Leveraging Race Conditions via Symbolic Links |
Attack Pattern ID: 27 (Detailed Attack Pattern Completeness: Complete) | Typical Severity: High | Status: Draft |
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 attacker's 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
Verify that target host's platform supports symbolic links.:
This attack pattern is only applicable on platforms that support symbolic links.
Attack Step Techniques
ID Attack Step Technique Description Environments 1 Research target platform to determine whether it supports symbolic links.
env-Embedded env-Local2 Create a symbolic link and ensure that it works as expected on the given platform.
env-Embedded env-LocalOutcomes
ID type Outcome Description 1 Success Target platform supports symbolic links (e.g. Linux, UNIX, etc.)2 Failure Target platform does not support symbolic links (e.g. MS Windows)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
ID Attack Step Technique Description Environments 1 Use kernel tracing utility such as ktrace to monitor application behavior
env-Local2 Use debugging utility such as File Monitor to monitor the application's filesystem I/O calls
env-Local3 Watch temporary directories to see when temporary files are created, modified and deleted.
env-Embedded env-Local4 Analyze source code for open-source systems like Linux, Apache, etc.
env-Embedded env-LocalIndicators
ID type Indicator Description Environments 1 Positive Attacker can watch files being created, modified and/or deleted by application.
env-Embedded env-Local2 Inconclusive Application does not seem to perform any filesystem I/O operations.
env-Embedded env-LocalOutcomes
ID type Outcome Description 1 Success Attacker identifies at least one reproducable file I/O operation performed by the application.2 Failure Attacker cannot identify any file I/O operations being performed by the application.
Verify ability to write to filesystem:
The attacker verifies ability to write to the target host's file system.
Attack Step Techniques
ID Attack Step Technique Description Environments -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-1 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-1 Verify permissions on target directory
env-Embedded env-LocalIndicators
ID type Indicator Description Environments -1 Positive Target directory is a globally writable temp directory (e.g. /tmp in many UNIX-like systems)
env-Embedded env-Local-1 Positive Target directory is writable by the attacker's effective user ID.
env-Embedded env-LocalOutcomes
ID type Outcome Description 0 Success Attacker can create and modify files in the target directory.0 Failure Attacker cannot create or modify files in the target directory.Security Controls
ID type Security Control Description 0 Preventative Store temporary files in a directory with limited permissions where malicious users cannot tamper with them.
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
ID Attack Step Technique Description Environments -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-1 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-LocalOutcomes
ID type Outcome Description 0 Success Sensitive file tampered with successfully.0 Failure Sensitive file could not be tampered with.Security Controls
ID type Security Control Description 0 Preventative 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.0 Preventative 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).0 Corrective Run application with minimal permissions. In particular, avoid running applications as root on UNIX-like systems and as Administrator on Windows systems.
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.
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.
(Source : Wikipedia (http://en.wikipedia.org/wiki/Symlink_race)).
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
CVE-2006-6939
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
CVE-2005-0894
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
CVE-2000-0972
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).
The attacker will certainly look for file system locations where he can write and create Symlink links.
The attacker may also observe the system and locate the temporary files created during a call to a certain function.
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.
The content of the temporary file which is copied to the file pointed to by the Symlink.
This attack can cause privilege escalation, modification of resources or denial of services.
Nature | Type | ID | Name | Description | View(s) this relationship pertains to |
---|---|---|---|---|---|
ChildOf | Attack Pattern | 26 | Leveraging Race Conditions | Mechanism of Attack (primary)1000 | |
ChildOf | Attack Pattern | 29 | Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions | Mechanism of Attack1000 |
Symlink Race, Wikipedia - http://en.wikipedia.org/wiki/Symlink_race
Safe temporary file creation with mkstemp - http://www.opengroup.org/onlinepubs/009695399/functions/mkstemp.html
Submissions | ||||
---|---|---|---|---|
Submitter | Organization | Date | Comments | |
Eric Dalci | Cigital, Inc | 2007-02-01 |
Modifications | |||||
---|---|---|---|---|---|
Modifier | Organization | Date | Comments | ||
Sean Barnum | Cigital, Inc | 2007-03-08 | Review and revise | ||
Richard Struse | VOXEM, Inc | 2007-03-26 | Review and feedback leading to changes in Name, Description, Likelihood and Related Attack Patterns | ||
Sean Barnum | Cigital, Inc | 2007-04-13 | Modified pattern content according to review and feedback | ||
Amit Sethi | Cigital, Inc. | 2007-10-29 | Added extended Attack Execution Flow |