small number of bits of information they contain.

Attacks on Passwords


How secure are passwords themselves? Passwords are somewhat limited as protection devices because of the relatively small number of bits of information they contain.


Here are some ways you might be able to determine a user's password, in decreasing order of difficulty.


·      Try all possible passwords.


·      Try frequently used passwords.


·      Try passwords likely for the user.


·      Search for the system list of passwords.


·      Ask the user.


Loose-Lipped Systems


So far the process seems secure, but in fact it has some vulnerabilities. To see why, consider the actions of a would-be intruder. Authentication is based on knowing the <name, password> pair A complete outsider is presumed to know nothing of the system. Suppose the intruder attempts to access a system in the following manner. (In the following examples, the system messages are in uppercase, and the user's responses are in lowercase.)










We assumed that the intruder knew nothing of the system, but without having to do much, the intruder found out that adams is not the name of an authorized user. The intruder could try other common names, first names, and likely generic names such as system or operator to build a list of authorized users.


An alternative arrangement of the login sequence is shown below.










This system notifies a user of a failure only after accepting both the user name and the password. The failure message should not indicate whether it is the user name or password that is unacceptable. In this way, the intruder does not know which failed.


These examples also gave a clue as to which computing system is being accessed. The true outsider has no right to know that, and legitimate insiders already know what system they have accessed. In the example below, the user is given no information until the system is assured of the identity of the user.













Exhaustive Attack


But the break-in time can be made more tractable in a number of ways. Searching for a single particular password does not necessarily require all passwords to be tried; an intruder needs to try only until the correct password is identified. If the set of all possible passwords were evenly distributed, an intruder would likely need to try only half of the password space: the expected number of searches to find any particular password. However, an intruder can also use to advantage the fact that passwords are not evenly distributed. Because a password has to be remembered, people tend to pick simple passwords. This feature reduces the size of the password space.


Probable Passwords


Think of a word.


Is the word you thought of long? Is it uncommon? Is it hard to spell or to pronounce? The answer to all three of these questions is probably no.


Penetrators searching for passwords realize these very human characteristics and use them to their advantage. Therefore, penetrators try techniques that are likely to lead to rapid success. If people prefer short passwords to long ones, the penetrator will plan to try all passwords


but to try them in order by length. There are only 261 + 262 + 26 3=18,278 passwords of length 3 or less. At the assumed rate of one password per millisecond, all of these passwords can be checked in 18.278 seconds, hardly a challenge with a computer. Even expanding the tries to 4 or 5 characters raises the count only to 475 seconds (about 8 minutes) or 12,356 seconds (about 3.5 hours), respectively.


This analysis assumes that people choose passwords such as vxlag and msms as often as they pick enter and beer. However, people tend to choose names or words they can remember. Many computing systems have spelling checkers that can be used to check for spelling errors and typographic mistakes in documents. These spelling checkers sometimes carry online dictionaries of the most common English words. One contains a dictionary of 80,000 words. Trying all of these words as passwords takes only 80 seconds.


Passwords Likely for a User


If Sandy is selecting a password, she is probably not choosing a word completely at random. Most likely Sandy's password is something meaningful to her. People typically choose personal passwords, such as the name of a spouse, a child, a brother or sister, a pet, a street name, or something memorable or familiar. If we restrict our password attempts to just names of people (first names), streets, projects, and so forth, we generate a list of only a few hundred possibilities at most. Trying this number of passwords takes under a second! Even a person working by hand could try ten likely candidates in a minute or two.


Thus, what seemed formidable in theory is in fact quite vulnerable in practice, and the likelihood of successful penetration is frightening. Morris and Thompson [MOR79] confirmed our fears in their report on the results of having gathered passwords from many users, shown in Table 4-2. Figure 4-15 (based on data from that study) shows the characteristics of the 3,289 passwords gathered. The results from that study are distressing, and the situation today is likely to be the same. Of those passwords, 86 percent could be uncovered in about one week's worth of 24-hour-a-day testing, using the very generous estimate of 1 millisecond per password check.

