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CAPEC-112: Brute Force

 
Brute Force
Definition in a New Window Definition in a New Window
Attack Pattern ID: 112
Abstraction: Meta
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

In this attack, some asset (information, functionality, identity, etc.) is protected by a finite secret value. The attacker attempts to gain access to this asset by using trial-and-error to exhaustively explore all the possible secret values in the hope of finding the secret (or a value that is functionally equivalent) that will unlock the asset. Examples of secrets can include, but are not limited to, passwords, encryption keys, database lookup keys, and initial values to one-way functions.

The key factor in this attack is the attackers' ability to explore the possible secret space rapidly. This, in turn, is a function of the size of the secret space and the computational power the attacker is able to bring to bear on the problem. If the attacker has modest resources and the secret space is large, the challenge facing the attacker is intractable. While the defender cannot control the resources available to an attacker, they can control the size of the secret space. Creating a large secret space involves selecting one's secret from as large a field of equally likely alternative secrets as possible and ensuring that an attacker is unable to reduce the size of this field using available clues or cryptanalysis. Doing this is more difficult than it sounds since elimination of patterns (which, in turn, would provide an attacker clues that would help them reduce the space of potential secrets) is difficult to do using deterministic machines, such as computers. Assuming a finite secret space, a brute force attack will eventually succeed. The defender must rely on making sure that the time and resources necessary to do so will exceed the value of the information. For example, a secret space that will likely take hundreds of years to explore is likely safe from raw-brute force attacks.

+ Attack Execution Flow
Explore
  1. Determine secret testing procedure:

    Determine how a potential guess of the secret may be tested. This may be accomplished by comparing some manipulation of the secret to a known value, use of the secret to manipulate some known set of data and determining if the result displays specific characteristics (for example, turning cryptotext into plaintext), or by submitting the secret to some external authority and having the external authority respond as to whether the value was the correct secret. Ideally, the attacker will want to determine the correctness of their guess independently since involvement of an external authority is usually slower and can provide an indication to the defender that a brute-force attack is being attempted.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    Determine if there is a way to parallelize the attack. Most brute force attacks can take advantage of parallel techniques by dividing the search space among available resources, thus dividing the average time to success by the number of resources available. If there is a single choke point, such as a need to check answers with an external authority, the attackers' position is significantly degraded.

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  2. Reduce search space:

    Find ways to reduce the secret space. The smaller the attacker can make the space they need to search for the secret value, the greater their chances for success. There are a great many ways in which the search space may be reduced.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    If possible, determine how the secret was selected. If the secret was determined algorithmically (such as by a random number generator) the algorithm may have patterns or dependencies that reduce the size of the secret space. If the secret was created by a human, behavioral factors may, if not completely reduce the space, make some types of secrets more likely than others. (For example, humans may use the same secrets in multiple places or use secrets that look or sound familiar for ease of recall.)

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    2

    If the secret was chosen algorithmically, cryptanalysis can be applied to the algorithm to discover patterns in this algorithm. (This is true even if the secret is not used in cryptography.) Periodicity, the need for seed values, or weaknesses in the generator all can result in a significantly smaller secret space.

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    3

    If the secret was chosen by a person, social engineering and simple espionage can indicate patterns in their secret selection. If old secrets can be learned (and a target may feel they have little need to protect a secret that has been replaced) hints as to their selection preferences can be gleaned. These can include character substitutions a target employs, patterns in sources (dates, famous phrases, music lyrics, family members, etc.). Once these patterns have been determined, the initial efforts of a brute-force attack can focus on these areas.

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    4

    Some algorithmic techniques for secret selection may leave indicators that can be tested for relatively easily and which could then be used to eliminate large areas of the search space for consideration. For example, it may be possible to determine that a secret does or does not start with a given character after a relatively small number of tests. Alternatively, it might be possible to discover the length of the secret relatively easily. These discoveries would significantly reduce the search space, thus increasing speed with which the attacker discovers the secret.

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  3. Expand victory conditions:

    It is sometimes possible to expand victory conditions. For example, the attacker might not need to know the exact secret but simply needs a value that produces the same result using a one-way function. While doing this does not reduce the size of the search space, the presence of multiple victory conditions does reduce the likely amount of time that the attacker will need to explore the space before finding a workable value.

Exploit
  1. Gather information so attack can be performed independently.:

    If possible, gather the necessary information so a successful search can be determined without consultation of an external authority. This can be accomplished by capturing cryptotext (if the goal is decoding the text) or the encrypted password dictionary (if the goal is learning passwords).

+ Attack Prerequisites
  • The attacker must be able to determine when they have successfully guessed the secret. As such, one-time pads are immune to this type of attack since there is no way to determine when a guess is correct.

+ Typical Severity

High

+ Methods of Attack
  • Brute Force
+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

The attack simply requires basic scripting ability to automate the exploration of the search space. More sophisticated attackers may be able to use more advanced methods to reduce the search space and increase the speed with which the secret is located.

+ Resources Required

Ultimately, the speed with which an attacker discovers a secret is directly proportional to the computational resources the attacker has at their disposal. This attack method is resource expensive: having large amounts of computational power do not guarantee timely success, but having only minimal resources makes the problem intractable against all but the weakest secret selection procedures.

+ Indicators-Warnings of Attack

Repeated submissions of incorrect secret values may indicate a brute force attack. For example, repeated bad passwords when accessing user accounts or repeated queries to databases using non-existent keys.

Attempts to download files protected by secrets (usually using encryption) may be a precursor to an offline attack to break the file's encryption and read its contents. This is especially significant if the file itself contains other secret values, such as password files.

If the attacker is able to perform the checking offline then there will likely be no indication that an attack is ongoing.

+ Obfuscation Techniques

The attack is impossible to detect if the attacker can test for successful discovery of the secret value independently, without needing to consult an external authority.

If an external authority must be consulted, the attacker can attempt to space out their guesses to avoid a large number of failed guesses in a short period of time, but doing so slows the attack to the point of making it unworkable against all but the most trivial secret spaces. As such, if an external authority must be consulted the attacked is unlikely to be able to keep the attack secret.

+ Solutions and Mitigations

Select a provably large secret space for selection of the secret. Provably large means that the procedure by which the secret is selected does not have artifacts that significantly reduce the size of the total secret space.

Do not provide the means for an attacker to determine success independently. This forces the attacker to check their guesses against an external authority, which can slow the attack and warn the defender. This mitigation may not be possible if testing material must appear externally, such as with a transmitted cryptotext.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Confidentiality
Read application data
Confidentiality
Access_Control
Authorization
Gain privileges / assume identity
+ Relevant Security Requirements

Protect sensitive data, even when the data is encrypted. If an attacker can gain access to encrypted data, they can mount a brute-force attack independently. The defender will not be aware of this attack or be able to do anything about it and at that point it is purely a function of the attackers' available resources as to how long it takes them to learn the secret.

Monitor activity logs for suspicious activity. An attacker that must use an external authority to check their brute-force guesses is easy to detect, but only if that external authority is monitoring activity and detects the abnormally large number of failed guesses.

+ Purposes
  • Penetration
+ Content History
Submissions
SubmitterOrganizationDateSource
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team

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Page Last Updated or Reviewed: December 07, 2015