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CWE-732: Incorrect Permission Assignment for Critical Resource

 
Incorrect Permission Assignment for Critical Resource
Weakness ID: 732 (Weakness Class)Status: Draft
+ Description

Description Summary

The software specifies permissions for a security-critical resource in a way that allows that resource to be read or modified by unintended actors.

Extended Description

When a resource is given a permissions setting that provides access to a wider range of actors than required, it could lead to the exposure of sensitive information, or the modification of that resource by unintended parties. This is especially dangerous when the resource is related to program configuration, execution or sensitive user data.

+ Time of Introduction
  • Architecture and Design
  • Implementation
  • Installation
  • Operation
+ Applicable Platforms

Languages

Language-independent

+ Modes of Introduction

The developer may set loose permissions in order to minimize problems when the user first runs the program, then create documentation stating that permissions should be tightened. Since system administrators and users do not always read the documentation, this can result in insecure permissions being left unchanged.

The developer might make certain assumptions about the environment in which the software runs - e.g., that the software is running on a single-user system, or the software is only accessible to trusted administrators. When the software is running in a different environment, the permissions become a problem.

+ Common Consequences
ScopeEffect
Confidentiality

Technical Impact: Read application data; Read files or directories

An attacker may be able to read sensitive information from the associated resource, such as credentials or configuration information stored in a file.

Access Control

Technical Impact: Gain privileges / assume identity

An attacker may be able to modify critical properties of the associated resource to gain privileges, such as replacing a world-writable executable with a Trojan horse.

Integrity
Other

Technical Impact: Modify application data; Other

An attacker may be able to destroy or corrupt critical data in the associated resource, such as deletion of records from a database.

+ Likelihood of Exploit

Medium to High

+ Detection Methods

Automated Static Analysis

Automated static analysis may be effective in detecting permission problems for system resources such as files, directories, shared memory, device interfaces, etc. Automated techniques may be able to detect the use of library functions that modify permissions, then analyze function calls for arguments that contain potentially insecure values.

However, since the software's intended security policy might allow loose permissions for certain operations (such as publishing a file on a web server), automated static analysis may produce some false positives - i.e., warnings that do not have any security consequences or require any code changes.

When custom permissions models are used - such as defining who can read messages in a particular forum in a bulletin board system - these can be difficult to detect using automated static analysis. It may be possible to define custom signatures that identify any custom functions that implement the permission checks and assignments.

Automated Dynamic Analysis

Automated dynamic analysis may be effective in detecting permission problems for system resources such as files, directories, shared memory, device interfaces, etc.

However, since the software's intended security policy might allow loose permissions for certain operations (such as publishing a file on a web server), automated dynamic analysis may produce some false positives - i.e., warnings that do not have any security consequences or require any code changes.

When custom permissions models are used - such as defining who can read messages in a particular forum in a bulletin board system - these can be difficult to detect using automated dynamic analysis. It may be possible to define custom signatures that identify any custom functions that implement the permission checks and assignments.

Manual Analysis

This weakness can be detected using tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session.

These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.

Manual Static Analysis

Manual static analysis may be effective in detecting the use of custom permissions models and functions. The code could then be examined to identifying usage of the related functions. Then the human analyst could evaluate permission assignments in the context of the intended security model of the software.

Manual Dynamic Analysis

Manual dynamic analysis may be effective in detecting the use of custom permissions models and functions. The program could then be executed with a focus on exercising code paths that are related to the custom permissions. Then the human analyst could evaluate permission assignments in the context of the intended security model of the software.

Fuzzing

Fuzzing is not effective in detecting this weakness.

Black Box

Use monitoring tools that examine the software's process as it interacts with the operating system and the network. This technique is useful in cases when source code is unavailable, if the software was not developed by you, or if you want to verify that the build phase did not introduce any new weaknesses. Examples include debuggers that directly attach to the running process; system-call tracing utilities such as truss (Solaris) and strace (Linux); system activity monitors such as FileMon, RegMon, Process Monitor, and other Sysinternals utilities (Windows); and sniffers and protocol analyzers that monitor network traffic.