Lest you dismiss these results as dated (they were reported in 1979), Klein repeated the experiment in 1990 [KLE90] and Spafford in 1992 [SPA92]. Each collected approximately 15,000 passwords. Klein reported that 2.7 percent of the passwords were guessed in only 15 minutes of machine time and 21 percent were guessed within a week! Spafford found the average password length was 6.8 characters, and 28.9 percent consisted of only lowercase alphabetic characters. Notice that both these studies were done after the Internet worm (described in Chapter 3) succeeded, in part by breaking weak passwords.


Even in 2002, the British online bank Egg found users still choosing weak passwords [BUX02]. A full 50 percent of passwords for their online banking service were family members' names: 23 percent children's names, 19 percent a spouse or partner, and 9 percent their own. Alas, pets came in at only 8 percent, while celebrities and football (soccer) stars tied at 9 percent each. And in 1998, Knight and Hartley [KNI98] reported that approximately 35 percent of passwords are deduced from syllables and initials of the account owner's name.

Two friends we know have told us their passwords as we helped them administer their systems, and their passwords would both have been among the first we would have guessed. But, you say, these are amateurs unaware of the security risk of a weak password. At a recent meeting, a security expert related this experience: He thought he had chosen a solid password, so he invited a class of students to ask him a few questions and offer some guesses as to his password. He was amazed that they asked only a few questions before they had deduced the password. And this was a security expert.


Several news articles have claimed that the four most common passwords are "God," "sex," "love,"and "money" (the order among those is unspecified). The perhaps apocryphal list of common passwords at geodsoft.com/howto/password/common.htm appears at several other places on the Internet. Or see the default password list at www.phenoelit.de/dpl/dpl.html. Whether these are really passwords we do not know. Still, it warrants a look because similar lists are bound to be built into some hackers' tools.


Several network sites post dictionaries of phrases, science fiction characters, places, mythological names, Chinese words, Yiddish words, and other specialized lists. All these lists are posted to help site administrators identify users who have chosen weak passwords, but the same dictionaries can also be used by attackers of sites that do not have such attentive administrators. The COPS [FAR90], Crack [MUF92], and SATAN [FAR95] utilities allow an administrator to scan a system for weak passwords. But these same utilities, or other homemade ones, allow attackers to do the same. Now Internet sites offer so-called password recovery software as freeware or shareware for under $20. (These are password-cracking programs.)


People think they can be clever by picking a simple password and replacing certain characters, such as 0 (zero) for letter O, 1 (one) for letter I or L, 3 (three) for letter E or @ (at) for letter A. But users aren't the only people who could think up these substitutions. Knight and Hartley [KNI98] list, in order, 12 steps an attacker might try in order to determine a password. These steps are in increasing degree of difficulty (number of guesses), so they indicate the amount of work to which the attacker must go to derive a password. Here are their password guessing steps:


·        no password


·        the same as the user ID


·        is, or is derived from, the user's name


·        common word list (for example, "password," "secret," "private") plus common names and patterns (for example, "asdfg," "aaaaaa")


·        short college dictionary


·        complete English word list


·        common non-English language dictionaries


·        short college dictionary with capitalizations (PaSsWorD) and substitutions (0 for O, and so forth)


·        complete English with capitalizations and substitutions


·        common non-English dictionaries with capitalization and substitutions


·        brute force, lowercase alphabetic characters


·        brute force, full character set


Although the last step will always succeed, the steps immediately preceding it are so time consuming that they will deter all but the dedicated attacker for whom time is not a limiting factor.


Plaintext System Password List


To validate passwords, the system must have a way of comparing entries with actual passwords. Rather than trying to guess a user's password, an attacker may instead target the system password file. Why guess when with one table you can determine all passwords with total accuracy?


On some systems, the password list is a file, organized essentially as a two-column table of user IDs and corresponding passwords. This information is certainly too obvious to leave out in the open. Various security approaches are used to conceal this table from those who should not see it.

You might protect the table with strong access controls, limiting access to the operating system. But even this tightening of control is looser than it should be, because not every operating system module needs or deserves access to this table. For example, the operating system scheduler, accounting routines, or storage manager have no need to know the table's contents. Unfortunately, in some systems, there are n+1 known users: n regular users and the operating system. The operating system is not partitioned, so all its modules have access to all privileged information. This monolithic view of the operating system implies that a user who exploits a flaw in one section of the operating system has access to all the system's deepest secrets. A better approach is to limit table access to the modules that need access: the user authentication module and the parts associated with installing new users, for example.


