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          CWE-94 對生成代碼的控制不恰當(代碼注入)

          Improper Control of Generation of Code ('Code Injection')

          結構: Simple

          Abstraction: Base

          狀態: Draft

          被利用可能性: Medium

          基本描述

          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.

          擴展描述

          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.

          相關缺陷

          • cwe_Nature: ChildOf cwe_CWE_ID: 74 cwe_View_ID: 1000 cwe_Ordinal: Primary

          • cwe_Nature: ChildOf cwe_CWE_ID: 74 cwe_View_ID: 1003 cwe_Ordinal: Primary

          • cwe_Nature: ChildOf cwe_CWE_ID: 74 cwe_View_ID: 699 cwe_Ordinal: Primary

          • cwe_Nature: ChildOf cwe_CWE_ID: 913 cwe_View_ID: 1000

          • cwe_Nature: ChildOf cwe_CWE_ID: 691 cwe_View_ID: 1000

          適用平臺

          Language: {'cwe_Class': 'Interpreted', 'cwe_Prevalence': 'Sometimes'}

          常見的影響

          范圍 影響 注釋
          Access Control Bypass Protection Mechanism In some cases, injectable code controls authentication; this may lead to a remote vulnerability.
          Access Control Gain Privileges or Assume Identity Injected code can access resources that the attacker is directly prevented from accessing.
          ['Integrity', 'Confidentiality', 'Availability'] 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.
          Non-Repudiation Hide Activities Often the actions performed by injected control code are unlogged.

          可能的緩解方案

          Architecture and Design

          策略:

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

          Architecture and Design

          策略:

          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.

          MIT-5 Implementation

          策略: 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().

          Testing

          策略:

          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.

          Testing

          策略:

          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.

          MIT-32 Operation

          策略: Compilation or Build 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).

          MIT-32 Operation

          策略: 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).

          示例代碼

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

          bad 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.

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

          bad 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.

          分析過的案例

          標識 說明 鏈接
          CVE-2008-5071 Eval injection in PHP program. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-5071
          CVE-2002-1750 Eval injection in Perl program. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2002-1750
          CVE-2008-5305 Eval injection in Perl program using an ID that should only contain hyphens and numbers. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-5305
          CVE-2002-1752 Direct code injection into Perl eval function. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2002-1752
          CVE-2002-1753 Eval injection in Perl program. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2002-1753
          CVE-2005-1527 Direct code injection into Perl eval function. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-1527
          CVE-2005-2837 Direct code injection into Perl eval function. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-2837
          CVE-2005-1921 MFV. code injection into PHP eval statement using nested constructs that should not be nested. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-1921
          CVE-2005-2498 MFV. code injection into PHP eval statement using nested constructs that should not be nested. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-2498
          CVE-2005-3302 Code injection into Python eval statement from a field in a formatted file. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-3302
          CVE-2007-1253 Eval injection in Python program. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-1253
          CVE-2001-1471 chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2001-1471
          CVE-2002-0495 Perl code directly injected into CGI library file from parameters to another CGI program. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2002-0495
          CVE-2005-1876 Direct PHP code injection into supporting template file. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-1876
          CVE-2005-1894 Direct code injection into PHP script that can be accessed by attacker. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-1894
          CVE-2003-0395 PHP code from User-Agent HTTP header directly inserted into log file implemented as PHP script. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2003-0395

          Notes

          分類映射

          映射的分類名 ImNode ID Fit Mapped Node Name
          PLOVER CODE Code Evaluation and Injection

          相關攻擊模式

          • CAPEC-242
          • CAPEC-35
          • CAPEC-77

          引用

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