Attach the monitor to the process and watch for library functions or system calls on OS resources such as files, directories, and shared memory. Examine the arguments to these calls to infer which permissions are being used.

Note that this technique is only useful for permissions issues related to system resources. It is not likely to detect application-level business rules that are related to permissions, such as if a user of a blog system marks a post as "private," but the blog system inadvertently marks it as "public."

+ Demonstrative Examples

Example 1

The following code sets the umask of the process to 0 before creating a file and writing "Hello world" into the file.

(Bad Code)
Example Language:
#define OUTFILE "hello.out"

umask(0);
FILE *out;
/* Ignore CWE-59 (link following) for brevity */
out = fopen(OUTFILE, "w");
if (out) {
fprintf(out, "hello world!\n");
fclose(out);
}

After running this program on a UNIX system, running the "ls -l" command might return the following output:

(Result)
 
-rw-rw-rw- 1 username 13 Nov 24 17:58 hello.out

The "rw-rw-rw-" string indicates that the owner, group, and world (all users) can read the file and write to it.

Example 2

This code creates a home directory for a new user, and makes that user the owner of the directory. If the new directory cannot be owned by the user, the directory is deleted.

(Bad Code)
Example Language: PHP 
function createUserDir($username){
$path = '/home/'.$username;
if(!mkdir($path)){
return false;
}
if(!chown($path,$username)){
rmdir($path);
return false;
}
return true;
}

Because the optional "mode" argument is omitted from the call to mkdir(), the directory is created with the default permissions 0777. Simply setting the new user as the owner of the directory does not explicitly change the permissions of the directory, leaving it with the default. This default allows any user to read and write to the directory, allowing an attack on the user's files. The code also fails to change the owner group of the directory, which may result in access by unexpected groups.

This code may also be vulnerable to Path Traversal (CWE-22) attacks if an attacker supplies a non alphanumeric username.

Example 3

The following code snippet might be used as a monitor to periodically record whether a web site is alive. To ensure that the file can always be modified, the code uses chmod() to make the file world-writable.

(Bad Code)
Example Language: Perl 
$fileName = "secretFile.out";

if (-e $fileName) {
chmod 0777, $fileName;
}

my $outFH;
if (! open($outFH, ">>$fileName")) {
ExitError("Couldn't append to $fileName: $!");
}
my $dateString = FormatCurrentTime();
my $status = IsHostAlive("cwe.mitre.org");
print $outFH "$dateString cwe status: $status!\n";
close($outFH);

The first time the program runs, it might create a new file that inherits the permissions from its environment. A file listing might look like:

(Result)
 
-rw-r--r-- 1 username 13 Nov 24 17:58 secretFile.out

This listing might occur when the user has a default umask of 022, which is a common setting. Depending on the nature of the file, the user might not have intended to make it readable by everyone on the system.

The next time the program runs, however - and all subsequent executions - the chmod will set the file's permissions so that the owner, group, and world (all users) can read the file and write to it:

(Result)
 
-rw-rw-rw- 1 username 13 Nov 24 17:58 secretFile.out

Perhaps the programmer tried to do this because a different process uses different permissions that might prevent the file from being updated.

Example 4

The following command recursively sets world-readable permissions for a directory and all of its children:

(Bad Code)
Example Language: Shell 
chmod -R ugo+r DIRNAME

If this command is run from a program, the person calling the program might not expect that all the files under the directory will be world-readable. If the directory is expected to contain private data, this could become a security problem.

