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CAPEC-27: Leveraging Race Conditions via Symbolic Links

 
Leveraging Race Conditions via Symbolic Links
Definition in a New Window Definition in a New Window
Attack Pattern ID: 27
Abstraction: Detailed
Status: Draft
Completeness: Complete
+ Description

Summary

This attack leverages the use of symbolic links (Symlinks) in order to write to sensitive files. An attacker can create a Symlink link to a target file not otherwise accessible to her. When the privileged program tries to create a temporary file with the same name as the Symlink link, it will actually write to the target file pointed to by the attackers' Symlink link. If the attacker can insert malicious content in the temporary file she will be writing to the sensitive file by using the Symlink. The race occurs because the system checks if the temporary file exists, then creates the file. The attacker would typically create the Symlink during the interval between the check and the creation of the temporary file.

Attack Execution Flow

Explore
  1. Verify that target host's platform supports symbolic links.:

    This attack pattern is only applicable on platforms that support symbolic links.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    Research target platform to determine whether it supports symbolic links.

    env-Embedded env-Local
    2

    Create a symbolic link and ensure that it works as expected on the given platform.

    env-Embedded env-Local

    Outcomes

    IDTypeOutcome Description
    1Success
    Target platform supports symbolic links (e.g. Linux, UNIX, etc.)
    2Failure
    Target platform does not support symbolic links (e.g. MS Windows)
  2. Examine application's file I/O behavior:

    Analyze the application's file I/O behavior to determine where it stores files, as well as the operations it performs to read/write files.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    Use kernel tracing utility such as ktrace to monitor application behavior

    env-Local
    2

    Use debugging utility such as File Monitor to monitor the application's filesystem I/O calls

    env-Local
    3

    Watch temporary directories to see when temporary files are created, modified and deleted.

    env-Embedded env-Local
    4

    Analyze source code for open-source systems like Linux, Apache, etc.

    env-Embedded env-Local

    Indicators

    IDTypeIndicator DescriptionEnvironments
    1Positive

    Attacker can watch files being created, modified and/or deleted by application.

    env-Embedded env-Local
    2Inconclusive

    Application does not seem to perform any filesystem I/O operations.

    env-Embedded env-Local

    Outcomes

    IDTypeOutcome Description
    1Success
    Attacker identifies at least one reproducible file I/O operation performed by the application.
    2Failure
    Attacker cannot identify any file I/O operations being performed by the application.
Experiment
  1. Verify ability to write to filesystem:

    The attacker verifies ability to write to the target host's file system.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    Create a file that does not exist in the target directory (e.g. "touch temp.txt" in UNIX-like systems)

    env-Embedded env-Local
    2

    On platforms that differentiate between file creation and file modification, if the target file that the application writes to already exists, attempt to modify it.

    env-Embedded env-Local
    3

    Verify permissions on target directory

    env-Embedded env-Local

    Indicators

    IDTypeIndicator DescriptionEnvironments
    1Positive

    Target directory is a globally writable temp directory (e.g. /tmp in many UNIX-like systems)

    env-Embedded env-Local
    2Positive

    Target directory is writable by the attackers' effective user ID.

    env-Embedded env-Local

    Outcomes

    IDTypeOutcome Description
    1Success
    Attacker can create and modify files in the target directory.
    2Failure
    Attacker cannot create or modify files in the target directory.

    Security Controls

    IDTypeSecurity Control Description
    1Preventative
    Store temporary files in a directory with limited permissions where malicious users cannot tamper with them.
Exploit
  1. Replace file with a symlink to a sensitive system file.:

    Between the time that the application checks to see if a file exists (or if the user has access to it) and the time the application actually opens the file, the attacker replaces the file with a symlink to a sensitive system file.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    Create an infinite loop containing commands such as "rm -f tempfile.dat; ln -s /etc/shadow tempfile.dat". Wait for an instance where the following steps occur in the given order: (1) Application ensures that tempfile.dat exists and that the user has access to it, (2) "rm -f tempfile.dat; ln -s /etc/shadow tempfile.dat", and (3) Application opens tempfile.dat for writing, and inadvertently opens /etc/shadow for writing instead.

    env-Embedded env-Local
    2

    Use other techniques with debugging tools to replace the file between the time the application checks the file and the time the application opens it.

    env-Embedded env-Local

    Outcomes

    IDTypeOutcome Description
    1Success
    Sensitive file tampered with successfully.
    2Failure
    Sensitive file could not be tampered with.

