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cwe:cn:definition:250

CWE-250:带着不必要的权限执行

Description Summary

The software performs an operation at a privilege level that is higher than the minimum level required, which creates new weaknesses or amplifies the consequences of other weaknesses.

Extended Description

New weaknesses can be exposed because running with extra privileges, such as root or Administrator, can disable the normal security checks being performed by the operating system or surrounding environment. Other pre-existing weaknesses can turn into security vulnerabilities if they occur while operating at raised privileges.

Privilege management functions can behave in some less-than-obvious ways, and they have different quirks on different platforms. These inconsistencies are particularly pronounced if you are transitioning from one non-root user to another. Signal handlers and spawned processes run at the privilege of the owning process, so if a process is running as root when a signal fires or a sub-process is executed, the signal handler or sub-process will operate with root privileges.

Modes of Introduction

If an application has this design problem, then it can be easier for the developer to make implementation-related errors such as CWE-271 (Privilege Dropping / Lowering Errors). In addition, the consequences of Privilege Chaining (CWE-268) can become more severe.

Likelihood of Exploit

Medium

Common Consequences

Scope Technical Impace Note
Confidentiality
Integrity
Availability
Access_Control
Gain privileges / assume identity
Execute unauthorized code or commands
Read application data
DoS: crash / exit / restart
An attacker will be able to gain access to any resources that are allowed by the extra privileges. Common results include executing code, disabling services, and reading restricted data.

Detection Methods

Detection Method - 1

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.

Detection Method - 2

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 perform a login. Look for library functions and system calls that indicate when privileges are being raised or dropped. Look for accesses of resources that are restricted to normal users.

Note that this technique is only useful for privilege issues related to system resources. It is not likely to detect application-level business rules that are related to privileges, such as if a blog system allows a user to delete a blog entry without first checking that the user has administrator privileges.
2013/05/30 09:37

Detection Method - 3

Automated Static Analysis - Binary / Bytecode

According to SOAR, the following detection techniques may be useful:

Detection Method - 4

Manual Static Analysis - Binary / Bytecode

According to SOAR, the following detection techniques may be useful:

Detection Method - 5

=== Dynamic Analysis with automated results interpretation === According to SOAR, the following detection techniques may be useful:

Detection Method - 6

Dynamic Analysis with manual results interpretation

According to SOAR, the following detection techniques may be useful:

Detection Method - 7

Manual Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Detection Method - 8

Automated Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Detection Method - 9

Automated Static Analysis

According to SOAR, the following detection techniques may be useful:

Detection Method - 10

Architecture / Design Review

According to SOAR, the following detection techniques may be useful:

Potential Mitigations

Mitigation - 1

Architecture and Design Operation

Strategy:Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.98.2]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

2013/05/30 09:37

Mitigation - 2

Architecture and Design

Strategy:Separation of Privilege

Identify and Reduce Attack Surface

Identify the functionality that requires additional privileges, such as access to privileged operating system resources. Wrap and centralize this functionality if possible, and isolate the privileged code as much as possible from other code [R.250.2]. Raise privileges as late as possible, and drop them as soon as possible to avoid CWE-271. Avoid weaknesses such as CWE-288 and CWE-420 by protecting all possible communication channels that could interact with the privileged code, such as a secondary socket that is only intended to be accessed by administrators.

2013/05/30 09:37

Mitigation - 3

Implementation

Perform extensive input validation for any privileged code that must be exposed to the user and reject anything that does not fit your strict requirements.

Mitigation - 4

Implementation

When dropping privileges, ensure that they have been dropped successfully to avoid CWE-273. As protection mechanisms in the environment get stronger, privilege-dropping calls may fail even if it seems like they would always succeed.

2013/05/30 09:37

Mitigation - 5

Implementation

If circumstances force you to run with extra privileges, then determine the minimum access level necessary. First identify the different permissions that the software and its users will need to perform their actions, such as file read and write permissions, network socket permissions, and so forth. Then explicitly allow those actions while denying all else [R.250.2]. Perform extensive input validation and canonicalization to minimize the chances of introducing a separate vulnerability. This mitigation is much more prone to error than dropping the privileges in the first place.