+ Observed Examples
ReferenceDescription
CVE-2009-3482Anti-virus product sets insecure "Everyone: Full Control" permissions for files under the "Program Files" folder, allowing attackers to replace executables with Trojan horses.
CVE-2009-3897Product creates directories with 0777 permissions at installation, allowing users to gain privileges and access a socket used for authentication.
CVE-2009-3489Photo editor installs a service with an insecure security descriptor, allowing users to stop or start the service, or execute commands as SYSTEM.
CVE-2009-3289Library function copies a file to a new target and uses the source file's permissions for the target, which is incorrect when the source file is a symbolic link, which typically has 0777 permissions.
CVE-2009-0115Device driver uses world-writable permissions for a socket file, allowing attackers to inject arbitrary commands.
CVE-2009-1073LDAP server stores a cleartext password in a world-readable file.
CVE-2009-0141Terminal emulator creates TTY devices with world-writable permissions, allowing an attacker to write to the terminals of other users.
CVE-2008-0662VPN product stores user credentials in a registry key with "Everyone: Full Control" permissions, allowing attackers to steal the credentials.
CVE-2008-0322Driver installs its device interface with "Everyone: Write" permissions.
CVE-2009-3939Driver installs a file with world-writable permissions.
CVE-2009-3611Product changes permissions to 0777 before deleting a backup; the permissions stay insecure for subsequent backups.
CVE-2007-6033Product creates a share with "Everyone: Full Control" permissions, allowing arbitrary program execution.
CVE-2007-5544Product uses "Everyone: Full Control" permissions for memory-mapped files (shared memory) in inter-process communication, allowing attackers to tamper with a session.
CVE-2005-4868Database product uses read/write permissions for everyone for its shared memory, allowing theft of credentials.
CVE-2004-1714Security product uses "Everyone: Full Control" permissions for its configuration files.
CVE-2001-0006"Everyone: Full Control" permissions assigned to a mutex allows users to disable network connectivity.
CVE-2002-0969Chain: database product contains buffer overflow that is only reachable through a .ini configuration file - which has "Everyone: Full Control" permissions.
+ Potential Mitigations

Phase: Implementation

When using a critical resource such as a configuration file, check to see if the resource has insecure permissions (such as being modifiable by any regular user) [R.732.1], and generate an error or even exit the software if there is a possibility that the resource could have been modified by an unauthorized party.

Phase: Architecture and Design

Divide the software into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully defining distinct user groups, privileges, and/or roles. Map these against data, functionality, and the related resources. Then set the permissions accordingly. This will allow you to maintain more fine-grained control over your resources. [R.732.2]

Effectiveness: Moderate

This can be an effective strategy. However, in practice, it may be difficult or time consuming to define these areas when there are many different resources or user types, or if the applications features change rapidly.

Phases: Architecture and Design; Operation

Strategy: Sandbox or Jail

Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.

OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.

This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.

Be careful to avoid CWE-243 and other weaknesses related to jails.

Effectiveness: Limited

The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

Phases: Implementation; Installation

During program startup, explicitly set the default permissions or umask to the most restrictive setting possible. Also set the appropriate permissions during program installation. This will prevent you from inheriting insecure permissions from any user who installs or runs the program.

Effectiveness: High

Phase: System Configuration

For all configuration files, executables, and libraries, make sure that they are only readable and writable by the software's administrator.

Effectiveness: High

Phase: Documentation

Do not suggest insecure configuration changes in documentation, especially if those configurations can extend to resources and other programs that are outside the scope of the application.

Phase: Installation

Do not assume that a system administrator will manually change the configuration to the settings that are recommended in the software's manual.