    Security Controls

    IDTypeSecurity Control Description
    1Preventative
    Use file handles to check existence of files, to check permissions and to open them. Do not use filename except to obtain a handle initially.
    2Preventative
    Drop application's permissions to the current user's permissions before performing any file I/O operations (e.g. using Process.as_uid() in Ruby).
    3Corrective
    Run application with minimal permissions. In particular, avoid running applications as root on UNIX-like systems and as Administrator on Windows systems.
+ Attack Prerequisites
  • The attacker is able to create Symlink links on the target host.

  • Tainted data from the attacker is used and copied to temporary files.

  • The target host does insecure temporary file creation.

+ Typical Severity

High

+ Typical Likelihood of Exploit

Likelihood: Medium

+ Methods of Attack
  • Injection
  • Time and State
  • Modification of Resources
+ Examples-Instances

Description

In this naive example, the Unix program foo is setuid. Its function is to retrieve information for the accounts specified by the user. For "efficiency," it sorts the requested accounts into a temporary file (/tmp/foo naturally) before making the queries.

The directory /tmp is world-writable. Malicious user Mallory creates a symbolic link to the file /.rhosts named /tmp/foo. Then, she invokes foo with + + as the requested account. The program creates the (temporary) file /tmp/foo (really creating /.rhosts) and puts the requested account (+ +) in it. It removes the temporary file (merely removing the symbolic link).

Now the /.rhosts contains + +, which is the incantation necessary to allow anyone to use rlogin to log into the computer as the superuser.

[R.27.1]

Description

GNU ed before 0.3 allows local users to overwrite arbitrary files via a symlink attack on temporary files, possibly in the open_sbuf function.

Related Vulnerabilities

Description

OpenmosixCollector and OpenMosixView in OpenMosixView 1.5 allow local users to overwrite or delete arbitrary files via a symlink attack on (1) temporary files in the openmosixcollector directory or (2) nodes.tmp.

Related Vulnerabilities

Description

Setuid product allows file reading by replacing a file being edited with a symlink to the targeted file, leaking the result in error messages when parsing fails.

Related Vulnerabilities

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Medium

This attack is sophisticated because the attacker has to overcome a few challenges such as creating symlinks on the target host during a precise timing, inserting malicious data in the temporary file and have knowledge about the temporary files created (file name and function which creates them).

+ Probing Techniques

Description

The attacker will certainly look for file system locations where he can write and create Symlink links.

Description

The attacker may also observe the system and locate the temporary files created during a call to a certain function.

+ Solutions and Mitigations

Use safe libraries when creating temporary files. For instance the standard library function mkstemp can be used to safely create temporary files. For shell scripts, the system utility mktemp does the same thing.

Access to the directories should be restricted as to prevent attackers from manipulating the files. Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file.

Follow the principle of least privilege when assigning access rights to files.

Ensure good compartmentalization in the system to provide protected areas that can be trusted.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Integrity
Modify application data
Confidentiality
Access_Control
Authorization
Gain privileges / assume identity
Availability
DoS: resource consumption (other)
Denial of Service
+ Injection Vector

The content of the temporary file which is copied to the file pointed to by the Symlink.

+ Payload

The content of the file overwritten when writing to the Symlink.

+ Activation Zone

The new content of the targeted file.

+ Payload Activation Impact

Description

This attack can cause privilege escalation, modification of resources or denial of services.

+ Purposes
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Low
+ Technical Context
Architectural Paradigms
All
Frameworks
All
Platforms
All
Languages
All
+ References
[R.27.1] [REF-6] "Wikipedia". Symlink race. The Wikimedia Foundation, Inc. <http://en.wikipedia.org/wiki/Symlink_race>.
[R.27.2] "mkstemp". IEEE Std 1003.1, 2004 Edition. The Open Group Base Specifications Issue 6. <http://www.opengroup.org/onlinepubs/009695399/functions/mkstemp.html>.
+ Content History
Submissions
SubmitterOrganizationDateComments
Eric DalciCigital, Inc2007-02-01
Modifications
ModifierOrganizationDateCommentsSource
Sean BarnumCigital, Inc2007-03-08Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Name, Description, Likelihood and Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback
Amit SethiCigital, Inc.2007-10-29Added extended Attack Execution Flow
CAPEC Content TeamThe MITRE Corporation2013-06-21Updated Example-Instance_DescriptionInternal
CAPEC Content TeamThe MITRE Corporation2013-12-18Updated Attack_PhasesInternal
CAPEC Content TeamThe MITRE Corporation2014-02-06Updated Attack_Phases, Description Summary, PayloadInternal

Page Last Updated: April 10, 2014