This attack exploits terminal devices that allow themselves to be written to by other users. The attacker sends command strings to the target terminal device hoping that the target user will hit enter and thereby execute the malicious command with their privileges. The attacker can send the results (such as copying /etc/passwd) to a known directory and collect once the attack has succeeded.
Likelihood Of Attack
The table below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
Meta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.
Identify attacker-writable terminals: Determine if users TTYs are writable by the attacker.
Determine the permissions for the TTYs found on the system. Any that allow user write to the TTY may be vulnerable.
Attempt to write to other user TTYs. This approach could leave a trail or alert a user.
Execute malicious commands: Using one or more vulnerable TTY, execute commands to achieve various impacts.
Commands that allow reading or writing end user files can be executed.
User terminals must have a permissive access control such as world writeable that allows normal users to control data on other user's terminals.
Ability to discover permissions on terminal devices. Of course, brute force can also be used.
Access to a terminal on the target network
The table below specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Execute Unauthorized Commands
Design: Ensure that terminals are only writeable by named owner user and/or administrator
Design: Enforce principle of least privilege
"Any system that allows other peers to write directly to its terminal process is vulnerable to this type of attack. If the terminals are available through being over-privileged (i.e. world-writable) or the attacker is an administrator, then a series of commands in this format can be used to echo commands out to victim terminals.
"$echo -e "\033[30m\033\132" > /dev/ttyXX
where XX is the tty number of the user under attack. This will paste the characters to another terminal (tty). Note this technique works only if the victim's tty is world writable (which it may not be). That is one reason why programs like write(1) and talk(1) in UNIX systems need to run setuid." [R.40.1][REF-2]
If the victim continues to hit "enter" and execute the commands, there are an endless supply of vectors available to the attacker, copying files, open up network connections, ftp out to servers, and so on.
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
Updated Attacker_Skills_or_Knowledge_Required, Related_Attack_Patterns, Type (Attack_Pattern -> Relationship)
CAPEC Content Team
The MITRE Corporation
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Page Last Updated or Reviewed:
September 30, 2019