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CWE-94: Improper Control of Generation of Code ('Code Injection')

 
Improper Control of Generation of Code ('Code Injection')
Weakness ID: 94 (Weakness Class)Status: Draft
+ Description

Description Summary

The software constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.

Extended Description

When software allows a user's input to contain code syntax, it might be possible for an attacker to craft the code in such a way that it will alter the intended control flow of the software. Such an alteration could lead to arbitrary code execution.

Injection problems encompass a wide variety of issues -- all mitigated in very different ways. For this reason, the most effective way to discuss these weaknesses is to note the distinct features which classify them as injection weaknesses. The most important issue to note is that all injection problems share one thing in common -- i.e., they allow for the injection of control plane data into the user-controlled data plane. This means that the execution of the process may be altered by sending code in through legitimate data channels, using no other mechanism. While buffer overflows, and many other flaws, involve the use of some further issue to gain execution, injection problems need only for the data to be parsed. The most classic instantiations of this category of weakness are SQL injection and format string vulnerabilities.

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

Languages

Interpreted languages: (Sometimes)

+ Common Consequences
ScopeEffect

Technical Impact: Bypass protection mechanism

In some cases, injectable code controls authentication; this may lead to a remote vulnerability.

Technical Impact: Gain privileges / assume identity

Injected code can access resources that the attacker is directly prevented from accessing.

Technical Impact: Execute unauthorized code or commands

Code injection attacks can lead to loss of data integrity in nearly all cases as the control-plane data injected is always incidental to data recall or writing. Additionally, code injection can often result in the execution of arbitrary code.

Technical Impact: Hide activities

Often the actions performed by injected control code are unlogged.

+ Likelihood of Exploit

Medium

+ Demonstrative Examples

Example 1

This example attempts to write user messages to a message file and allow users to view them.

(Bad Code)
Example Language: PHP 
$MessageFile = "cwe-94/messages.out";
if ($_GET["action"] == "NewMessage") {
$name = $_GET["name"];
$message = $_GET["message"];
$handle = fopen($MessageFile, "a+");
fwrite($handle, "<b>$name</b> says '$message'<hr>\n");
fclose($handle);
echo "Message Saved!<p>\n";
}
else if ($_GET["action"] == "ViewMessages") {
include($MessageFile);
}

While the programmer intends for the MessageFile to only include data, an attacker can provide a message such as:

(Attack)
 
name=h4x0r
message=%3C?php%20system(%22/bin/ls%20-l%22);?%3E

which will decode to the following:

(Attack)
 
<?php system("/bin/ls -l");?>

The programmer thought they were just including the contents of a regular data file, but PHP parsed it and executed the code. Now, this code is executed any time people view messages.

Notice that XSS (CWE-79) is also possible in this situation.

Example 2

edit-config.pl: This CGI script is used to modify settings in a configuration file.

(Bad Code)
Example Language: Perl 
use CGI qw(:standard);

sub config_file_add_key {
my ($fname, $key, $arg) = @_;

# code to add a field/key to a file goes here
}

sub config_file_set_key {
my ($fname, $key, $arg) = @_;

# code to set key to a particular file goes here
}

sub config_file_delete_key {
my ($fname, $key, $arg) = @_;

# code to delete key from a particular file goes here
}

sub handleConfigAction {
my ($fname, $action) = @_;
my $key = param('key');
my $val = param('val');

# this is super-efficient code, especially if you have to invoke
# any one of dozens of different functions!

my $code = "config_file_$action_key(\$fname, \$key, \$val);";
eval($code);
}

$configfile = "/home/cwe/config.txt";
print header;
if (defined(param('action'))) {
handleConfigAction($configfile, param('action'));
}
else {
print "No action specified!\n";
}

The script intends to take the 'action' parameter and invoke one of a variety of functions based on the value of that parameter - config_file_add_key(), config_file_set_key(), or config_file_delete_key(). It could set up a conditional to invoke each function separately, but eval() is a powerful way of doing the same thing in fewer lines of code, especially when a large number of functions or variables are involved. Unfortunately, in this case, the attacker can provide other values in the action parameter, such as: add_key(",","); system("/bin/ls"); This would produce the following string in handleConfigAction(): config_file_add_key(",","); system("/bin/ls"); Any arbitrary Perl code could be added after the attacker has "closed off" the construction of the original function call, in order to prevent parsing errors from causing the malicious eval() to fail before the attacker's payload is activated. This particular manipulation would fail after the system() call, because the "_key(\$fname, \$key, \$val)" portion of the string would cause an error, but this is irrelevant to the attack because the payload has already been activated.

