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CAPEC-132: Symlink Attack

Attack Pattern ID: 132
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
An attacker positions a symbolic link in such a manner that the targeted user or application accesses the link's endpoint, assuming that it is accessing a file with the link's name. The endpoint file may be either output or input. If the file is output, the result is that the endpoint is modified, instead of a file at the intended location. Modifications to the endpoint file may include appending, overwriting, corrupting, changing permissions, or other modifications. In some variants of this attack the attacker may be able to control the change to a file while in other cases they cannot. The former is especially damaging since the attacker may be able to grant themselves increased privileges or insert false information, but the latter can also be damaging as it can expose sensitive information or corrupt or destroy vital system or application files. Alternatively, the endpoint file may serve as input to the targeted application. This can be used to feed malformed input into the target or to cause the target to process different information, possibly allowing the attacker to control the actions of the target or to cause the target to expose information to the attacker. Moreover, the actions taken on the endpoint file are undertaken with the permissions of the targeted user or application, which may exceed the permissions that the attacker would normally have.
+ Likelihood Of Attack

Low

+ Typical Severity

High

+ Relationships

The table(s) below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf, ParentOf, MemberOf 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.

+ Relevant to the view "Mechanisms of Attack" (CAPEC-1000)
NatureTypeIDName
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.159Redirect Access to Libraries
+ Execution Flow
Explore
  1. Identify Target: Attacker identifies the target application by determining whether there is sufficient check before writing data to a file and creating symlinks to files in different directories. The attacker writes to files in different directories to check whether the application has sufficient checking before file operations. The attacker creates symlinks to files in different directories.

    Techniques
    The attacker writes to files in different directories to check whether the application has sufficient checking before file operations.
    The attacker creates symlinks to files in different directories.
Experiment
  1. Try to create symlinks to different files: The attacker then uses a variety of techniques, such as monitoring or guessing to create symlinks to the files accessed by the target application in the directories which are identified in the explore phase. The attacker monitors the file operations performed by the target application using a tool like dtrace or FileMon. And the attacker can delay the operations by using "sleep(2)" and "usleep()" to prepare the appropriate conditions for the attack, or make the application perform expansive tasks (large files parsing, etc.) depending on the purpose of the application. The attacker may need a little guesswork on the filenames on which the target application would operate. The attacker tries to create symlinks to the various filenames.

    Techniques
    The attacker monitors the file operations performed by the target application using a tool like dtrace or FileMon. And the attacker can delay the operations by using "sleep(2)" and "usleep()" to prepare the appropriate conditions for the attack, or make the application perform expansive tasks (large files parsing, etc.) depending on the purpose of the application.
    The attacker may need a little guesswork on the filenames on which the target application would operate.
    The attacker tries to create symlinks to the various filenames.
Exploit
  1. Target application operates on created symlinks to sensitive files: The attacker is able to create symlinks to sensitive files while the target application is operating on the file. Create the symlink to the sensitive file such as configuration files, etc.

    Techniques
    Create the symlink to the sensitive file such as configuration files, etc.
+ Prerequisites
The targeted application must perform the desired activities on a file without checking whether the file is a symbolic link or not. The attacker must be able to predict the name of the file the target application is modifying and be able to create a new symbolic link where that file would appear.
+ Skills Required
[Level: Low]
To create symlinks
[Level: High]
To identify the files and create the symlinks during the file operation time window
+ Resources Required
None: No specialized resources are required to execute this type of attack. The only requirement is the ability to create the necessary symbolic link.
+ Consequences

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.

ScopeImpactLikelihood
Confidentiality
Other
Integrity
Modify Data
Confidentiality
Read Data
Integrity
Modify Data
Confidentiality
Read Data
Integrity
Modify Data
Confidentiality
Read Data
Authorization
Execute Unauthorized Commands
Accountability
Authentication
Authorization
Non-Repudiation
Gain Privileges
Access Control
Authorization
Bypass Protection Mechanism
Availability
Unreliable Execution
+ Mitigations
Design: Check for the existence of files to be created, if in existence verify they are neither symlinks nor hard links before opening them.
Implementation: Use randomly generated file names for temporary files. Give the files restrictive permissions.
+ Example Instances

The attacker creates a symlink with the "same" name as the file which the application is intending to write to. The application will write to the file- "causing the data to be written where the symlink is pointing". An attack like this can be demonstrated as follows:

root# vulprog myFile
{...program does some processing...]
attacker# ln –s /etc/nologin myFile
[...program writes to 'myFile', which points to /etc/nologin...]

In the above example, the root user ran a program with poorly written file handling routines, providing the filename "myFile" to vulnprog for the relevant data to be written to. However, the attacker happened to be looking over the shoulder of "root" at the time, and created a link from myFile to /etc/nologin. The attack would make no user be able to login.

+ Taxonomy Mappings
Relevant to the ATT&CK taxonomy mapping
Entry IDEntry Name
1023Shortcut Modification
+ References
[REF-13] Shaun Colley. "Crafting Symlinks for Fun and Profit". <http://www.infosecwriters.com/texts.php?op=display&id=159>.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2015-11-09CAPEC Content TeamThe MITRE Corporation
Updated References
2017-08-04CAPEC Content TeamThe MITRE Corporation
Updated Resources_Required

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Page Last Updated or Reviewed: July 31, 2018