CWE-94: Improper Control of Generation of Code ('Code Injection')Weakness ID: 94 Vulnerability Mapping:
ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction: BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. |
Description The product 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 a product 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 product. 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. Common Consequences This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.Scope | Impact | Likelihood |
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Access Control
| Technical Impact: Bypass Protection Mechanism In some cases, injectable code controls authentication; this may lead to a remote vulnerability. | | Access Control
| Technical Impact: Gain Privileges or Assume Identity Injected code can access resources that the attacker is directly prevented from accessing. | | Integrity Confidentiality Availability
| 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. | | Non-Repudiation
| Technical Impact: Hide Activities Often the actions performed by injected control code are unlogged. | |
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 product. 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 list 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. This 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, denylists 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 allowlists 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 product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash, or generate incorrect results. |
Phase: Operation Strategy: 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). |
Phase: Operation Strategy: 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). |
Phase: Implementation For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [ REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [ REF-1373]. Effectiveness: Discouraged Common Practice |
Relationships This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Research Concepts" (CWE-1000) Nature | Type | ID | Name |
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ChildOf | Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 913 | Improper Control of Dynamically-Managed Code Resources | ChildOf | Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 74 | Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection') | ParentOf | Variant - a weakness
that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource. | 95 | Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection') | ParentOf | Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 96 | Improper Neutralization of Directives in Statically Saved Code ('Static Code Injection') | ParentOf | Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 1336 | Improper Neutralization of Special Elements Used in a Template Engine | CanFollow | Variant - a weakness
that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource. | 98 | Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion') |
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Software Development" (CWE-699) Nature | Type | ID | Name |
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MemberOf | Category - a CWE entry that contains a set of other entries that share a common characteristic. | 137 | Data Neutralization Issues |
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (CWE-1003) This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Architectural Concepts" (CWE-1008) Nature | Type | ID | Name |
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MemberOf | Category - a CWE entry that contains a set of other entries that share a common characteristic. | 1019 | Validate Inputs |
Modes Of Introduction The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.Phase | Note |
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Implementation | REALIZATION: This weakness is caused during implementation of an architectural security tactic. |
Likelihood Of Exploit 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
$MessageFile = "messages.out"; if ($_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. 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. Example 3 This simple script asks a user to supply a list of numbers as input and adds them together. (bad code) Example Language: Python
def main():
sum = 0
numbers = eval(input("Enter a space-separated list of numbers: "))
for num in numbers:
sum = sum + num
print(f"Sum of {numbers} = {sum}")
main()
The eval() function can take the user-supplied list and convert it into a Python list object, therefore allowing the programmer to use list comprehension methods to work with the data. However, if code is supplied to the eval() function, it will execute that code. For example, a malicious user could supply the following string:
__import__('subprocess').getoutput('rm -r *')
This would delete all the files in the current directory. For this reason, it is not recommended to use eval() with untrusted input. A way to accomplish this without the use of eval() is to apply an integer conversion on the input within a try/except block. If the user-supplied input is not numeric, this will raise a ValueError. By avoiding eval(), there is no opportunity for the input string to be executed as code. (good code) Example Language: Python
def main():
sum = 0
numbers = input("Enter a space-separated list of numbers: ").split(" ")
try:
for num in numbers:
sum = sum + int(num)
print(f"Sum of {numbers} = {sum}")
except ValueError:
print("Error: invalid input")
main()
An alternative, commonly-cited mitigation for this kind of weakness is to use the ast.literal_eval() function, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373]. Observed Examples Reference | Description |
| Math component in an LLM framework translates user input into a Python expression that is input into the Python exec() method, allowing code execution - one variant of a "prompt injection" attack. |
| Python-based library uses an LLM prompt containing user input to dynamically generate code that is then fed as input into the Python exec() method, allowing code execution - one variant of a "prompt injection" attack. |
| Framework for LLM applications allows eval injection via a crafted response from a hosting provider. |
| Python compiler uses eval() to execute malicious strings as Python code. |
| Chain: regex in EXIF processor code does not correctly determine where a string ends ( CWE-625), enabling eval injection ( CWE-95), as exploited in the wild per CISA KEV. |
| "Code injection" in VPN product, as exploited in the wild per CISA KEV. |
| 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. |
Detection Methods
Automated Static Analysis Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.) |
Memberships This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources. Vulnerability Mapping Notes Usage: ALLOWED (this CWE ID could be used to map to real-world vulnerabilities) | Reason: Acceptable-Use | Rationale: This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities. | Comments: Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction. |
Taxonomy Mappings Mapped Taxonomy Name | Node ID | Fit | Mapped Node Name |
PLOVER | CODE | | Code Evaluation and Injection |
ISA/IEC 62443 | Part 4-2 | | Req CR 3.5 |
ISA/IEC 62443 | Part 3-3 | | Req SR 3.5 |
ISA/IEC 62443 | Part 4-1 | | Req SVV-1 |
ISA/IEC 62443 | Part 4-1 | | Req SVV-3 |
References
[REF-44] 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.