+ Observed Examples
ReferenceDescription
Eval injection in PHP program.
Eval injection in Perl program.
Eval injection in Perl program using an ID that should only contain hyphens and numbers.
Direct code injection into Perl eval function.
Eval injection in Perl program.
Direct code injection into Perl eval function.
Direct code injection into Perl eval function.
MFV. code injection into PHP eval statement using nested constructs that should not be nested.
MFV. code injection into PHP eval statement using nested constructs that should not be nested.
Code injection into Python eval statement from a field in a formatted file.
Eval injection in Python program.
chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement.
Perl code directly injected into CGI library file from parameters to another CGI program.
Direct PHP code injection into supporting template file.
Direct code injection into PHP script that can be accessed by attacker.
PHP code from User-Agent HTTP header directly inserted into log file implemented as PHP script.
+ Potential Mitigations

Phase: Architecture and Design

Refactor your program so that you do not have to dynamically generate code.

Phase: Architecture and Design

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

Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection.

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

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

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

To reduce the likelihood of code injection, use stringent whitelists that limit which constructs are allowed. If you are dynamically constructing code that invokes a function, then verifying that the input is alphanumeric might be insufficient. An attacker might still be able to reference a dangerous function that you did not intend to allow, such as system(), exec(), or exit().

Phase: Testing

Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Phase: Testing

Use dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Operation

Strategies: Compilation or Build Hardening; Environment Hardening

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).

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfWeakness ClassWeakness Class691Insufficient Control Flow Management
Research Concepts1000
ChildOfCategoryCategory7522009 Top 25 - Risky Resource Management
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ChildOfCategoryCategory896SFP Cluster: Tainted Input
Software Fault Pattern (SFP) Clusters (primary)888
ChildOfWeakness ClassWeakness Class913Improper Control of Dynamically-Managed Code Resources
Research Concepts1000
ParentOfWeakness BaseWeakness Base95Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base96Improper Neutralization of Directives in Statically Saved Code ('Static Code Injection')
Development Concepts (primary)699
Research Concepts (primary)1000
MemberOfViewView635Weaknesses Used by NVD
Weaknesses Used by NVD (primary)635
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
CanFollowWeakness BaseWeakness Base98Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
Development Concepts699
Research Concepts1000
+ Research Gaps

Many of these weaknesses are under-studied and under-researched, and terminology is not sufficiently precise.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERCODECode Evaluation and Injection
+ References
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 3: Web-Client Related Vulnerabilities (XSS)." Page 63. McGraw-Hill. 2010.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01CigitalExternal
updated Time_of_Introduction
2008-09-08MITREInternal
updated Applicable_Platforms, Relationships, Research_Gaps, Taxonomy_Mappings
2009-01-12MITREInternal
updated Common_Consequences, Demonstrative_Examples, Description, Likelihood_of_Exploit, Name, Potential_Mitigations, Relationships
2009-03-10MITREInternal
updated Potential_Mitigations
2009-05-27MITREInternal
updated Demonstrative_Examples, Name
2010-02-16MITREInternal
updated Potential_Mitigations
2010-06-21MITREInternal
updated Description, Potential_Mitigations
2011-03-29MITREInternal
updated Name
2011-06-01MITREInternal
updated Common_Consequences
2012-05-11MITREInternal
updated Common_Consequences, Demonstrative_Examples, Observed_Examples, References, Relationships
2012-10-30MITREInternal
updated Potential_Mitigations
2013-02-21MITREInternal
updated Relationships
Previous Entry Names
Change DatePrevious Entry Name
2009-01-12Code Injection
2009-05-27Failure to Control Generation of Code (aka 'Code Injection')
2011-03-29Failure to Control Generation of Code ('Code Injection')
Page Last Updated: June 23, 2014