Mitigation - 6

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.

2013/05/30 13:23

Maintenance Notes

Maintenance Note - 1

CWE-271, CWE-272, and CWE-250 are all closely related and possibly overlapping. CWE-271 is probably better suited as a category. Both CWE-272 and CWE-250 are in active use by the community. The “least privilege” phrase has multiple interpretations.

Demonstrative Examples

Example - 1

This code temporarily raises the program's privileges to allow creation of a new user folder.

def makeNewUserDir(username): 
if invalidUsername(username): 
#avoid CWE-22 and CWE-78 
print('Usernames cannot contain invalid characters') 
return False 
 
try: 
raisePrivileges() 
os.mkdir('/home/' + username) 
lowerPrivileges() 
 
except OSError: 
print('Unable to create new user directory for user:' + username) 
return False 
 
return True 
 

While the program only raises its privilege level to create the folder and immediately lowers it again, if the call to os.mkdir() throws an exception, the call to lowerPrivileges() will not occur. As a result, the program is indefinitely operating in a raised privilege state, possibly allowing further exploitation to occur.

Example - 2

The following code calls chroot() to restrict the application to a subset of the filesystem below APP_HOME in order to prevent an attacker from using the program to gain unauthorized access to files located elsewhere. The code then opens a file specified by the user and processes the contents of the file.

chroot(APP_HOME); 
chdir("/"); 
FILE* data = fopen(argv[1], "r+"); 
... 

Constraining the process inside the application's home directory before opening any files is a valuable security measure. However, the absence of a call to setuid() with some non-zero value means the application is continuing to operate with unnecessary root privileges. Any successful exploit carried out by an attacker against the application can now result in a privilege escalation attack because any malicious operations will be performed with the privileges of the superuser. If the application drops to the privilege level of a non-root user, the potential for damage is substantially reduced.

Example - 3

This application intends to use a user's location to determine the timezone the user is in:

locationClient = new LocationClient(this, this, this); 
locationClient.connect(); 
Location userCurrLocation; 
userCurrLocation = locationClient.getLastLocation(); 
setTimeZone(userCurrLocation); 

This is unnecessary use of the location API, as this information is already available using the Android Time API. Always be sure there is not another way to obtain needed information before resorting to using the location API.

Example - 4

This code uses location to determine the user's current US State location.

First the application must declare that it requires the ACCESS_FINE_LOCATION permission in the application's manifest.xml:

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/> 

During execution, a call to getLastLocation() will return a location based on the application's location permissions. In this case the application has permission for the most accurate location possible:

locationClient = new LocationClient(this, this, this); 
locationClient.connect(); 
Location userCurrLocation; 
userCurrLocation = locationClient.getLastLocation(); 
deriveStateFromCoords(userCurrLocation); 

While the application needs this information, it does not need to use the ACCESS_FINE_LOCATION permission, as the ACCESS_COARSE_LOCATION permission will be sufficient to identify which US state the user is in.

2014/09/04 16:55

Observed Examples

Reference Description
CVE-2007-4217FTP client program on a certain OS runs with setuid privileges and has a buffer overflow. Most clients do not need extra privileges, so an overflow is not a vulnerability for those clients.
CVE-2008-1877Program runs with privileges and calls another program with the same privileges, which allows read of arbitrary files.
CVE-2007-5159OS incorrectly installs a program with setuid privileges, allowing users to gain privileges.
CVE-2008-4638Composite: application running with high privileges allows user to specify a restricted file to process, which generates a parsing error that leaks the contents of the file.
CVE-2008-0162Program does not drop privileges before calling another program, allowing code execution.
CVE-2008-0368setuid root program allows creation of arbitrary files through command line argument.
CVE-2007-3931Installation script installs some programs as setuid when they shouldn't be.
cwe/cn/definition/250.txt · 最后更改: 2014/09/04 14:29 (外部编辑)