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Content History Submissions |
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Submission Date | Submitter | Organization |
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2006-07-19 (CWE Draft 3, 2006-07-19) | PLOVER | | | Contributions |
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Contribution Date | Contributor | Organization |
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2023-06-29 (CWE 4.12, 2023-06-29) | "Mapping CWE to 62443" Sub-Working Group | CWE-CAPEC ICS/OT SIG | Suggested mappings to ISA/IEC 62443. | 2024-02-29 (CWE 4.15, 2024-07-16) | Abhi Balakrishnan | | Provided diagram to improve CWE usability | Modifications |
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Modification Date | Modifier | Organization |
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2008-07-01 | Eric Dalci | Cigital | updated Time_of_Introduction | 2008-09-08 | CWE Content Team | MITRE | updated Applicable_Platforms, Relationships, Research_Gaps, Taxonomy_Mappings | 2009-01-12 | CWE Content Team | MITRE | updated Common_Consequences, Demonstrative_Examples, Description, Likelihood_of_Exploit, Name, Potential_Mitigations, Relationships | 2009-03-10 | CWE Content Team | MITRE | updated Potential_Mitigations | 2009-05-27 | CWE Content Team | MITRE | updated Demonstrative_Examples, Name | 2010-02-16 | CWE Content Team | MITRE | updated Potential_Mitigations | 2010-06-21 | CWE Content Team | MITRE | updated Description, Potential_Mitigations | 2011-03-29 | CWE Content Team | MITRE | updated Name | 2011-06-01 | CWE Content Team | MITRE | updated Common_Consequences | 2012-05-11 | CWE Content Team | MITRE | updated Common_Consequences, Demonstrative_Examples, Observed_Examples, References, Relationships | 2012-10-30 | CWE Content Team | MITRE | updated Potential_Mitigations | 2013-02-21 | CWE Content Team | MITRE | updated Relationships | 2014-07-30 | CWE Content Team | MITRE | updated Relationships | 2015-12-07 | CWE Content Team | MITRE | updated Relationships | 2017-11-08 | CWE Content Team | MITRE | updated Demonstrative_Examples, Modes_of_Introduction, Relationships | 2019-06-20 | CWE Content Team | MITRE | updated Related_Attack_Patterns, Type | 2019-09-19 | CWE Content Team | MITRE | updated Relationships | 2020-02-24 | CWE Content Team | MITRE | updated Potential_Mitigations, Relationships | 2020-06-25 | CWE Content Team | MITRE | updated Potential_Mitigations | 2020-08-20 | CWE Content Team | MITRE | updated Relationships | 2021-03-15 | CWE Content Team | MITRE | updated Demonstrative_Examples | 2021-07-20 | CWE Content Team | MITRE | updated Relationships | 2021-10-28 | CWE Content Team | MITRE | updated Relationships | 2022-04-28 | CWE Content Team | MITRE | updated Research_Gaps | 2022-06-28 | CWE Content Team | MITRE | updated Observed_Examples, Relationships | 2022-10-13 | CWE Content Team | MITRE | updated Observed_Examples | 2023-01-31 | CWE Content Team | MITRE | updated Demonstrative_Examples, Description, Potential_Mitigations, Relationships | 2023-04-27 | CWE Content Team | MITRE | updated Demonstrative_Examples, Detection_Factors, Relationships, Time_of_Introduction | 2023-06-29 | CWE Content Team | MITRE | updated Mapping_Notes, Relationships, Taxonomy_Mappings | 2024-02-29 (CWE 4.14, 2024-02-29) | CWE Content Team | MITRE | updated Demonstrative_Examples, Potential_Mitigations, References | 2024-07-16 (CWE 4.15, 2024-07-16) | CWE Content Team | MITRE | updated Applicable_Platforms, Observed_Examples | Previous Entry Names |
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Change Date | Previous Entry Name |
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2009-01-12 | Code Injection | | 2009-05-27 | Failure to Control Generation of Code (aka 'Code Injection') | | 2011-03-29 | Failure to Control Generation of Code ('Code Injection') | |
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