If the table is stored in plain sight, an intruder can simply dump memory at a convenient time to access it. Careful timing may enable a user to dump the contents of all of memory and, by exhaustive search, find values that look like the password table.


System backups can also be used to obtain the password table. To be able to recover from system errors, system administrators periodically back up the file space onto some auxiliary medium for safe storage. In the unlikely event of a problem, the file system can be reloaded from a backup, with a loss only of changes made since the last backup. Backups often contain only file contents, with no protection mechanism to control file access. (Physical security and access controls to the backups themselves are depended on to provide security for the contents of backup media.) If a regular user can access the backups, even ones from several weeks, months, or years ago, the password tables stored in them may contain entries that are still valid.


Finally, the password file is a copy of a file stored on disk. Anyone with access to the disk or anyone who can overcome file access restrictions can obtain the password file.


Encrypted Password File


There is an easy way to foil an intruder seeking passwords in plain sight: encrypt them. Frequently, the password list is hidden from view with conventional encryption or one-way ciphers.


With conventional encryption, either the entire password table is encrypted or just the password column. When a user's password is received, the stored password is decrypted, and the two are compared.


Even with encryption, there is still a slight exposure because for an instant the user's password is available in plaintext in main memory. That is, the password is available to anyone who could obtain access to all of memory.


A safer approach uses one-way encryption, defined in Chapter 2. The password table's entries are encrypted by a one-way encryption and then stored. When the user enters a password, it is also encrypted and then compared with the table. If the two values are equal, the authentication succeeds. Of course, the encryption has to be such that it is unlikely that two passwords would encrypt to the same ciphertext, but this characteristic is true for most secure encryption algorithms.


With one-way encryption, the password file can be stored in plain view. For example, the password table for the Unix operating system can be read by any user unless special access controls have been installed. Because the contents are encrypted, backup copies of the password table are no longer a problem.


There is always the possibility that two people might choose the same password, thus creating two identical entries in the password file. Even though the entries are encrypted, each user will know the plaintext equivalent. For instance, if Bill and Kathy both choose their passwords on April 1, they might choose APRILFOOL as a password. Bill might read the password file and notice that the encrypted version of his password is the same as Kathy's.


Unix+ circumvents this vulnerability by using a password extension, called the salt. The salt is a 12-bit number formed from the system time and the process identifier. Thus, the salt is likely to be unique for each user, and it can be stored in plaintext in the password file. The salt is concatenated to Bill's password (pw) when he chooses it; E(pw+saltB) is stored for Bill, and his salt value is also stored. When Kathy

chooses her password, the salt is different because the time or the process number is different. Call this new one saltK. For her, E (pw+saltK) and saltK are stored. When either person tries to log in, the system fetches the appropriate salt from the password table and

combines that with the password before performing the encryption. The encrypted versions of (pw+salt) are very different for these two users. When Bill looks down the password list, the encrypted version of his password will not look at all like Kathy's.


Storing the password file in a disguised form relieves much of the pressure to secure it. Better still is to limit access to processes that legitimately need access. In this way, the password file is protected to a level commensurate with the protection provided by the password itself. Someone who has broken the controls of the file system has access to data, not just passwords, and that is a serious threat. But if an attacker successfully penetrates the outer security layer, the attacker still must get past the encryption of the password file to access the useful information in it.


Indiscreet Users


Guessing passwords and breaking encryption can be tedious or daunting. But there is a simple way to obtain a password: Get it directly from the user! People often tape a password to the side of a terminal or write it on a card just inside the top desk drawer. Users are afraid they will forget their passwords, or they cannot be bothered trying to remember them. It is particularly tempting to write the passwords down when users have several accounts.


Users sharing work or data may also be tempted to share passwords. If someone needs a file, it is easier to say "my password is x; get the file yourself" than to arrange to share the file. This situation is a result of user laziness, but it may be brought about or exacerbated by a system that makes sharing inconvenient.


In an admittedly unscientific poll done by Verisign [TEC05], two-thirds of people approached on the street volunteered to disclose their password for a coupon good for a cup of coffee, and 79 percent admitted they used the same password for more than one system or web site.

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