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
Scope
Effect
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: C
#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.
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.
Anti-virus product sets insecure "Everyone: Full
Control" permissions for files under the "Program Files" folder, allowing
attackers to replace executables with Trojan
horses.
Library 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.
Product uses "Everyone: Full Control" permissions
for memory-mapped files (shared memory) in inter-process communication,
allowing attackers to tamper with a session.
Chain: 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.
[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.
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).
Description