Phases: Operation; System Configuration

Strategy: Environment Hardening

Ensure that the software runs properly under the Federal Desktop Core Configuration (FDCC) [R.732.4] or an equivalent hardening configuration guide, which many organizations use to limit the attack surface and potential risk of deployed software.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory275Permission Issues
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class285Improper Authorization
Research Concepts (primary)1000
ChildOfWeakness ClassWeakness Class668Exposure of Resource to Wrong Sphere
Research Concepts1000
ChildOfCategoryCategory743CERT C Secure Coding Section 09 - Input Output (FIO)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory7532009 Top 25 - Porous Defenses
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ChildOfCategoryCategory8032010 Top 25 - Porous Defenses
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
ChildOfCategoryCategory815OWASP Top Ten 2010 Category A6 - Security Misconfiguration
Weaknesses in OWASP Top Ten (2010) (primary)809
ChildOfCategoryCategory840Business Logic Errors
Development Concepts699
ChildOfCategoryCategory857CERT Java Secure Coding Section 12 - Input Output (FIO)
Weaknesses Addressed by the CERT Java Secure Coding Standard844
ChildOfCategoryCategory859CERT Java Secure Coding Section 14 - Platform Security (SEC)
Weaknesses Addressed by the CERT Java Secure Coding Standard844
ChildOfCategoryCategory860CERT Java Secure Coding Section 15 - Runtime Environment (ENV)
Weaknesses Addressed by the CERT Java Secure Coding Standard (primary)844
ChildOfCategoryCategory8662011 Top 25 - Porous Defenses
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory877CERT C++ Secure Coding Section 09 - Input Output (FIO)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory899SFP Cluster: Access Control
Software Fault Pattern (SFP) Clusters (primary)888
RequiredByCompound Element: CompositeCompound Element: Composite689Permission Race Condition During Resource Copy
Research Concepts1000
ParentOfWeakness VariantWeakness Variant276Incorrect Default Permissions
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant277Insecure Inherited Permissions
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant278Insecure Preserved Inherited Permissions
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant279Incorrect Execution-Assigned Permissions
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base281Improper Preservation of Permissions
Research Concepts (primary)1000
ParentOfCompound Element: CompositeCompound Element: Composite689Permission Race Condition During Resource Copy
Research Concepts (primary)1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CERT Java Secure CodingFIO03-JCreate files with appropriate access permission
CERT Java Secure CodingSEC01-JDo not allow tainted variables in privileged blocks
CERT Java Secure CodingENV03-JDo not grant dangerous combinations of permissions
CERT C++ Secure CodingFIO06-CPPCreate files with appropriate access permissions
CERT C Secure CodingFIO06-CCreate files with appropriate access permissions
+ References
[R.732.1] [REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "File Permissions." Page 495.. 1st Edition. Addison Wesley. 2006.
[R.732.2] [REF-9] John Viega and Gary McGraw. "Building Secure Software: How to Avoid Security Problems the Right Way". Chapter 8, "Access Control." Page 194.. 1st Edition. Addison-Wesley. 2002.
[R.732.3] Jason Lam. "Top 25 Series - Rank 21 - Incorrect Permission Assignment for Critical Response". SANS Software Security Institute. 2010-03-24. <http://software-security.sans.org/blog/2010/03/24/top-25-series-rank-21-incorrect-permission-assignment-for-critical-response>.
[R.732.4] [REF-24] NIST. "Federal Desktop Core Configuration". <http://nvd.nist.gov/fdcc/index.cfm>.
+ Maintenance Notes

The relationships between privileges, permissions, and actors (e.g. users and groups) need further refinement within the Research view. One complication is that these concepts apply to two different pillars, related to control of resources (CWE-664) and protection mechanism failures (CWE-396).

+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2008-09-08Internal CWE Team
new weakness-focused entry for Research view.
Modifications
Modification DateModifierOrganizationSource
2009-01-12CWE Content TeamMITREInternal
updated Description, Likelihood_of_Exploit, Name, Potential_Mitigations, Relationships
2009-03-10CWE Content TeamMITREInternal
updated Potential_Mitigations, Related_Attack_Patterns
2009-05-27CWE Content TeamMITREInternal
updated Name
2009-12-28CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Demonstrative_Examples, Detection_Factors, Modes_of_Introduction, Observed_Examples, Potential_Mitigations, References
2010-02-16CWE Content TeamMITREInternal
updated Relationships
2010-04-05CWE Content TeamMITREInternal
updated Potential_Mitigations, Related_Attack_Patterns
2010-06-21CWE Content TeamMITREInternal
updated Common_Consequences, Detection_Factors, Potential_Mitigations, References, Relationships
2010-09-27CWE Content TeamMITREInternal
updated Potential_Mitigations, Relationships
2010-12-13CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-03-29CWE Content TeamMITREInternal
updated Demonstrative_Examples, Description, Relationships
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITREInternal
updated Relationships
2011-09-13CWE Content TeamMITREInternal
updated Potential_Mitigations, References, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated References, Related_Attack_Patterns, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2013-07-17CWE Content TeamMITREInternal
updated References
Previous Entry Names
Change DatePrevious Entry Name
2009-01-12Insecure Permission Assignment for Resource
2009-05-27Insecure Permission Assignment for Critical Resource
Page Last Updated: February 18, 2014