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Description
The software constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.
Modes of Introduction:
– Architecture and Design
Likelihood of Exploit: High
Related Weaknesses
CWE-77
CWE-74
CWE-77
CWE-77
CWE-88
Consequences
Confidentiality, Integrity, Availability, Non-Repudiation: Execute Unauthorized Code or Commands, DoS: Crash, Exit, or Restart, Read Files or Directories, Modify Files or Directories, Read Application Data, Modify Application Data, Hide Activities
Attackers could execute unauthorized commands, which could then be used to disable the software, or read and modify data for which the attacker does not have permissions to access directly. Since the targeted application is directly executing the commands instead of the attacker, any malicious activities may appear to come from the application or the application’s owner.
Potential Mitigations
Phase: Architecture and Design
Effectiveness:
Description:
If at all possible, use library calls rather than external processes to recreate the desired functionality.
Phase: Architecture and Design, Operation
Effectiveness: Limited
Description:
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.
Phase: Architecture and Design
Effectiveness:
Description:
For any data that will be used to generate a command to be executed, keep as much of that data out of external control as possible. For example, in web applications, this may require storing the data locally in the session’s state instead of sending it out to the client in a hidden form field.
Phase: Architecture and Design
Effectiveness:
Description:
For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Phase: Architecture and Design
Effectiveness:
Description:
Phase: Implementation
Effectiveness:
Description:
While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments. The most conservative approach is to escape or filter all characters that do not pass an extremely strict allowlist (such as everything that is not alphanumeric or white space). If some special characters are still needed, such as white space, wrap each argument in quotes after the escaping/filtering step. Be careful of argument injection (CWE-88).
Phase: Implementation
Effectiveness:
Description:
If the program to be executed allows arguments to be specified within an input file or from standard input, then consider using that mode to pass arguments instead of the command line.
Phase: Architecture and Design
Effectiveness:
Description:
Phase: Implementation
Effectiveness:
Description:
Phase: Architecture and Design
Effectiveness:
Description:
When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
Phase: Operation
Effectiveness:
Description:
Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl’s “-T” switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
Phase: Operation
Effectiveness:
Description:
Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl’s “-T” switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
Phase: Implementation
Effectiveness:
Description:
Phase: Operation
Effectiveness:
Description:
Use runtime policy enforcement to create an allowlist of allowable commands, then prevent use of any command that does not appear in the allowlist. Technologies such as AppArmor are available to do this.
Phase: Operation
Effectiveness: Moderate
Description:
Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth.
An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.
Phase: Architecture and Design, Operation
Effectiveness:
Description:
Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. 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.
Phase: Operation, Implementation
Effectiveness:
Description:
When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.
CVE References
- CVE-1999-0067
- Canonical example of OS command injection. CGI program does not neutralize “|” metacharacter when invoking a phonebook program.
- CVE-2001-1246
- Language interpreter’s mail function accepts another argument that is concatenated to a string used in a dangerous popen() call. Since there is no neutralization of this argument, both OS Command Injection (CWE-78) and Argument Injection (CWE-88) are possible.
- CVE-2002-0061
- Web server allows command execution using “|” (pipe) character.
- CVE-2003-0041
- FTP client does not filter “|” from filenames returned by the server, allowing for OS command injection.
- CVE-2008-2575
- Shell metacharacters in a filename in a ZIP archive
- CVE-2002-1898
- Shell metacharacters in a telnet:// link are not properly handled when the launching application processes the link.
- CVE-2007-3572
- Chain: incomplete denylist for OS command injection
- CVE-2012-1988
- Product allows remote users to execute arbitrary commands by creating a file whose pathname contains shell metacharacters.
