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cwe:cn:definition:94

CWE-94:对生成代码的控制不恰当(代码注入)

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.

Likelihood of Exploit

Medium

Common Consequences

Scope Technical Impace Note
Access_ControlBypass protection mechanismIn some cases, injectable code controls authentication; this may lead to a remote vulnerability.
Access_ControlGain privileges / assume identityInjected code can access resources that the attacker is directly prevented from accessing.
Integrity
Confidentiality
Availability
Execute unauthorized code or commandsCode 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.
Non-RepudiationHide activitiesOften the actions performed by injected control code are unlogged.

Potential Mitigations

Mitigation - 1

Architecture and Design

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

Mitigation - 2

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.

Mitigation - 3

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.

2013/05/30 09:37

Mitigation - 4

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.

Mitigation - 5

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.

Mitigation - 6

Operation

Strategy: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).

2013/05/30 09:37

Demonstrative Examples

Example - 1

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

$MessageFile = "cwe-94/messages.out"; 
if ($_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:

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

which will decode to the following:

<?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.

2013/05/30 09:37

Example - 2

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

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.

2013/05/30 09:37

Observed Examples

Reference Description
CVE-2008-5071Eval injection in PHP program.
CVE-2002-1750Eval injection in Perl program.
CVE-2008-5305Eval injection in Perl program using an ID that should only contain hyphens and numbers.
CVE-2002-1752Direct code injection into Perl eval function.
CVE-2002-1753Eval injection in Perl program.
CVE-2005-1527Direct code injection into Perl eval function.
CVE-2005-2837Direct code injection into Perl eval function.
CVE-2005-1921MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-2498MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-3302Code injection into Python eval statement from a field in a formatted file.
CVE-2007-1253Eval injection in Python program.
CVE-2001-1471chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement.
CVE-2002-0495Perl code directly injected into CGI library file from parameters to another CGI program.
CVE-2005-1876Direct PHP code injection into supporting template file.
CVE-2005-1894Direct code injection into PHP script that can be accessed by attacker.
CVE-2003-0395PHP code from User-Agent HTTP header directly inserted into log file implemented as PHP script.
cwe/cn/definition/94.txt · 最后更改: 2014/09/04 15:03 (外部编辑)